metrology & instrumentation

7/27/2019 Metrology & Instrumentation

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MetrologyA scientific paper reports informationregarding an experimental or theoreticalwork.

Experimental works involve measurements.

Theoretical works include parametersobtained experimentally, and most often,

they are validated through experimentalwork.

Hence, measurements are essential for ascientific work, which is essential in ascientific paper.

General procedure for

the scientific method


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MetrologyMetrology: field of knowledge concerned withmeasurement.

Measurement: Process of experimentally obtainingone or more quantities that can reasonably beattributed to another quantity.

Measurement implies comparison of quantities or countingof entities.

Measurement presupposes description of the quantitycommensurate with the intended use of the measurementresult.

Measurement presupposes a measurement procedure.

Measurement presupposes a calibrated measuring systemoperating according to a specified measurement procedure.

Metrology

A measurement includes:

A numeric value.

An unit associated to the numeric value.

An uncertainty associated to the numeric value.


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We use sensors to measureSensor: A device which provides a usable output in response toa specified measurand

A sensor acquires a physical quantity and converts it into asignal suitable for processing (e.g. optical, electrical,mechanical)

Common sensors convert measurement of physical phenomenainto an electrical signal

Active element of a sensor is called transducer

The TransducerA device that converts one form of energy to another

When input is a physical quantity and output electrical Sensor

When input is electrical and output a physical quantity Actuator

ActuatorsSensors

Physical

parameter

Electrical

Output

Electrical

Input

Physical

Output

e.g. Piezoelectric:

Force -> voltage

Voltage-> Force


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Commonly Measured Quantities

Temperature, Flux, Specific Heat, Thermal ConductivityThermal

Position, Velocity, Acceleration, Force, Strain, Stress,Pressure, Torque

Mechanical

Refractive Index, Reflectivity, AbsorptionOptical

Magnetic Field (amplitude, phase, polarization), Flux,Permeability

Magnetic

Charge, Voltage, Current, Electric Field (amplitude, phase,polarization), Conductivity, Permittivity

Electric

Fluid Concentrations (Gas or Liquid)Biological & Chemical

Wave (amplitude, phase, polarization), Spectrum, WaveVelocity

Acoustic

QuantityStimulus

Spatial Variables Measurement

Displacement Measurement, Linear and Angular

Thickness Measurement

Distance

Position, Location, Altitude Measurement

Level Measurement

Area Measurement

Volume Measurement

Angle Measurement

Tilt Measurement

Velocity and Acceleration Measurement

Vibration and Shock Measurement


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Time and Frequency Measurement

Time Measurement

Frequency Measurement

Mechanical Variables Measurement (Solid)

Mass and Weight Measurement

Density measurement

Strain Measurement

Force Measurement

Torque Measurement

Mechanical Variables Measurement (Fluid)

Pressure and Sound Measurement

Flow Measurement

Point Velocity Measurement

Viscosity MeasurementSurface Tension Measurement


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Thermal Variables Measurement

Temperature Measurement

Thermal Conductivity Measurement

Heat Flux

Calorimetry Measurement

Electromagnetic Variables Measurement

Voltage Measurement

Current Measurement

Power Measurement

Power Factor MeasurementPhase Measurement

Energy Measurement

Electrical Conductivity and Resistivity

Charge Measurement

Capacitance and Capacitance Measurements


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Electromagnetic Variables MeasurementPermittivity Measurement

Electric Field Strength

Magnetic Field Measurement

Permeability and Hysteresis Measurement

Inductance Measurement

Immittance Measurement

Q Factor Measurement

Distortion Measurement

Noise Measurement

Microwave Measurement

Optical Variables Measurement

Photometry and Radiometry

Densitometry Measurement

Colorimetry

Optical LossPolarization Measurement

Refractive Index Measurement

Turbidity Measurement

Laser Output Measurement

Vision and Image Sensors


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Radiation MeasurementRadioactivity Measurement

Charged Particle Measurement

Neutron Measurement

Dosimetry Measurement

Chemical Variables Measurement

Composition Measurement

pH Measurement

Humidity and Moisture Measurement

Selection of a Sensor


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Selection of a SensorAccuracy

The degree of agreement of the measured dimensionwith its true magnitude.

