CONDITION BASED

MONITORING Part 3

A compilation of technical articles and papers prepared by:

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Temperature Sensors

ContentsArticles

Sensor 1

RTD 5

Resistance thermometer 5

Themocouples 13

Thermocouple 13

ReferencesArticle Sources and Contributors 22Image Sources, Licenses and Contributors 23

Article LicensesLicense 24

Sensor 1

Sensor

Thermocouple sensor for high temperaturemeasurement

A sensor is a device that measures a physical quantity and converts itinto a signal which can be read by an observer or by an instrument. Forexample, a mercury-in-glass thermometer converts the measuredtemperature into expansion and contraction of a liquid which can beread on a calibrated glass tube. A thermocouple converts temperatureto an output voltage which can be read by a voltmeter. For accuracy,all sensors need to be calibrated against known standards.

UseSensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim orbrighten by touching the base. There are also innumerable applications for sensors of which most people are neveraware. Applications include cars, machines, aerospace, medicine, manufacturing and robotics.A sensor is a device which receives and responds to a signal or stimulus. Here, the term "stimulus" means a propertyor a quantity that needs to be converted into electrical form. Hence, sensor can be defined as a device which receivesa signal and converts it into electrical form which can be further used for electronic devices. A sensor differs from atransducer in the way that a transducer converts one form of energy into other form whereas a sensor converts thereceived signal into electrical form only.A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. Forinstance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is1 cm/°C. Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact onwhat they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquidwhile the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured,making the sensor smaller often improves this and may introduce other advantages. Technological progress allowsmore and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. Inmost cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopicapproaches.

Classification of measurement errorsA good sensor obeys the following rules:• Is sensitive to the measured property• Is insensitive to any other property likely to be encountered in its application• Does not influence the measured propertyIdeal sensors are designed to be linear or linear to some simple mathematical fuction of the measurement, typicallylogrithmic. The output signal of such a sensor is linearly proportional to the value or simple function of the measuredproperty. The sensitivity is then defined as the ratio between output signal and measured property. For example, if asensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor islinear because the ratio is constant at all points of measurement.

Sensor 2

Sensor deviationsIf the sensor is not ideal, several types of deviations can be observed:• The sensitivity may in practice differ from the value specified. This is called a sensitivity error, but the sensor is

still linear.• Since the range of the output signal is always limited, the output signal will eventually reach a minimum or

maximum when the measured property exceeds the limits. The full scale range defines the maximum andminimum values of the measured property.

• If the output signal is not zero when the measured property is zero, the sensor has an offset or bias. This is definedas the output of the sensor at zero input.

• If the sensitivity is not constant over the range of the sensor, this is called nonlinearity. Usually this is defined bythe amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage ofthe full range.

• If the deviation is caused by a rapid change of the measured property over time, there is a dynamic error. Often,this behaviour is described with a bode plot showing sensitivity error and phase shift as function of the frequencyof a periodic input signal.

• If the output signal slowly changes independent of the measured property, this is defined as drift(telecommunication).

• Long term drift usually indicates a slow degradation of sensor properties over a long period of time.• Noise is a random deviation of the signal that varies in time.• Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in

time for the sensor to respond, creating a different offset error in one direction than in the other.• If the sensor has a digital output, the output is essentially an approximation of the measured property. The

approximation error is also called digitization error.• If the signal is monitored digitally, limitation of the sampling frequency also can cause a dynamic error, or if the

variable or added noise noise changes periodically at a frequency near a multiple of the sampling rate may induceailiasing errors.

• The sensor may to some extent be sensitive to properties other than the property being measured. For example,most sensors are influenced by the temperature of their environment.

All these deviations can be classified as systematic errors or random errors. Systematic errors can sometimes becompensated for by means of some kind of calibration strategy. Noise is a random error that can be reduced bysignal processing, such as filtering, usually at the expense of the dynamic behaviour of the sensor.

ResolutionThe resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digitaldisplay, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. Theresolution is related to the precision with which the measurement is made. For example, a scanning tunneling probe(a fine tip near a surface collects an electron tunnelling current) can resolve atoms and molecules.

Sensors in NatureAll living organisms contain biological sensors with functions similar to those of the mechanical devices described.Most of these are specialized cells that are sensitive to:• Light, motion, temperature, magnetic fields, gravity, humidity, vibration, pressure, electrical fields, sound, and

other physical aspects of the external environment• Physical aspects of the internal environment, such as stretch, motion of the organism, and position of appendages

(proprioception)• Environmental molecules, including toxins, nutrients, and pheromones

Sensor 3

• Estimation of biomolecules interaction and some kinetics parameters• Internal metabolic milieu, such as glucose level, oxygen level, or osmolality• Internal signal molecules, such as hormones, neurotransmitters, and cytokines• Differences between proteins of the organism itself and of the environment or alien creatures

BiosensorIn biomedicine and biotechnology, sensors which detect analytes thanks to a biological component, such as cells,protein, nucleic acid or biomimetic polymers, are called biosensors. Whereas a non-biological sensor, even organic(=carbon chemistry), for biological analytes is referred to as sensor or nanosensor (such a microcantilevers). Thisterminology applies for both in vitro and in vivo applications. The encapsulation of the biological component inbiosensors, presents with a slightly different problem that ordinary sensors, this can either be done by means of asemipermeable barrier, such as a dialysis membrane or a hydrogel, a 3D polymer matrix, which either physicallyconstrains the sensing macromolecule or chemically (macromolecule is bound to the scaffold. [1]

See also• Actuator• Capacitive displacement sensor• Data acquisition• Data logger• Detection theory• Fully automatic time• Lateral line• Limen• List of sensors• Machine olfaction• Nanoelectronics• Nanosensor• Printed electronics• Receiver operating characteristic• Transducer• Wireless sensor network• Sensor Web

External links• Capacitive Position/Displacement Sensor Theory/Tutorial [2]

• Capacitive Position/Displacement Overview [3]

• Comparing Capacitive and Eddy-Current Sensors [4]

• M. Kretschmar and S. Welsby (2005), Capacitive and Inductive Displacement Sensors, in Sensor TechnologyHandbook, J. Wilson editor, Newnes: Burlington, MA.

• C. A. Grimes, E. C. Dickey, and M. V. Pishko (2006), Encyclopedia of Sensors (10-Volume Set), AmericanScientific Publishers. ISBN 1-58883-056-X

• Sensors [5] - Open access journal of MDPI [6]

• M. Pohanka, O. Pavlis, and P. Skladal. Rapid Characterization of Monoclonal Antibodies using the PiezoelectricImmunosensor [7]. Sensors 2007, 7, 341-353

• SensEdu; how sensors work [8]

Sensor 4

• Clifford K. Ho, Alex Robinson, David R. Miller and Mary J. Davis. Overview of Sensors and Needs forEnvironmental Monitoring [9]. Sensors 2005, 5, 4-37

• Wireless hydrogen sensor [10]

• Sensors and Actuators [11] - Elsevier journal• Automated Deformation Monitoring system [12]

• Droid's Sensors [13] - About mobile sensors for iPhone, Android, Symbian etc

References[1] Wolfbeis, O. S. (2000). "Fiber-optic chemical sensors and biosensors." Anal Chem 72(12): 81R-89R[2] http:/ / www. capsensortheory. com[3] http:/ / www. capsensors. com[4] http:/ / www. lionprecision. com/ tech-library/ technotes/ article-0011-cve. html[5] http:/ / www. mdpi. com/ journal/ sensors[6] http:/ / www. mdpi. net[7] http:/ / www. mdpi. org/ sensors/ papers/ s7030341. pdf[8] http:/ / www. sensedu. com/[9] http:/ / www. mdpi. net/ sensors/ papers/ s5010004. pdf[10] http:/ / news. ufl. edu/ 2006/ 05/ 24/ hydrogen-sensor/[11] http:/ / www1. elsevier. com/ homepage/ saa/ sensors[12] http:/ / www. measurandgeotechnical. com/[13] http:/ / www. droidsensors. com/

5

RTD

Resistance thermometerResistance thermometers, also called resistance temperature detectors or resistive thermal devices (RTDs), aretemperature sensors that exploit the predictable change in electrical resistance of some materials with changingtemperature. As they are almost invariably made of platinum, they are often called platinum resistancethermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below600 °C, due to higher accuracy and repeatability.[1]

General descriptionThere are many categories; carbon resistors, film, and wire-wound types are the most widely used.• Carbon resistors are widely available and are very inexpensive. They have very reproducible results at low

temperatures. They are the most reliable form at extremely low temperatures. They generally do not suffer fromhysteresis or strain gauge effects. Carbon resistors have been employed by researchers for years because of themany advantages associated with them.

• Film thermometers have a layer of platinum on a substrate; the layer may be extremely thin, perhaps onemicrometer. Advantages of this type are relatively low cost and fast response. Such devices have improvedperformance although the different expansion rates of the substrate and platinum give "strain gauge" effects andstability problems.

• Wire-wound thermometers can have greater accuracy, especially for wide temperature ranges. The coil diameterprovides a compromise between mechanical stability and allowing expansion of the wire to minimize strain andconsequential drift.