Precision

Repeatability.

Resolution

The smallest dimension that can be read on aninstruments.

SensitivityThe input required to produce a response in anmeasuring device, expressed as the ratio of the responseto the magnitude of the input quantity.

Stability

Capability to maintain calibrated status.

Precision vs. Accuracy


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Metrology

Calibration: the act of checking oradjusting (by comparison with a standard)the accuracy of a measuring instrument.

No instrument can properly operate for longperiods.

Periodic calibration.

Displacement MeasurementLinear and Angular

Resistive Displacement Sensors

Inductive Displacement Sensors

Capacitive Displacement Sensors

Piezoelectric Transducers and SensorsLaser Interferometer Displacement Sensors

Bore Gaging Displacement Sensors

Time-of-Flight Ultrasonic Displacement Sensors

Optical Encoder Displacement Sensors

Magnetic Displacement Sensors

Synchro/Resolver Displacement Sensors

Optical Fiber Displacement Sensors

Optical Beam Deflection Sensing


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Displacement MeasurementKey Selection Criteria:

Define clearly what you need to measure and why?

What type of environment will the sensor operate in(harsh, vacuum, high pressure, dusty, etc.)?

Are there space restrictions?

Compromising on resolution and accuracy may saveyou time and money, but will the sensor perform wellenough in the application?

Displacement MeasurementKey Selection Criteria:

Custom versus standard off-the-shelf sensors?

When considering standard versus custom sensors,improved sensor accuracy often comes from re-calibration, intelligent integrated sensor software,

improving the mechanical mounting or by manufacturingthe sensor from better components or materials.

http://www.micro-epsilon.co.uk/download/products/cat--Micro-Epsilon--products--en.pdf

http://www.micro-epsilon.co.uk/download/products/T001--en--precise-non-contact-sensors.pdf


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Potentiometer sensors (resistive)Potentiometer consists of wire wound around a rod with fixedresistor R

Movement of wiper change the resistance of thepotentiometer

Potentiometer sensors (resistive)


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Potentiometer sensors (resistive)

WEBSTER, J. G. The

Measurement,

Instrumentation, and

Sensors Handbook. 1a ed.

Boca Raton:CRC Press, 1999.

Inductive Displacement Sensors

The Linear Variable DifferentialTransformer (LVDT) is the most used variable-inductance transducer in industry.

It is an electro-mechanical device designed toproduce an AC voltage output proportional to therelative displacement of the transformer and thearmature, as illustrated in the figure below.


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LVDT: Inductive Displacement

SensorsPros:Relative low cost due to its popularity.

Solid and robust, capable of working in a widevariety of environments.

No friction resistance, since the iron core does notcontact the transformer coils, resulting in aninfinite (very long) service life.

High signal to noise ratio and low outputimpedance.

LVDT: Inductive DisplacementSensors

Pros:

Negligible hysteresis.

Infinitesimal resolution (theoretically).In reality, displacement resolution is limited by theresolution of the amplifiers and voltage meters used to

process the output signal.Short response time.

No permanent damage to the LVDT ifmeasurements exceed the designed range.


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LVDT: Inductive Displacement

SensorsCons:The core must contact directly or indirectly withthe measured surface which is not alwayspossible or desirable.

Dynamic measurements are limited to no morethan 1/10 of the LVDT resonant frequency.


Pressure

Absolute or manometric (gage)

Above or below atmosferic (vacuum)


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Pressure sensing elements

Pressure

Detection methodsRequired to convert the deformation of the sensing elementinto a pressure readout.

In the simplest approach, the displacements of a sensingelement can be amplified mechanically by lever and flexurelinkages to drive a pointer over a graduated scale.

Some pressure sensors employed a Bourdon tube to drive thewiper arm over a potentiometric resistance element.

Inpiezoelectricpressure sensors, the strains associated withthe deformation of a sensing element are converted into anelectrical charge output by a piezoelectric crystal.

Piezoelectric pressure sensors are useful for measuring high-pressuretransient events, for example, explosive pressures.Not suitable for static pressure measurement (continuous discharge ofthe crystal).


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The sensing diaphragm and capacitor form a differentialvariable separation capacitor.

When the two input pressures are equal the diaphragm ispositioned centrally and the capacitance are equal. Adifference in the two input pressure causesdisplacement of the sensing diaphragm and is sensed asa difference between the two capacitances

Example: Capacitive Pressure Transducer

Large application range:

high-pressure sensors with full-scalepressures above 10 MPa

vacuum sensors (commonly referred to ascapacitive manometers) for pressuremeasurements < 10 mPa

Accurate within 0.1 % of reading or 0.01 %of full scale.

Corrosion resistant

Capacitive Pressure Transducer


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Pressure range

Pressure


Flow

What is the fluid being measured by theflowmeter: liquid or gas?

Do you require rate measurement and/ortotalization from the flow meter?

What viscosity is the liquid?

Is the fluid clean?

What is the minimum and maximum flowrate forthe flow meter?


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Flow

What is the minimum and maximum processpressure?

What is the minimum and maximum processtemperature?

Is the fluid chemically compatible with theflowmeter wetted parts?

If this is a process application, what is the size ofthe pipe?


FlowTypes of flow meters

Differential Pressure (DP) Flowmeters

Variable Area (VA) Flowmeters

Positive Displacement (PD) Flowmeters

Turbine and Vane Flowmeters

Impeller Flowmeters

Electromagnetic Flowmeters

Ultrasonic Flowmeters

Vortex Shedding Flowmeters

Thermal Mass Flow Sensors

Coriolis Effect Mass Flowmeters

Drag Force Flowmeters


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Measuring volume or mass rate?PD flowmeters are the only ones that directlymeasure volumetric flow.

Although techniques like turbine, ultrasonic and vortexmeasure the velocity of the gas stream in order todetermine volumetric flow.

Inferential flowmeters, such as DP and variablerea (VA) sensors, measure neither volumetric normass flow, but infer its rate from other

parameters, like a drop in pressure or thedisplacement of a float.

Coriolis and thermal instruments are the only onesthat measure the mass flow of gases.

Measuring volume or mass rate?Volumetric and mass measurements can beconverted between one another if the fluiddensity is known

Density of gases is equally sensitive to

pressure and temperature, unlike liquidsthat are less susceptible to changingconditions.

Volumetric flow measurement still has itsplace in many processes, however it is moreconvenient to measure mass flow in gasesand steam than volumetric flow.


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Flow sensorsCoriolis

Use the Coriolis effect, in which a vibrating tubeis caused to distort, for measuring the flow ratedirectly, eliminating the need to compensate fortemperature, pressure and density.

Can measure a mixture of gases, unknowngases, and fluids moving between gaseous and

liquid states.High measuring accuracy, which is unaffected byflow profile, even down to very low flow rates.

CoriolisPros:

Higher accuracy than most flowmeters.

Can be used in a wide range of liquid flow conditions.

Capable of measuring hot (e.g., molten sulphur, liquidtoffee) and cold (e.g., cryogenic helium, liquid nitrogen)

fluid flow.Low pressure drop.

Suitable for bi-directional flow.

Cons:High initial set up cost.

Clogging may occur and difficult to clean.

Larger in over-all size compared to other flowmeters.

Limited line size availability (6 ~ 200 mm ).


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Coriolis

Flow sensors

Differential pressureThe most common type of flowmeter.

DP measures the flow of gases inferentially,employing the Bernoulli equation that interpretsthe relationship between pressure and flow rate.

These flowmeters introduce a constriction orobstruction in the pipeline, creating a pressuredrop from which velocity and volumetric flowcan be calculated.

Various types of DP flowmeters are used, themost popular being the orifice plate, which canbe subject to wear.


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Differential pressure sensor

Orifice Meters

Venturi Meters

Flow Nozzles

P

A

A

YCAQ

FlowalFor

IdealP

A

A

AvAQ

RateFlow

=

==

2

1

1

Re

2

1

1

2

1

2

2

2

1

2

222


Y = Compressibility Factor

= 1 for incompressible flow or

when P


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Flow sensors

Positive displacement

PD flowmeters measure volumes of fluid byrepeatedly filling and discharging compartmentsof known volume, with fluid from theflowstream.