Resistance thermometer 6

• Coil Elements have largely replaced wire wound elements in the industry. This design allows the wire coil toexpand more freely over temperature while still provided the necessary support for the coil. This design is similarto that of a SPRT, the primary standard which ITS-90 is based on, while still providing the durability necessaryfor an industrial process.

The current international standard which specifies tolerance and the temperature to electrical resistance relationshipfor platinum resistance thermometers is IEC 751:1983. By far the most common devices used in industry have anominal resistance of 100 ohms at 0 °C, and are called Pt-100 sensors ('Pt' is the symbol for platinum). Thesensitivity of a standard 100 ohm sensor is a nominal 0.385 ohm/°C. RTDs with a sensitivity of 0.375 and 0.392ohm/°C as well as a variety of others are also available.

FunctionResistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatabilityin some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistancethermometers use electrical resistance and require a power source to operate. The resistance ideally varies linearlywith temperature.Resistance thermometers are usually made using platinum, because of its linear resistance-temperature relationshipand its chemical inertness. The platinum detecting wire needs to be kept free of contamination to remain stable. Aplatinum wire or film is supported on a former in such a way that it gets minimal differential expansion or otherstrains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are alsoused in some applications.Commercial platinum grades are produced which exhibit a change of resistance of 0.385 ohms/°C (European Fundamental Interval) The sensor is usually made to have a resistance of 100Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995) . The American Fundamental Interval is 0.392 Ω/°C, based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result the "American standard"

Resistance thermometer 7

is hardly the standard even in the US.Resistance thermometers require a small current to be passed through in order to determine the resistance. This cancause resistive heating, and manufacturers' limits should always be followed along with heat path considerations indesign. Care should also be taken to avoid any strains on the resistance thermometer in its application. Lead wireresistance should be considered, and adopting three and four wire connections can eliminate connection leadresistance effects from measurements - industrial practice is almost universally to use 3-wire connection. 4-wireconnections need to be used for precise applications.

Advantages and limitationsAdvantages of platinum resistance thermometers:• High accuracy• Low drift• Wide operating range• Suitability for precision applicationsLimitations:

• RTDs in industrial applications are rarely used above 660 °C. At temperatures above 660 °C it becomesincreasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath ofthe thermometer. This is why laboratory standard thermometers replace the metal sheath with a glassconstruction. At very low temperatures, say below -270 °C (or 3 K), due to the fact that there are very fewphonons, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basicallyindependent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.

• Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slowerresponse time. However, thermistors have a smaller temperature range and stability.

Common Error Sources:

The common error sources of a PRT are:• Interchangeability: the “closeness of agreement” between the specific PRT's Resistance vs. Temperature

relationship and a predefined Resistance vs. Temperature relationship, commonly defined by IEC 60751.[2]

• Insulation Resistance: Error caused by the inability to measure the actual resistance of element. Current leaks intoor out of the circuit through the sheath, between the element leads, or the elements.[3]

• Stability: Ability to maintain R vs T over time as a result of thermal exposure.[4]

• Repeatability: Ability to maintain R vs T under the same conditions after experiencing thermal cyclingthroughout a specified temperature range.[5]

• Hysteresis: Change in the characteristics of the materials from which the RTD is built due to exposures to varyingtemperatures.[6]

• Stem Conduction: Error that results from the PRT sheath conducting heat into or out of the process.• Calibration/Interpolation: Errors that occur due to calibration uncertainty at the cal points, or between cal point

due to propagation of uncertainty or curve fit errors.• Lead Wire: Errors that occur because a 4 wire or 3 wire measurement is not used, this is greatly increased by

higher gauge wire.• 2 wire connection adds lead resistance in series with PRT element.• 3 wire connection relies on all 3 leads having equal resistance.

• Self Heating: Error produced by the heating of the PRT element due to the power applied.• Time Response: Errors are produced during temperature transients because the PRT cannot respond to changes

fast enough.

Resistance thermometer 8

• Thermal EMF: Thermal EMF errors are produced by the EMF adding to or subtracting from the applied sensingvoltage, primarily in DC systems.

When to use RTDs or thermocouplesThe two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs)and thermocouples. But when should control engineers use a Thermocouple and when should they use an RTD? Theanswer is usually determined by four factors: Factors: - Temperature, time, size, and overall accuracy requirements.• What are the temperature requirements? If process temperatures fall from -200 to 500 °C (-328 to 932 °F), then

an industrial RTD is the preferred option. Thermocouples have a range of -180 to 2320 °C (-292 to 4208 °F),[7]so for extremely high temperatures they are the only contact temperature measurement choice.

• What are the time-response requirements? If the process requires a very fast response to temperaturechanges—fractions of a second as opposed to seconds (i.e. 2.5 to 10 s)—then a thermocouple is the best choice.Keep in mind that time response is measured by immersing the sensor in water moving at 1 m/s (3 ft/s) with a63.2% step change.

• What are the size requirements? A standard RTD sheath is 3.175 to 6.35 mm (0.1250 to 0.250 in) in diameter,while sheath diameters for thermocouples can be less than 1.6 mm (0.063 in).

• What are the overall requirements for accuracy? If the process only requires a tolerance of 2 °C or greater, then athermocouple is appropriate. If the process needs less than 2 °C tolerance, then an RTD is sometimes the onlychoice. Keep in mind, unlike RTDs that can maintain stability for many years, thermocouples can drift within thefirst few hours of use

ElementsResistance thermometer elements are available in a number of forms. The most common are:• Unsupported Wire wound - a wire coil minimally supported within a sealed housing and backfilled with an inert

gas. These sensors work up to 961.78 °C and are used in the SPRT’s that define ITS-90• Wire wound in a ceramic insulator – a wire coil sealed in a ceramic cylinder, works with temperatures to 850 °C• Wire encapsulated in glass - wire around glass core with glass fused homogenously around, resists vibration,

more protection to the detecting wire but smaller usable range• Thin film - platinum film on ceramic substrate, small and inexpensive to mass produce, fast response to

temperature change but smaller temperature range and lower accuracy.

Construction

These elements nearly always require insulated leads attached. At low temperatures PVC, silicon rubber or PTFEinsulators are common to 250°C. Above this, glass fibre or ceramic are used. The measuring point and usually mostof the leads require a housing or protection sleeve. This is often a metal alloy which is inert to a particular process.Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer thatmust withstand chemical or physical attack and offer convenient process attachment points.

Resistance thermometer 9

Wiring configurations

Two-wire configuration

The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not requiredas the resistance of the connecting wires is always included with that of the sensor leading to errors in the signal.Using this configuration you will be able to use 100 meters of cable. This applies equally to balanced bridge andfixed bridge system.

Three-wire configuration

In order to minimize the effects of the lead resistances a three wire configuration can be used. Using this method thetwo leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore thelead resistance is cancelled out. High quality connection cables should be used for this type of configuration becausean assumption is made that the two lead resistances are the same. This configuration allows for up to 600 meters ofcable.

Four-wire configuration

The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured. In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better alternative configuration four-wire Kelvin connection that

Resistance thermometer 10

should be used in all RTDs. It provides full cancellation of spurious effects and cable resistance of up to 15 Ω can behandled. Actually in four wire measurement the resistance error due to lead wire resistance is zero.

HistoryThe application of the tendency of electrical conductors to increase their electrical resistance with rising temperaturewas first described by Sir William Siemens at the Bakerian Lecture of 1871 before the Royal Society of GreatBritain. The necessary methods of construction were established by Callendar, Griffiths, Holborn and Wein between1885 and 1900.

Standard resistance thermometer dataTemperature sensors are usually supplied with thin-film elements. The resisting elements are rated in accordancewith BS EN 60751:2008 as:

Tolerance Class Valid Range

Tolerance class F 0.3 -50 to +500°C

Tolerance class F0.15

-30 to +300°C

Tolerance class F 0.1 0 to +150 °C

Resistance thermometer elements can be supplied which function up to 1000 °C. The relation between temperatureand resistance is given by the Callendar-Van Dusen equation,

Here, is the resistance at temperature T, is the resistance at 0 °C, and the constants (for an alpha=0.00385platinum RTD) are

Since the B and C coefficients are relatively small, the resistance changes almost linearly with the temperature.

Resistance thermometer 11

Values for various popular resistance thermometers

Values for various popular resistance thermometers

Temperaturein °C

Pt100in Ω

Pt1000in Ω

PTCin Ω

NTCin Ω

NTCin Ω

NTCin Ω

NTCin Ω

NTCin Ω

Typ: 404 Typ: 501 Typ: 201 Typ: 101 Typ: 102 Typ: 103 Typ: 104 Typ: 105

−50 80,31 803,1 1032

−45 82,29 822,9 1084

−40 84,27 842,7 1135 50475

−35 86,25 862,5 1191 36405

−30 88,22 882,2 1246 26550

−25 90,19 901,9 1306 26083 19560

−20 92,16 921,6 1366 19414 14560

−15 94,12 941,2 1430 14596 10943

−10 96,09 960,9 1493 11066 8299

−5 98,04 980,4 1561 31389 8466

0 100,00 1000,0 1628 23868 6536

5 101,95 1019,5 1700 18299 5078

10 103,90 1039,0 1771 14130 3986

15 105,85 1058,5 1847 10998

20 107,79 1077,9 1922 8618

25 109,73 1097,3 2000 6800 15000

30 111,67 1116,7 2080 5401 11933

35 113,61 1136,1 2162 4317 9522

40 115,54 1155,4 2244 3471 7657

45 117,47 1174,7 2330 6194

50 119,40 1194,0 2415 5039

55 121,32 1213,2 2505 4299 27475

60 123,24 1232,4 2595 3756 22590

65 125,16 1251,6 2689 18668

70 127,07 1270,7 2782 15052

75 128,98 1289,8 2880 12932

80 130,89 1308,9 2977 10837

85 132,80 1328,0 3079 9121

90 134,70 1347,0 3180 7708

95 136,60 1366,0 3285 6539

100 138,50 1385,0 3390

105 140,39 1403,9

110 142,29 1422,9

Resistance thermometer 12

150 157,31 1573,1

200 175,84 1758,4

See also• Thermowell• Thermistor• Thermostat• Thermocouple• Platinum

References[1] Frequently asked questions about RTD’s (http:/ / www. burnsengineering. com/ pgd. asp?pgid=docfaq), , retrieved 2009-09-18[2] Interchangeability (http:/ / www. burnsengineering. com/ document/ pdf/ interchangeability. pdf), , retrieved 2009-09-18[3] Insulation Resistance (http:/ / www. burnsengineering. com/ document/ pdf/ a080211. pdf), , retrieved 2009-09-18[4] Stability (http:/ / www. burnsengineering. com/ document/ pdf/ a080306. pdf), , retrieved 2009-09-18[5] Repeatability (http:/ / www. burnsengineering. com/ document/ papers/ PRT_Error_Sources_Part_4_Repeatability. pdf), , retrieved

2009-09-18[6] Hysteresis (http:/ / www. burnsengineering. com/ document/ papers/ PRT_Error_Sources_Part_5_Hysteresis. pdf), , retrieved 2009-09-18[7] http:/ / www. omega. com/ temperature/ Z/ pdf/ z241-245. pdf

Text and images used by permission of Peak Sensors Ltd: Platinum Resistance Thermometer (http:/ / www.peaksensors. co. uk/ rtddatasheets. html) Information

External links• Practical computational RTD linearization techniques for DAQ and embedded systems. (http:/ / garga. iet. unipi.

it/ II/ Linearizing RTD. pdf)• Information on RTDs and Calibration (http:/ / www. burnsengineering. com) from Burns Engineering• The Callendar – van Dusen coefficients (http:/ / www. uniteksys. com/ Graphics/ CalVan. pdf) - How to calculate

Callendar – van Dusen coefficients

13

Themocouples

Thermocouple

Thermocouple plugged to a multimeterdisplaying room temperature in °C.

A thermocouple is a junction between two different metals thatproduces a voltage related to a temperature difference. Thermocouplesare a widely used type of temperature sensor for measurement andcontrol[1] and can also be used to convert heat into electric power.They are inexpensive[2] and interchangeable, are supplied fitted withstandard connectors, and can measure a wide range of temperatures.The main limitation is accuracy: system errors of less than one degreeCelsius (C) can be difficult to achieve.[3]

Any junction of dissimilar metals will produce an electric potentialrelated to temperature. Thermocouples for practical measurement oftemperature are junctions of specific alloys which have a predictableand repeatable relationship between temperature and voltage. Differentalloys are used for different temperature ranges. Properties such asresistance to corrosion may also be important when choosing a type ofthermocouple. Where the measurement point is far from the measuringinstrument, the intermediate connection can be made by extensionwires which are less costly than the materials used to make the sensor.Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instrumentsuse electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals.Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve theprecision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gasturbine exhaust, diesel engines, and other industrial processes.

Principle of operationIn 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected toa thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Anyattempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additionalconductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose theoriginal. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to completethe circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltageavailable for measurement. That difference increases with temperature, and is between 1 and 70 microvolts perdegree Celsius (µV/°C) for standard metal combinations.

Thermocouple 14

Practical use

A thermocouple measuring circuit with a heatsource, cold junction and a measuring instrument.

Voltage–temperature relationship

Polynomial Coefficients 0-500 °C[4]

n Type K

1 25.08355

2 7.860106x10−2

3 -2.503131x10−1

4 8.315270x10−2

5 -1.228034x10−2

6 9.804036x10−4

7 -4.413030x10−5

8 1.057734x10−6

9 -1.052755x10−8

The nonlinear relationship between the temperature difference (ΔT) and the output voltage (mV) of a thermocouplecan be approximated by a polynomial:

The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals. In some cases betteraccuracy is obtained with additional non-polynomial terms[4] . A database of voltage as a function of temperature,and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple isavailable online[4] .In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;[5] olderdevices use analog circuits.

Thermocouple 15

Cold junction compensationThermocouples measure the temperature difference between two points, not absolute temperature. To measure asingle temperature one of the junctions—normally the cold junction—is maintained at a known referencetemperature, and the other junction is at the temperature to be sensed.Having a junction of known temperature, while useful for laboratory calibration, is not convenient for mostmeasurement and control applications. Instead, they incorporate an artificial cold junction using a thermally sensitivedevice such as a thermistor or diode to measure the temperature of the input connections at the instrument, withspecial care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a knowncold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.It is worth noting that the EMF (or voltage) is NOT generated at the junction of the two metals of the thermocouplebut rather along that portion of the length of the two dissimilar metals that is subjected to a temperature gradient.Alternatively cold junction compensation can be performed by computation using look-up tables[5] and polynomialinterpolation.

Power productionA thermocouple can produce current, which means it can be used to drive some processes directly, without the needfor extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when atemperature difference arises. The electrical energy generated by a thermocouple is converted from the heat energywhich must be supplied to the hot side to maintain the electric potential. A continuous flow of heat is necessarybecause the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side toheat up (the Peltier effect).Thermocouples can be connected in series to form a thermopile, where all the hot junctions are exposed to a higherand all the cold junctions to a lower temperature. The output is the sum of the voltages across the individualjunctions, giving larger voltage and power output. Using the radioactive decay of transuranic elements as a heatsource, this arrangement has been used to power spacecraft on missions too far from the Sun to utilize solar power.

GradesThermocouple wire is available in several different metallurgical formulations per type, typically, in decreasinglevels of accuracy and cost: special limits of error, standard, and extension grades.

Extension wire

Extension grade wires made of the same metals as a higher-grade thermocouple are used to connect it to a measuringinstrument some distance away without introducing additional junctions between dissimilar materials which wouldgenerate unwanted voltages; the connections to the extension wires, being of like metals, do not generate a voltage.In the case of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive touse platinum for extension wires. The extension wire is specified to have a very similar thermal coefficient of EMFto the thermocouple, but only over a narrow range of temperatures; this reduces the cost significantly.The temperature-measuring instrument must have high input impedance to prevent any significant current draw fromthe thermocouple, to prevent a resistive voltage drop across the wire. Changes in metallurgy along the length of thethermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouplejunction which affects measurement accuracy.

Thermocouple 16

TypesCertain combinations of alloys have become popular as industry standards. Selection of the combination is driven bycost, availability, convenience, melting point, chemical properties, stability, and output. Different types are bestsuited for different applications. They are usually selected based on the temperature range and sensitivity needed.Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selectioncriteria include the inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocoupletypes are listed below with the positive electrode first, followed by the negative electrode.

KType K (chromel–alumel) is the most common general purpose thermocouple with a sensitivity of approximately41 µV/°C, chromel positive relative to alumel.[6] It is inexpensive, and a wide variety of probes are available in its−200 °C to +1350 °C / -328 °F to +2462 °F range. Type K was specified at a time when metallurgy was lessadvanced than it is today, and consequently characteristics vary considerably between samples. One of theconstituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that theyundergo a step change in output when the magnetic material reaches its Curie point (around 354 °C for type Kthermocouples).

EType E (chromel–constantan)[5] has a high output (68 µV/°C) which makes it well suited to cryogenic use.Additionally, it is non-magnetic.

JType J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about55 µV/°C.[2] The Curie point of the iron (770 °C) causes an abrupt change in the characteristic, which determines theupper temperature limit.

NType N (Nicrosil–Nisil) (Nickel-Chromium-Silicon/Nickel-Silicon) thermocouples are suitable for use at hightemperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity isabout 39 µV/°C at 900 °C, slightly lower than type K. Designed to be an improved type K, it is becoming morepopular.

Platinum types B, R, and STypes B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These are amongthe most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, andS thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.B

Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodiumwhile the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type Bthermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.R

Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinumfor the other conductor. Type R thermocouples are used up to 1600 °C.S

Thermocouple 17

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire)and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

TType T (copper–constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used asa differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, thereis no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about43 µV/°C.