There are various types of PD meter, usingvanes, gears, pistons, paddles or diaphragms toseparate the fluid.

They provide high accuracy, but cannot handledirty fluids, and they incorporate moving partsthat are subject to wear.


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Flow sensors

Positive displacement

Flow sensors

Thermal

Thermal flowmeters measure the mass flow ofgases, employing a combination of heatedelements and temperature sensors, withthermodynamic principles used to derive actualmass flow.

They do not require correction for changes ingas temperature, pressure or density and areextremely accurate, especially when measuringlow and very low flow rates.


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Flow sensors

TurbineFluid passing through a turbine flowmeter spinsan axial rotor, the rotational speed of whichindicates flow velocity.

They have a wide flow range and offer areasonable level of accuracy at an affordableprice, although they are restricted in use toclean, non-corrosive fluids.

Similar comments apply to paddle wheel and pinwheel flowmeters, which translate themechanical action of paddles/wheels intovolumetric flow.

Flow sensors

Variable Area

VA flowmeters typically comprise a tapered glass orplastic tube and an internal metering float, with thevolumetric flow rate proportional to the displacement ofthe float.

Among the oldest flow technologies, it is inexpensiveand easy to install, although historically had to be fittedvertically, and is sensitive to changes in temperature,pressure and density.

Recently introduced a digital alternative, which offersgreatly improved accuracy, electronic output signals andno fragile glass components in the flow path.


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Flow sensors

http://www.omega.com/prodinfo/FlowmeterSelection.pdf

Common temperature sensors

Liquid-in-glass thermometres (normally usedfor calibration of other temperature sensors).

Thermocouple (cheap, small size, resistant).

Thermoresistance (more accurate).


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Liquid-in-glass thermometres

Liquid-in-glass thermometres

The traditional thermometres

Measurement scale from -190 C to

+600 CUsed mainly in calibration


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Causes of inaccuraties

Temperaturedifferences in theliquid

Glass temperaturealso affects

The amount ofimmersion (vs.calibration)

ThermocouplesSeebeck effect

If two wires of dissimilar metals are joined atboth ends and one end is heated, current willflow.

If the circuit is broken, there will be an opencircuit voltage across the wires.

Voltage is a function of temperature and metaltypes.

For small Ts, the relationship with temperatureis linear

For larger Ts, non-linearities may occur.

V T =


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Thermocouple Advantages andDisadvantages

Advantages:Self Powered (doesnot require a currentor voltage source)

Rugged

Inexpensive

Simple

Disadvantages:Extremely Low

Voltage output (mV)

Not very stable

Needs a referencepoint

Thermocouple


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Thermocouple

One wire cannot form a thermocouple: Net voltage = 0

Thermocouple


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Thermocouple

Thermocouple


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Measuring the Thermocouple VoltageIf you attach the thermocouple directly to a voltmeter,you may have problem.

You have just created another junction! Your displayedvoltage will be proportional to the difference between J1and J2 (and hence T1 and T2). Note that this is Type Tthermocouple.

( )231213 5.6355.6 TTT +

( ) ( )

( ) ( )

( )21

2121

122313

5.41

355.6

355.6

TT

TTTT

TTT

+

External Reference JunctionA solution is to put J2 in an ice-bath; thenyou know T2, and your output voltage willbe proportional to T1-T2.

( )231213 5.6355.6 TTT +

( ) ( )

( ) ( )

( )21

2121

122313

5.41

355.6

355.6

TT

TTTT

TTT

+


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Thermocouple

Other types of thermocouplesMany thermocouples dont have one copperwire. Shown below is a Type Jthermocouple.


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( )

( ) ( ) ( ) ( ) ( )[ ]

( ) ( ) ( ) ( ) ( )

( ) ( ) ( ) ( )4321

21424331

21424331

12244313

TTACTTBA

TTBTTATTCTTCTTA

TTBTTATTCTTCTTA

TBTATCTCTA

CC

CC

CC

+

+

+++

+++

Isothermal Block

The block is an electrical insulator but goodheat conductor. This way the voltages for J3and J4 cancel out. Thermocouple dataacquisition set-ups include these isothermal

blocks.