CType C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must neverbe used in the presence of oxygen at temperatures above 260 °C.

MType M thermocouples use a nickel alloy for each wire. The positive wire contains 18% molybdenum while thenegative wire contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as withtype C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.

Chromel-gold/ironIn chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction(0.03–0.15 atom percent) of iron. It can be used for cryogenic applications (1.2–300 K and even up to 600 K). Boththe sensitivity and the temperature range depends on the iron concentration. The sensitivity is typically around 15µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.

Laws for thermocouples

Law of homogeneous materialA thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heatalone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring betweenthe input and output do not affect the output voltage, provided all wires are made of the same materials as thethermocouple.

Law of intermediate materialsThe algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar materials is zero ifall of the junctions are at a uniform temperature. So If a third metal is inserted in either wire and if the two newjunctions are at the same temperature, there will be no net voltage generated by the new metal.

Thermocouple 18

Law of successive or intermediate temperaturesIf two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and T2 and producethermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will beemf1 + emf2 .

Aging of thermocouplesThermoelements are often used at high temperatures and in reactive furnace atmospheres. In this case the practicallifetime is determined by aging. The thermoelectric coefficients of the wires in the area of high temperature changewith time and the measurement voltage drops. The simple relationship between the temperature difference of thejoints and the measurement voltage is only correct if each wire is homogeneous. With an aged thermocouple this isnot the case. Relevant for the generation of the measurement voltage are the properties of the metals at a temperaturegradient. If an aged thermocouple is pulled partly out of the furnace, the aged parts from the region previously athigh temperature enter the area of temperature gradient and the measurement error is significantly increased.However an aged thermocouple that is pushed deeper into the furnace gives a more accurate reading.

Thermocouple comparisonThe table below describes properties of several different thermocouple types. Within the tolerance columns, Trepresents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.

Type Temperaturerange °C

(continuous)

Temperaturerange °C

(short term)

Toleranceclass one

(°C)

Toleranceclass two

(°C)

IEC Color code BS Color code ANSI Color code

K 0 to +1100 −180 to+1300

±1.5between−40 °Cand 375 °C±0.004×Tbetween375 °C and1000 °C

±2.5between−40 °C and333 °C±0.0075×Tbetween333 °C and1200 °C

J 0 to +700 −180 to +800 ±1.5between−40 °Cand 375 °C±0.004×Tbetween375 °C and750 °C

±2.5between−40 °C and333 °C±0.0075×Tbetween333 °C and750 °C

N 0 to +1100 −270 to+1300

±1.5between−40 °Cand 375 °C±0.004×Tbetween375 °C and1000 °C

±2.5between−40 °C and333 °C±0.0075×Tbetween333 °C and1200 °C

Thermocouple 19

R 0 to +1600 −50 to +1700 ±1.0between 0°C and1100 °C±[1 +0.003×(T− 1100)]between1100 °Cand 1600°C

±1.5between 0°C and 600°C±0.0025×Tbetween600 °C and1600 °C

Not defined.

S 0 to 1600 −50 to +1750 ±1.0between 0°C and1100 °C±[1 +0.003×(T− 1100)]between1100 °Cand 1600°C

±1.5between 0°C and 600°C±0.0025×Tbetween600 °C and1600 °C

Not defined.

B +200 to+1700

0 to +1820 NotAvailable

±0.0025×Tbetween600 °C and1700 °C

No standard usecopper wire

No standard usecopper wire

Not defined.

T −185 to +300 −250 to +400 ±0.5between−40 °Cand 125 °C±0.004×Tbetween125 °C and350 °C

±1.0between−40 °C and133 °C±0.0075×Tbetween133 °C and350 °C

E 0 to +800 −40 to +900 ±1.5between−40 °Cand 375 °C±0.004×Tbetween375 °C and800 °C

±2.5between−40 °C and333 °C±0.0075×Tbetween333 °C and900 °C

Chromel/AuFe −272 to +300 n/a Reproducibility 0.2% ofthe voltage; each sensorneeds individualcalibration.

ApplicationsThermocouples are suitable for measuring over a large temperature range, up to 2300 °C. They are less suitable forapplications where smaller temperature differences need to be measured with high accuracy, for example the range0–100 °C with 0.1 °C accuracy. For such applications thermistors and resistance temperature detectors are moresuitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and otherindustrial processes.

Thermocouple 20

Steel industryType B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures andchemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used inthe electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of asmall steel sample can be analyzed and used to estimate the carbon content of molten steel.

Heating appliance safetyMany gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gasburner when required. If it goes out gas may be released, which is a fire risk and a health hazard. To prevent thissome appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of thethermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas tothe pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. Ifthe pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop andthe valve to close.Some systems, known as millivolt control systems, extend this concept to the main gas valve as well. Not only doesthe voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat topower the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system describedabove, and a thermopile is used rather than a single thermocouple. Such a system requires no external source ofelectricity for its operation and so can operate during a power failure, provided all the related system componentsallow for this. Note that this excludes common forced air furnaces because external power is required to operate theblower motor, but this feature is especially useful for un-powered convection heaters.A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burnerignites within a certain time period, shutting off the main burner gas supply valve should that not happen.Out of concern for energy wasted by the standing pilot, designers of many newer appliances have switched to anelectronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is norisk of gas buildup should the flame go out, so these appliances do not need thermocouple-based safety pilot safetyswitches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots arestill used in some appliances. The exception is later model instantaneous water heaters that utilise the flow of waterto generate the current required to ignite the gas burner, in conjunction with a thermocouple as a safety cut-offdevice in the event the gas fails to ignite, or the flame is extinguished.

Thermopile radiation sensorsThermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heatsthe hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only afew μW/cm2 with commercially available thermopile sensors. For example, some laser power meters are based onsuch sensors.

ManufacturingThermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example,switchgear under test for its current carrying capacity may have thermocouples installed and monitored during a heatrun test, to confirm that the temperature rise at rated current does not exceed designed limits.

Thermocouple 21

Radioisotope thermoelectric generatorsThermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.

Process plantsChemical production and petroleum refineries will usually employ computers for logging and limit testing the manytemperatures associated with a process, typically numbering in the hundreds. For such cases a number ofthermocouple leads will be brought to a common reference block (a large block of copper) containing the secondthermocouple of each circuit. The temperature of the block is in turn measured by a thermistor. Simple computationsare used to determine the temperature at each measured location.

See also• Bolometer• Giuseppe Domenico Botto• Resistance thermometer• Thermistor• List of sensors• International Temperature Scale of 1990

External links• Thermocouple Operating Principle - University Of Cambridge [7]

• Thermocouple Drift - University Of Cambridge [8]

• Thermocouple design guide [9]

• Mineral-Insulated Thermocouple Know-How [10]

• Thermocouple Color Code Chart and Specifications [11]

• Thermocouple Attachment - A Primer [12]

• Thermocouple design [13]

References[1] "Thermocouple temperature sensors" (http:/ / www. temperatures. com/ tcs. html). Temperatures.com. . Retrieved 2007-11-04.[2] Ramsden, Ed (September 1, 2000). "Temperature measurement" (http:/ / www. sensorsmag. com/ sensors/ temperature/

temperature-measurement-1030). Sensors. . Retrieved 2010-02-19.[3] "Technical Notes: Thermocouple Accuracy" (http:/ / www. microlink. co. uk/ tctable. html). IEC 584-2(1982)+A1(1989). . Retrieved

2010-04-28.[4] "NIST ITS-90 Thermocouple Database" (http:/ / srdata. nist. gov/ its90/ main/ ). .[5] Baker, Bonnie C. (September 1, 2000). "Designing the embedded temperature circuit to meet the system's requirements" (http:/ / www.

sensorsmag. com/ sensors/ temperature/ designing-embedded-temperature-circuit-meet-system039s-requi-1089?print=1). Sensors. . Retrieved2010-04-26.

[6] Manual on the Use of Thermocouples in Temperature Measurements. ASTM, 1974[7] http:/ / www. msm. cam. ac. uk/ UTC/ thermocouple/ pages/ ThermocouplesOperatingPrinciples. html[8] http:/ / www. msm. cam. ac. uk/ UTC/ thermocouple/ pages/ Drift. html[9] http:/ / www. peaksensors. co. uk/ designguide. html[10] http:/ / www. isomil. de/ en/ mineral-insulated-cable. htm[11] http:/ / www. thermalcorp. com/ documents/ TCCHART. pdf[12] http:/ / www. ecd. com/ news/ articles/ tc-attachment. asp[13] http:/ / www. keyosens. com/ tcletter. php

Article Sources and Contributors 22

Article Sources and ContributorsSensor  Source: http://en.wikipedia.org/w/index.php?oldid=368639227  Contributors: 01101001, 16@r, A.K., AK Auto, Achalmeena, Alexandrov, Allstarecho, Alteripse, Amaltheus,AmyKondo, Analogauthority, Andrewpmk, Anlace, Anthony Appleyard, Atlan, Atlantia, Atul44885, Axel.mulder, Barkeep, Beetstra, Benvogel, BillC, Binksternet, Blainster, Blobglob, Bobo192,Boing! said Zebedee, Bongwarrior, CENSI, Calltech, Caltas, Caltrop, Can't sleep, clown will eat me, CanadianLinuxUser, Capricorn42, Chaosdruid, ChemGardener, Clan-destine, Cowman109,Cpl Syx, D climacus, DH85868993, DanaRuff, Dancter, Darth Panda, Dawnseeker2000, Dbcv, Decltype, DerHexer, Dhiva1, Dicklyon, Dieselbub, Digresser, Dina, Dmr2, E Wing, ERK,Eaolson, Edebraal, Ehynes, Electricsforlife, EncMstr, Epbr123, Eric119, Evilrabidplotbunnies, EyeSerene, Fabartus, Falcon8765, Femto, Finejon, Flowerpotman, Funandtrvl, Galoubet, Geekstuff,Geodesic42, Gilliam, Glaurung, Glenn, Grubber, Guidod, Gzkn, HalfShadow, Hankwang, Harriv, Haukurth, Helix84, Henkjanvanderpol, Heron, Hukseflux, Iautomation, Ikalogic, Indurilo,Jackelfive, Jaeger5432, Jaxhere, Jc3s5h, Jlmerrill, Jni, JoanneB, Jon Awbrey, JonHarder, Jondel, Jreferee, Karafias, Kaverin, Kender, Keyence, Kingpin13, Kisss256, Kkolodziej,KnowledgeOfSelf, Korg, LOL, LeaveSleaves, Leonard G., Levineps, Light current, Lights, Love Krittaya, Mac, Madurasn, Mako098765, Malo, Manavbhardwaj, Mark Kretschmar,Masgatotkaca, MauriceKA, Mdd, Meaghan, Meekywiki, Mikr18, Minakshimeena, Mion, Miroslavpohanka, Momergil, Momusufan, Monkey230, Mortenoesterlundjoergensen, Mrh30,NawlinWiki, Ncmvocalist, Neelix, Nemilar, Newbi, Nornen3, Nosleep, Nposs, Oli Filth, Omair.majid, Onebravemonkey, Ot, Patrick, Patrick Berry, Pavithrabtech, Payno, Pb30, Pearle,Peruvianllama, Phantom784, Pigletpoglet, Pit, Pro66, Publunch, Qatter, RJFJR, Raasgat, Rabbiz, Radudobra, RainbowOfLight, Rajeev1988, RalphWiggum, Rdsmith4, Renaissancee, Rettetast,ReturnofPenitron, Rigadoun, Rpyle731, Rubin joseph 10, Sadik ulubey, Sanders muc, Sapphic, Semiwiki, Sergio.ballestrero, Shappy, Shawnhath, SilentC, Silivrenion, Sinbaddasail0r, SirNicholas de Mimsy-Porpington, Sjorford, Slowking Man, Smack, Snowdog, Snowolf, Solarmicrowavefive, Sole Soul, Sonett72, Sourcer66, Squidonius, Srleffler, Stephenb, Steve Pucci,Stoneygirl45, Stuartfost, SuperMono, Synthy, The Anome, The Photon, The flamingo, TheFeds, Tiddly Tom, TigerShark, Tohd8BohaithuGh1, Troy.peterson, Ummairsaeed, V8rik, Vactivity,Veinor, Vsmith, WikHead, WikiLeon, Wikifan798, YourEyesOnly, ZX81, Zzuuzz, 410 anonymous edits

Resistance thermometer  Source: http://en.wikipedia.org/w/index.php?oldid=366951672  Contributors: Ahoerstemeier, AlphaTeller, Anonymous6494, ArséniureDeGallium, Bassbonerocks,Bobblewik, Calebcs, Callunckleart, CanadianLinuxUser, Carnildo, Catapult, Chochopk, Closedmouth, Codeczero, EliV, Femto, Gabriel Kielland, Gaius Cornelius, Gene Nygaard, Grahamwild,Hankwang, Heron, Howcheng, Ihg, Ixfd64, Jaan513, Japanese Searobin, Jgoldfar, Jwortzel, Karpouzi, Kimmyb76, Kjkolb, Lankiveil, MER-C, Microfrost, Mpj28202, Mpj96, Nemilar,Nitecrawler, Noisy, PHermans, Phildong1339, Psanderson, Qqzzccdd, RainbowOfLight, Rasbach, Redeemer, Rjwilmsi, Robert - Northern VA, Roiwikiacct, Rorro, Rwl10267, Sadalmelik,SchuminWeb, Shikharsaxena, Shirifan, Sho Uemura, Skraz, Srleffler, Stokito, Suspekt, THEN WHO WAS PHONE?, Tasior, Tbhotch, Tom Hawkins, Toon05, Torrentcat, Turian, Vuo,WingkeeLEE, Xxslayer88xx, Yummy mummy, 145 anonymous edits

Thermocouple  Source: http://en.wikipedia.org/w/index.php?oldid=366702951  Contributors: Andante1980, ArchonMagnus, Ashgup, Atlant, AxelBoldt, Axeman89, BASF Catalysts, Bkell,Blazotron, Bmunden, Bobbis, Bobo192, Bobstay, Bogey97, Bookaddict, BritishWatcher, Brockert, Bromskloss, Bryan Derksen, Bubba hotep, Bushcarrot, Buster2058, Calebcs, Callunckleart,Caltas, CanadianLinuxUser, Cassavau, CesarB, CharlesC, Chinju, Chris the speller, ChrisHodgesUK, CommonsDelinker, Compellingelegance, Congruence, Conversion script, Cookingpro, Corti,Cybercobra, Danceswithzerglings, Davidagomezjr, Dcm684, Dcoggins, Dekimasu, Doradus, Dtwo, Emanuel.Blei, Filsorrow, Flatline, Fleet Pete, Fleminra, Gaius Cornelius, Garfield226, GeneNygaard, Gibbon prime, Gillyweed, Giraffedata, Glen, Gsmcolect, Guymacon, Hadal, Hankwang, Heron, Homeboyy, Hooperbloob, HopeChrist, Howcheng, Iammaxus, Icairns, Igfmnbo,Industrialinfo, Iridescent, JKoether, Jake8082, Jason Patton, JasonHolt, Jeffthejiff, JeremyLydellHaugen, Jgcalvop, Jim Swenson, JoanneB, Kaszeta, KeithB, Kjordahl, Kjramesh, Laportechicago,Laura schnak, Leonard G., Lightmouse, LilHelpa, Luis María Benítez, MC MasterChef, MER-C, MPerel, Masgatotkaca, MattH, Meaghan, Mechasheherezada, Mejor Los Indios, Michael Hardy,Mikiemike, Mion, Momusufan, MooseKin, Mpj28202, Mpj96, Ms737, Muhends, Nardinc, Neffk, Nemilar, Nikai, Nutiketaiel, Oda Mari, Ohnoitsjamie, Ojigiri, Omegatron, Othompson, PavelVozenilek, Philip Trueman, Pigman, Pmlineditor, Pol098, Ponder, Porqin, Proofreader77, Proofroadsrex, Psanderson, Pyrop, R9ainsworth, Rareandfirsts, RastaKins, Redadare10, RevRagnarok,ReyBrujo, Rjwilmsi, Robert K S, Romary, Rrburke, Rustyguts, Ryjaz, Sagsaw, Seoawebsite, SeymourSycamore, Shaddack, Skatebiker, Skäpperöd, Slipperyweasel, Sovxx, Srleffler, Stannered,StaticGull, Stephan Leeds, Stroppolo, THEN WHO WAS PHONE?, Tabletop, Tarchon, Techman224, That Guy, From That Show!, The Thing That Should Not Be, Theresa knott, Thumperward,Tom harrison, Toniold, Topory, TrygveFlathen, Useight, User A1, Utcursch, Uwguy, Vandergus, Vipinhari, Voidxor, Vsmith, W0lfie, Woohookitty, WordSurd, Wtshymanski, XJamRastafire,Yahadreas, Zaharous, 497 anonymous edits