( ) ( )21

1221

12243413

TTCFe

TCTFeTFe

TCTFeTCuTFe

II

+

( ) ( )I

II

TTCFe

TCTFe

TCTCuTFe

+

1

11

143413


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Thermocouple

( ) ( )I

II

TTCFe

TCTFe

TCTCuTFe

+

1

11

143413

Typically cold junction temperature is sensed by a precision thermistor in good

thermal contact with the input connectors of the measuring instrument.

Software CompensationHow can we find the temperature of the block? Usea thermister or RTD.

Once the temperature is known, the voltageassociated with that temperature can be subtractedoff.

Then why use thermocouples at all?Thermocouples are cheaper, smaller, more flexible andrugged, and operate over a wider temperature range.

Most data acquisition systems have softwarecompensation built in.


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Hardware CompensationWith hardware compensation, the temperatureof the isothermal block again is measured, andthen a battery is used to cancel out the voltageof the reference junction.

This is also called an electronic ice pointreference.

Thermocouple typesType E: Chromel + leg (nickel/10% chromium) and constantan - leg

(nickel/45% copper). 200


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Thermocouple typesType K: Chromel + leg and Alumel (nickel/5% aluminum andsilicon) - leg. Low cost, most popular. 200


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Sheathing and SLE

Special Limits of Error wire can be used to improveaccuracy.

Sheathing of wires protects them from theenvironment (fracture, oxidation, etc.) and shieldsthem from electrical interference.

The sheath should extend completely through themedium of interest. Outside the medium of interestit can be reduced.

Sometimes the bead is exposed and only the wire iscovered by the sheath. In harsher environments, thebead is also covered. This will increase the timeconstant

Thermocouple errors

Any error in the measurement of cold junction temperature will lead to the same error

in the measured temperature from the thermocouple tip.

If you do your own calibration, you can usually improve on the listed

uncertainties.


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Precautions and Considerationsfor Using Thermocouples

Most measurement problems and errors withthermocouples are due to a lack of understanding of howthermocouples work.

Thermocouples can suffer from ageing and accuracy mayvary consequently especially after prolonged exposure totemperatures at the extremities of their useful operatingrange.

Potential ProblemsPoor bead construction

Weld changed material characteristics because the weldtemperature was too high.

Large solder bead with temperature gradient across it

DecalibrationProcess of unintentionally altering the properties of the

thermocouple wire.The usual cause is the diffusion of atmospheric particles intothe metal at the extremes of operating temperature. Anothercause is impurities and chemicals from the insulationdiffusing into the thermocouple wire.

Inhomogeneities in the wire; these are especially bad inapplications with large temperature gradients. Common iniron wires.


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Potential ProblemsConnection problems

Errors caused by unintentional thermocouplejunctions.

Any junction of two different metals willcause a junction.

Use the correct type of thermocoupleextension wire to increase the length of the

leads from your thermocouple.Any connectors used must be made of thecorrect thermocouple material and correctpolarity must be observed.

Potential ProblemsNoise

The output from a thermocouple is a small signal, so itis prone to electrical noise pick up.

Most measuring instruments reject any common mode

noise (signals that are the same on both wires) sonoise can be minimized by twisting the cable togetherto help ensure both wires pick up the same noisesignal.

If operating in an extremely noisy environment, (suchas near a large motor) it is worthwhile consideringusing a screened extension cable.

If noise pickup is suspected first switch off all suspectequipment and see if the reading changes.


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Potential ProblemsThermal Shunting

All thermocouples have some mass. Heating this masstakes energy so will affect the temperature that you aretrying to measure.

Consider for example measuring the temperature of liquidin a test tube: the heat will travel up the thermocouplewire and dissipate to the atmosphere so reducing thetemperature of the liquid around the wires. If thethermocouple is not sufficiently immersed in the liquid,due to the cooler ambient air temperature on the wires,thermal conduction may cause the thermocouple junctionto be a different temperature to the liquid itself.

Thermocouple with thinner wires may help avoiding thisproblem, but attention to lead resistance.