Image Sources, Licenses and Contributors 23

Image Sources, Licenses and ContributorsFile:Type K and type S.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Type_K_and_type_S.jpg  License: Public Domain  Contributors: Skatebiker, 1 anonymous editsFile:Thin Film PRT.png  Source: http://en.wikipedia.org/w/index.php?title=File:Thin_Film_PRT.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:AlphaTellerFile:Wire Wound PRT.png  Source: http://en.wikipedia.org/w/index.php?title=File:Wire_Wound_PRT.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:AlphaTellerFile:Coil Element PRT.png  Source: http://en.wikipedia.org/w/index.php?title=File:Coil_Element_PRT.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:AlphaTellerImage:Rtdconstruction.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Rtdconstruction.gif  License: unknown  Contributors: Original uploader was Psanderson at en.wikipediaImage:twowire.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Twowire.gif  License: unknown  Contributors: Original uploader was Psanderson at en.wikipediaImage:threewire.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Threewire.gif  License: unknown  Contributors: Psanderson at en.wikipediaImage:fourwire.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Fourwire.gif  License: unknown  Contributors: Psanderson at en.wikipediaImage:4wirebetter.gif  Source: http://en.wikipedia.org/w/index.php?title=File:4wirebetter.gif  License: unknown  Contributors: Original uploader was Psanderson at en.wikipediaImage:Thermocouple0002.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Thermocouple0002.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: myselfFile:Thermocouple circuit.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Thermocouple_circuit.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:WtshymanskiImage:IEC Type K Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:IEC_Type_K_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:BS Type K Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BS_Type_K_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:MC 96.1 K Thermocouple Grade Color Code.svg  Source: http://en.wikipedia.org/w/index.php?title=File:MC_96.1_K_Thermocouple_Grade_Color_Code.svg  License: Public Domain Contributors: Original uploader was Sagsaw at en.wikipediaImage:IEC Type J Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:IEC_Type_J_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:BS Type J Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BS_Type_J_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:MC 96.1 J Thermocouple Grade Color Code.svg  Source: http://en.wikipedia.org/w/index.php?title=File:MC_96.1_J_Thermocouple_Grade_Color_Code.svg  License: Public Domain Contributors: Original uploader was Sagsaw at en.wikipediaImage:IEC Type N Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:IEC_Type_N_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:BS Type N Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BS_Type_N_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:MC 96.1 N Thermocouple Grade Color Code.svg  Source: http://en.wikipedia.org/w/index.php?title=File:MC_96.1_N_Thermocouple_Grade_Color_Code.svg  License: Public Domain Contributors: Original uploader was Sagsaw at en.wikipediaImage:BS Type R Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BS_Type_R_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:IEC Type T Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:IEC_Type_T_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:BS Type T Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BS_Type_T_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:MC 96.1 T Thermocouple Grade Color Code.svg  Source: http://en.wikipedia.org/w/index.php?title=File:MC_96.1_T_Thermocouple_Grade_Color_Code.svg  License: Public Domain Contributors: Original uploader was Sagsaw at en.wikipediaImage:IEC Type E Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:IEC_Type_E_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:BS Type E Thermocouple.svg  Source: http://en.wikipedia.org/w/index.php?title=File:BS_Type_E_Thermocouple.svg  License: unknown  Contributors: User:StanneredImage:MC 96.1 E Thermocouple Grade Color Code.svg  Source: http://en.wikipedia.org/w/index.php?title=File:MC_96.1_E_Thermocouple_Grade_Color_Code.svg  License: Public Domain Contributors: Original uploader was Sagsaw at en.wikipedia

License 24

LicenseCreative Commons Attribution-Share Alike 3.0 Unportedhttp:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/

1

Thermocouple Standards and Calibrations

Thermocouple Standards

As shown in Table I, there are seven thermocouple standards applicable to heat-treating furnaces.

These include Reference Standard, Primary Standard, Secondary Standard, Temperature Uniformity

Test Standard, System Accuracy Test plus Working and Load Standards. This chart summarizes the

type of thermocouples, which can be used for each category in addition to calibration frequency and

accuracy requirements. These standards were established by SAE (Society of Automotive Engineers)

specification SAE-AMS-2750 Rev. C in 1990. This specification has been adopted by the U.S.

Department of Defense. It is a valuable reference and we suggest that anyone who manufactures or

uses heat-treating furnaces should have a copy of this specification in their Quality Control Department.

Table I

Outline of Sensors

Nomenclature Description

Calibration Use/Max Error Limit

Period Against Correction Factor (°F)

Reference

Standard

Platinum

Platinum-Rhodium 5 years

NIST

Reference Standard

Primary Standard Calibration

None

Primary

Standard

Platinum

Platinum-Rhodium 3 years Reference Standard

Secondary Standard Calibration

±2.7° or ±0.25%**

Secondary

Standard Base or noble metal

1 year base

2 years noble Primary Standard

Test Sensor Calibration

Base: ±2° or ±0.4%**

Noble: ±2.7° or ±0.25%**

Temperature

Uniformity Test Base of noble metal

3 months base

6 months noble

Primary or

Secondary Standard

Temperature Uniformity Tests

±4° or ±0.75%**

System

Accuracy Test Base of noble metal

3 months base

6 months noble

Primary or

Secondary Standard

System Accuracy Tests

±2° or ±0.4%**

Working Base of noble metal Before installation Primary or

Secondary Standard

Installation in Equipment

Class 1: ±2° or ±0.4%**

Class 2: ±4° or ±0.75%**

Load Base of noble metal 3 months N, R, S

6 months other

Primary or

Secondary Standard

Insertion in Loads

±4° or ±0.75%**

* Sensors of Equivalent or Greater Accuracy are Acceptable

** Percent of Reading, if Greater Than Correction Factor in Degrees

Aerospace Material Specification - SAE AMS-2750 Rev. C. issued 1980-04-15, Revised 1990-04-01

Superceding AMS-2750B. Society of Automotive Engineers, Inc., 400 Commonwealth Drive, Warrendale, PA 15096 (1990).

Thermocouple Standards and Calibrations

2

Calibration Services

Nanmac’s calibration laboratory will calibrate bare or insulated thermocouple wire, assembled

thermocouples, RTD’s, thermistors and instruments. All of our calibration equipment is calibrated

against National Institute of Standards and Technology (NIST) standards. Our calibration data are

traceable to NIST standards. Calibration costs are listed in the chart below. The maximum temperature

range of our standard services is 2,100 degrees Fahrenheit.

Notes:

• All temperature sensors must be at least 12 inches long to minimize conduction errors.

• Calibrations to 2,950°F can be made on a special basis (contact factory for details).

• Also, calibrations at cryogenic ranges can also be made on a special basis. Your instruments

and sensors can also be calibrated and certified (contact factory for details).

1

Thermocouples in Furnaces and Ovens The temperature on the inside of furnaces and ovens are commonly monitored and controlled by thermocouples inserted into the heated chamber. The one common feature of all furnaces is the fact that there are isotherms within the heated chamber. Isotherms are regions of equal temperatures. They are similar to the contour lines on a map illustrating areas of equal altitude. Isotherms are caused by gradients within the furnace created by uneven heating, inadequate circulation, uneven distribution of the workload within the furnace, etc. There are also isotherms within the wall of the furnace since the outside surface temperature of the wall is close to ambient temperature and the inside surface temperature of the wall may be 3,000 degrees Fahrenheit or more. Thermocouples are installed in the furnace by machining an appropriate hole through the wall. The thermocouple now cuts across a myriad of isotherms and creates a conductive path for heat to flow from the hot area to the cool area. The thermocouple (since it measures its own temperature) is constantly being cooled by this conduction. The end result is the output of the thermocouple is always in equilibrium with the heat coming into the junction and the heat being carried away by conduction via the thermowell to the outer jacket of your furnace and the atmosphere. We call this error the “Stem Effect.” This error is influenced by the heat conduction in the wires, insulation and the sheath or thermowell of the thermocouple. It is virtually impossible to predict the magnitude of this error. Even if you could determine this error at a particular temperature, it would change as the temperature changes since the thermal conductivity of all materials varies with temperature. The basic physical principles involved are as follows:

1) Heat is always exchanged between two objects at different temperatures and it always flows from the hotter to the cooler object and. 2) Heat cannot be exchanged between two objects at the same temperature.

Our objective is to design and install a thermocouple into a furnace so that the thermocouple sensing tip is always in equilibrium with the temperature of interest and therefore accurate measurements are being made. In order to minimize the “Stem Effect” error one must install the thermocouple parallel to the plane of heat flow for a distance of at least 20 diameters of the probe. If you use an 1/8” OD probe, then 2-1/2” inches of the tip of this probe should be located parallel to the plane of heat flow. Furnace Probes Fig. 1 is a schematic of a furnace with heating elements at the bottom. The isotherms are shown as dotted tines. Fig. 2 is a similar sketch showing the isotherms in a furnace with heaters on all sides. In both sketches the correct orientation of the temperature sensors has been illustrated. In all instances, the correct orientation requires that the thermocouple extend parallel to the isotherms for a length equal to 20 times the probe diameter. A 3/8” diameter thermocouple probe thus should be inserted for a length of 7-1/2” inches along the isotherm.

Thermocouples in Furnaces and Ovens

2

Accuracy of temperature measurements made within furnaces can be greatly improved if the temperature sensors are installed parallel to the isotherms for a distance equal to 20 times the diameter of the protection sheath. This will reduce the “Stem Effect” (error caused by conduction in the sheath, wires and insulation) to an insignificant amount.

Figure 1 – Furnace with Bottom Heaters

Figure 2 – Furnace with Heaters on all Sides

1

Thermocouples and Extension Grade Wires

Thermocouple Wire

Thermocouple wires are made from very tightly controlled alloys and pure metals. Thermocouple wire

is available in two grades: standard grade and special grade. The special grade is sometimes referred to

as premium grade. All thermocouple wire and extension grade wire must conform to the American

National Standards Institute (ANSI) standard C96.1 which was originally approved in 1964, reaffirmed

without changes in 1969 and finally approved by ANSI on Nov. 26, 1975 with the designation

MC96.1-1975. Table I contains the limits of error on standard and special grades of thermocouple

wire.

Even though Table I shows the upper temperature limit of the thermocouple wires, the practical upper

temperature is limited by the wire diameter. Table II summarizes the recommended practical

temperature limits of various wire sizes for long life applications. Temperature limits for standard

thermocouples that are protected with a closed end protecting tube are shown. These limits are

suggested for continuous temperature sensing where insulation is not a factor. For unprotected

thermocouples where fast response is required, these limits should be reduced for equivalent service

life.

Referring to Table I again, note the Limits of Error for W/Re alloy wires. Above 800 degrees

Fahrenheit, you can expect an error of +/-1 % or a maximum of +/-42 degrees Fahrenheit at 4,200

degrees Fahrenheit. Calibrating a sample piece of wire and using this calibration for the remainder of

this lot of wire can reduce this error. Often it is necessary to calibrate an assembled thermocouple

(elements, insulation and sheath) up to its anticipated temperature usage range. Unfortunately, very

few laboratories will calibrate an assembled thermocouple. At Nanmac, we have modified one of our

furnaces so that we can calibrate an assembled thermocouple up to 1,600 degrees Celsius (2,912

degrees Fahrenheit) to an accuracy of +/-1 degrees Celsius. The only requirement is that the probe has

to be at least 12 inches long.

Thermocouple Extension Wire

Thermocouple extension wire has been developed primarily to reduce the overall cost of the

instrumentation. One could use thermocouple grade wire throughout, if the cost was not prohibitive.

Other requirements for the need for extension grade wires include electrical resistance limits, flexibility,

ductility and strength requirements, etc. In addition to lead wires, the extension grade alloys are used to

fabricate thermocouple connectors, buss bars, spade lugs and similar products.

Thermocouple extension grade wires are available from gauge size #8 (0.1285” diameter) down to as

small as gauge size #40 (0.003” diameter). All extension wires are also available with various types of

insulation to meet a wide range of applications.

It should be stressed that using extension grade wires at temperatures above the recommended

temperatures will produce very large errors in your temperature measurements. For example, the

maximum error for extension wire for platinum thermocouples is 12 degrees Fahrenheit up to 400

Thermocouples and Extension Grade Wires

2

degrees Fahrenheit. However at 1,000 degrees Fahrenheit, this error can be as high as 60 degrees

Fahrenheit. There are several “tricks-of-the-trade” which will hold these errors to a very low figure;

these are:

(1) For Base Metal thermocouples such as Types E, J, K, or T one can use thermocouple grade wires

for extension leads. Cost difference is insignificant.

(2) For Noble Metal thermocouples such as Types B, R, and S; also, Refractory Metal thermocouples

such as Types C, D, and G. It will be more economical to make the thermocouples slightly longer so

that the connectors and the lead wires are in a cooler area (below 400 degrees Fahrenheit). If this is not

possible, you can use a short length of thermocouple grade wire swaged into a 1/16” OD stainless steel

sheath for the length required to bring the lead wires safely below 400 degrees Fahrenheit. You can

still attach standard connectors to each end of this special swaged cable. Even though the connector

prongs are made from the extension grade alloy, they will operate accurately because of the Law of

Intermediate metals as applied to thermocouples. Since the temperature on both prongs of the

connector is the same (or vary together) the EMF generated by each of the two intermediate junctions

cancel each other out.

For more information on thermocouple wires, refer to Section G of the Nanmac Catalog. Also we

stock a large inventory of thermocouple wires both bare and insulated for immediate shipment.

Calibration services are available for thermocouple wires, assembled thermocouples and RTD’s.

Thermocouples and Extension Grade Wires

3

Table I

Thermocouple Wire Limits of Error

Type

Temperature

Range (°F)

Limits of Error

Wire Alloys

Standard

Grade

Special

Grade

Copper {+} vs

Constantan { - } T

-300 to -75

-150 to -75

-75 to 200

200 to 700

-

±2%

±1-1/2°F

±3/4%

±1%

±1%

±3/4°F

±3/8%

Iron {+} vs

Constantan { - } J

32 to 530

530 to 1400

±4°F

±3/4%

±2°F

±3/8%

Chromel {+} vs

Constantan { - } E

32 to 600

600 to 1600

±3°F

±1/2%

±2-1/4°F

±3/8%

Chromel {+} vs

Alumel { - } K

32 to 530

530 to 2300

±4°F

±3/4%

±2°F

±3/8%

Platinum { - } vs

Platinum Rhodium {+} R & S

32 to 1000

1000 to 2700

±2-1/2°F

±1/4%

-

±1/10%

Platinum 6% Rhodium { - } vs

Platinum 30% Rhodium {+} B 1600 to 3100 ±1/2% -

Tungsten {+} vs

Tungsten 26% Rhenium G

to 800

800 to 4200

±8°F

±1% -

Tungsten 3% Rhenium {+} vs

Tungsten 25% Rhenium { - } D

Tungsten 5% Rhenium {+} vs

Tungsten 25% Rhenium C

Thermocouples and Extension Grade Wires

4

Table II

Thermocouple Limits for Thermocouple Wire

Thermocouple Type

Wire Gauge (AWG)

8 14 20 24 28

Copper/Constantan T - 700°F 500°F 400°F 400°F

Iron/Constantan J 1,400°F 1,100°F 900°F 700°F 700°F

Chromel/Constantan E 1,600°F 1,200°F 1,000°F 800°F 800°F

Chromel/Alumel K 2,300°F 2,000°F 1,800°F 1,600°F 1,600°F

Platinum/

Platinum Rhodium R & S - - - 2,700°F -

Platinum 6% Rhodium/

Platinum 30% Rhodium B - - - 3,100°F -

Tungsten/

Tungsten Rhenium

G

D

C

- - - 4,200°F -

1

Eliminate Temperature Errors Caused by Conduction

Control of any heat treat process, whether basic or advanced, is only as good as the accuracy of

temperature measurement. The accuracy of contact-type temperature sensors is affected by calibration,

response time, and conduction.

Calibration errors are generally insignificant. Sensors can be calibrated to an accuracy of ±0.25°C

(0.5°F), and standards can be calibrated to the sixth decimal place at laboratories such as NIST

(National Institute of Standards and Technology). Response time of sensors is very important in

transient or cyclical applications, but in furnace applications, rapid transients are not usually

encountered. However, the conduction phenomenon is of primary concern in furnaces because it can

generate large errors in temperature measurements.

Causes of Conductance Errors

Conduction errors are caused by temperature gradients within the furnace and the furnace wall, and the

ambient temperature outside the furnace. Since the temperature sensor is installed through the furnace

wall, it creates a conductive path along which heat flows from the inside of the furnace to the exterior,

thus creating an error in the observed measurement. Uneven heating, poor circulation, and uneven

distribution of the workload within the furnace also cause temperature gradients. To provide accurate

measurement, a temperature sensor must:

• Have reproducible and stable calibration properties (temperature vs. EMF and temperature

vs. resistance) over the measured temperature span

• Not disturb the local temperature of the medium by its very existence; and

• Have sufficiently fast response to follow temperature changes accurately.

Eliminate Temperature Errors Caused by Conduction

2

Figure 1 illustrates the temperature profile across the wall of a heated chamber at any instant of time. In

this illustration, the wall is made of a homogeneous material such as steel or ceramic. If the wall

contains several materials, such as steel and insulation plus graphite or ceramic, each homogeneous

material will have its own temperature profile.

• Several observations can be made in connection with Figure 1:

• Heat always flows from the hotter medium to the cooler medium.

• Heat energy is continuously absorbed by the wall at its hot side and liberated at the cold side

to the cooling medium (air, water, etc.). Under steady-state conditions, the heat absorbed must

be equal to the heat liberated or the wall will begin to melt.

• The temperature profile at each interface approaches exponential conditions.

• No region exhibits constant temperature over a given cross-section. Constant temperature

zones are called isotherms, and they appear only parallel to the plane of heat flow.

Thermocouples are installed by machining an appropriate hole through the wall. The thermocouple

thus cuts across myriad isotherms, and creates a conductive path for heat to flow from the hot area to

the cool area. The thermocouple, which in reality measures its own temperature, is constantly being

cooled by this conduction.

The output of the thermocouple is always in equilibrium with the heat coming into the junction and the

heat being carried away by conduction via the thermowell to the outer jacket of the furnace and the

atmosphere. This is called the stem effect. This error is caused by the heat conduction

in the wires, the insulation, and the sheath or thermowell of the thermocouple. It is impossible to predict

the magnitude of this error. Even if you could determine this error at a particular temperature, it would

change as the temperature changes, because thermal conductivity of all materials varies with

temperature.

Therefore, the heat treater’s objective should be to install a thermocouple into a furnace so that the

sensing tip is always in equilibrium with the temperature of interest and, therefore, accurate

measurements are made. Ideally, the sensor should be an adiabatic probe — that is, it should be

thermally isolated from the wall.

Testing for conduction

We designed an experiment to determine the magnitude of errors produced by the stem effect. We used

a container of hot water surrounded by a reservoir of cold water. The hot water was kept hot by an

immersion-type heater; the cold water was kept cold by a simple circulating pump and cooling radiator.

The temperatures in both the hot and cold reservoirs were continuously monitored by laboratory

thermometers with an accuracy of ±0.25°C (0.5°F). The carbon steel hot water tank had a 12-cm (5-in.)

O.D. with a wall of 1 cm (0.5-in.) thickness.

Four thermocouples, each with a different style of thermal junction, were installed in the wall of the hot

water tank and aligned flush with the inner surface of the tank. Thus, all the thermocouples were in

direct contact with the hot water. All the thermocouples had iron/constantan (copper-nickel) elements,

Eliminate Temperature Errors Caused by Conduction

3

and their outputs were monitored by the same meter through a thermocouple selector switch. In

addition, all thermocouples were made from the same lot of wire, and resistances were matched.

The Four Thermal Junction Styles Were:

• T1 — Exposed-bead weld junction

• T2 — Grounded junction: sheath welded closed and the thermal junction electrically in

contact with the sheath.

• T3 — Insulated junction: sheath welded closed but the thermal junction electrically isolated

from the sheath. All RTDs and thermistors are similar in design to this insulated junction. The

sensing element is electrically insulated and then enclosed in a metallic sheath that protects the

sensing element from abuses of the environment such as corrosion, oxidation, erosion, and

chemical attack. This protection comes at the cost of reduced accuracy.

• T4 — Right-angle junction: exposed-ribbon welded junction.

T1, T2, and T3 are conventional thermal junction styles and T4 is a right-angle unit with ribbon

elements.

Measurements were taken from these four thermocouples over a 16-hour period. When the hot water

temperature was held at 83°C (181°F) and the cold water held at 38°C (101°F), thermocouple T1 read

78°C (173°F), T2 read 69°C (156°F) and T3 read 53°C (128°F), whereas the right-angle ribbon

thermocouple read 83°C (181°F).

A temperature gradient of 40 to 50°C (70 to 88°F) was maintained over the two days in which this test

was conducted (the test took place in an exhibit hall during an ISA show). The variations in this

Eliminate Temperature Errors Caused by Conduction

4

gradient were caused by several factors: our cooling system was not very well controlled by our simple

heat exchanger — the circulating water pump and our small radiator; and, as the ambient temperature

in the hall rose during the day, it caused the temperature of our cold water reservoir to rise, thus putting

an additional load on our heat exchanger system. Note that the insulated junction (T3) exhibited the

largest error in both absolute measurement and percent of temperature change between the hot and cold

water. The right-angle ribbon sensor has a junction thickness of 0.07 mm (0.003-in.). The extension

leads in the vicinity of the junction are also ribbons, and lie in the same plane as the sensing junction for

a distance of at least 20 times the thickness of the junction. Thus, the sensing tip and the adjacent

ribbons are a parallel to the plane of heat flow, and since the ribbons on both sides of the junction are

heated simultaneously with the junction, no significant error is caused by the stem effect.

To minimize the error caused by the stem effect in all sensors, this ratio of 20:1 must be maintained at

the sensing tip. For example, a 3-mm (0.125-in.) round stainless steel probe must be positioned in the

furnace so that it lies parallel to the plane of heat flow for 6 cm (2.5 in.) This can be accomplished by

one of two methods: make a 90° bend in the probe 6 cm (2.5 in.) from its tip, or position the mounting

so that the probe is parallel to the plane of heat flow.

Figure 4 is a schematic of a furnace with heating elements at the bottom. The isotherms are shown as

broken lines. Figure 5 is a similar sketch showing the isotherms in a furnace with heaters on all sides. In

all instances, the correct orientation requires that the thermocouple extend parallel to the isotherms for a

length equal to 20 times the probe diameter. A right-angle thermocouple such as the one used in the test

described above can be installed into a furnace without any 90° bends because the probe already

incorporates such a bend.

Eliminate Temperature Errors Caused by Conduction

5

1

The Stem Effect: Leading Cause of Error in Temperature Measurement

All contact type of temperature sensors such as thermocouples, RTD’s, thermistors, bi-metallic

thermometers, etc. are subject to an error we call the “STEM EFFECT’ (SE). When a probe is

immersed into a gas or a liquid environment, a new thermal conductive path is created by the probe’s

stem. Thus, heat is conducted away from the sensing tip via the probe’s stem to the outer atmosphere.

These results in the sensing tip reading a temperature, which is lower than the surrounding gas or

liquid.

Several years ago, NANMAC conducted an experiment to determine the magnitude of the errors

produced by the “Stem Effect.” The experiment consisted of a container of hot water surrounded by a

reservoir of cold water. The hot water was kept hot by an immersion type heater and the cold water

was kept cold by a simple circulating pump and cooling radiator. A gradient of about 80 degrees

Fahrenheit was thus maintained over extended time periods.

The temperatures in the hot and cold reservoirs were continuously monitored by laboratory

thermometers with an accuracy of + .5 degrees Fahrenheit. The carbon steel hot water tank had a

dimension of 5” OD with a wall of 1/2” thickness.

Four thermocouples, (Figs. 1 & 2) each with a different style of thermal junction, were installed in the

wall of the hot water tank and aligned flush with the inner surface of the tank. All thermocouples were

in direct contact with the hot water. All units had iron/constantan elements and the outputs were

monitored by the same pyrometer through a thermocouple selector switch. In addition, all

thermocouples were made from the same lot of wire and resistances were matched- Fig. 3 is a

schematic of the test setup.

Thermocouples T-1, T-2 and T-3 are conventional type thermocouples, and T-4 is the “Right Angle”

unit with ribbon elements. Table 1 shows the actual measurements made over a 16-hour period

covering two consecutive days. The conventional style thermal junctions produced errors of from 10%

to 66% in the observed temperature changes while the “Right Angle” thermocouple showed no

measurable errors due to conduction.

The Patented “Right Angle” thermocouple features a thermal junction at right angles to the longitudinal

axis of the probe. The thermal elements consist of ribbons electrically welded together at the sensing

tip. These ribbon elements are brought out to the opposing sides of the probes along the interface of a

rod and cone assembly made out of high temperature insulation. The ribbons in the vicinity of the

thermal junction are parallel to the plane of heat flow. Thus both the thermal junction and the ribbons

are heated simultaneously and conduction errors are minimized. The distance from the junction to the

corners of the ribbon elements divided by the junction thickness produces a ratio of 20 to 1. This ratio

is the reason why the ribbon thermocouples have an insignificant error caused by the “Stem Effect.”

Ribbon elements also have response times in the low milliseconds to transients.

The junction formed for microsecond response time will typically need to be remade after each use.

Keep in mind; the response time of any thermocouple is directly related to the size of the thermal

junction. In order to provide a device that has microsecond response, the junction must be microscopic.

The Stem Effect: Leading Cause of Error in Temperature Measurement

2

This size of a junction will not last long under “clean” conditions. In other words, if the temperature to

be measured is in a harsh environment, such as an interface temperature of two materials, of a “dirty”

gas flow, the thermal junction will be made and renewed by the interface of blast. If the temperature is

a “clean” temperature, then the junction may be swept away and require the aforementioned

maintenance. This does not mean that it will not react to and measure, the temperature of interest, it

may though require the maintenance described here after use. The device will continue to work after

maintenance is performed until about 1/2 inch has been removed from the sensing tip. Hence the name

“Eroding Junction Thermocouple.”

• T1 — Exposed-bead weld junction

• T2 — Grounded junction: sheath welded closed and the thermal junction electrically in

contact with the sheath.

• T3 — Insulated junction: sheath welded closed but the thermal junction electrically isolated

from the sheath. All RTDs and thermistors are similar in design to this insulated junction. The

sensing element is electrically insulated and then enclosed in a metallic sheath that protects the

sensing element from abuses of the environment such as corrosion, oxidation, erosion, and

chemical attack. This protection comes at the cost of reduced accuracy.

• T4 — Right-angle junction: exposed-ribbon welded junction.

The Stem Effect: Leading Cause of Error in Temperature Measurement

3

For more information on the Nanmac “Right Angle” thermocouples contact us.

Reading

No.

Temperature Measurement (°F)

Cold

Water Hot Water T T1 T2 T3 T4

1

2

3

4

5

6

7

104

104

104

104

102

102

102

174

174

175

175

174

175

175

70

70

71

71

71

73

73

169

166

167

167

167

167

167

154

157

151

151

151

152

151

127

127

127

124

125

126

125

175

175

175

175

174

175

175

8

9

10

11

12

13

14

90

93

102

102

100

102

103

175

185

190

190

188

189

189

85

92

88

88

88

87

86

167

178

182

180

180

180

180

150

157

165

163

160

160

160

120

127

135

133

130

130

132

175

185

191

190

188

189

188

Average 101.0 180.6 79.5 172.6 155.9 127.7 180.7

Error (°F) -8.0 -24.7 -52.9 0

% Error 10.1 31.1 66.5 0