Potential ProblemsLead Resistance

Thermocouples are made of thin wire to minimizethermal shunting and improve response times. Thus,thermocouple may have high resistance which can makeit sensitive to noise and can also cause errors due to theinput impedance of the measuring instrument.

If thermocouples with thin leads or long cables areneeded, it is worth keeping the thermocouple leads shortand then using thermocouple extension wire (which ismuch thicker, so has a lower resistance) to run betweenthe thermocouple and measuring instrument.


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Potential ProblemsLinearization

The measuring instrument must allow for the factthat the thermocouple output is non linear.

The relationship between temperature and outputvoltage is a complex polynomial equation (5th to9th order depending on thermocouple type).

Analogue methods of linearization are used in lowcost thermocouple meters.

High accuracy instruments store thermocoupletables in computer memory to eliminate thissource of error.

Thermoresistance sensorsThe resistance of several materials changes with the temperature.

In general, the resistance of metallic materials increase with T, whereas that ofsemiconductor decreases

Thermistor: Negative temperature coefficient

(NTC) and positive temperature coefficient (PTC)

are small, very sensitive and used in a smallrange of temperature

Resistance Temperature Detectors (RTDs):Wire wound or a thin film.Platinum is the most used material.International standard temperature sensor forlaboratory applications between -270< T


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ThermoresistanceIn the USA, ASTM Specification E1137 "Standards Specification for Industrial

Platinum Resistance Thermometers" gives many details and specifications forthem over the range from -200 C to 650C.

Two RTD grades: A and B with a resistance-temperature relationship that has thefollowing tolerances:

Grade A Tolerance = [0.13 +0.0017 *|t|] C

Grade B tolerance =[0.25 +0.0042 *|t|] Cwhere |t| is the absolute value of the RTD's temperature in C.

DIN Standard (German) recognizes three different tolerance classes:

Class A tolerance: [0.15 + 0.002*|t|] CClass B tolerance: [0.30 + 0.005*|t|] CClass C tolerance: [1.20 + 0.005*|t|] C

ThermoresistanceTemperature vs resistance equation (Callendar-Van Dusen equation)

Here, RT

is the resistance at T, R0

is the resistance at 0 C, and the constants (for =0.00385 platinum RTD) are:

Since the Band Ccoefficients are relatively small, the resistance changes almostlinearly with T.

RRR oo

0.100

01

=


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ThermoresistanceAdvantages of RTDs Stable output for long period of time.

Accurate readings over relatively narrow temperature spans.

No need of special wire.

Change in resistance linear.

Disadvantages, compared to the thermocouples: Smaller overall temperature range.

Higher initial cost.

More sensible in high vibration environments. Higher response time.

Current source required.

Small change in resistance.

Self heating.

Less rugged than thermocouples.


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RTD geometry

Sheathing: stainless steel or iconel, glass, alumina,quartz

Metal sheath can cause contamination at hightemperatures and are best below 250C.

At very high temperatures, quartz and high-purityalumina are best to prevent contamination.

Thermoresistance


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ThermoresistanceRequire an electrical current to produce a voltage drop across the

sensor that can be then measured by a calibrated read-out device.

Wheatstone bridge:

R4.R2 = R3.R1 VAB=0,no current passing through the

voltmeter OR

SAB VRR

R

RR

RV

+

+=

21

2

43

4

ThermoresistanceTwo wires connection

(R4+RL1+RL2).R2 = R3.R1


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ThermoresistanceThree wires connection

(R4+RL2).R2 = (R3+RL1).R1

R4.R2 +RL2.R2= R3.R1+RL1.R1

R2=R1=R, and if RL2=RL1

R4= R3

ThermoresistanceFour wires connection

A power source A supplies a stable current S

throught the thermoresistance, and the voltage

drop is measured with a voltmeter.

Negligeble effect of the conduction wires.


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ThermoresistanceThree wires sensor measure through a 4 wires connection measure

Error caused by RL; thus, create the forth

wire, as close as possible to the RTD.

When a 4 wires connection is available, a

two wires sensor can be used as 4 wires to

reduce wire errors.

Potential ProblemsRTDs are more fragile than thermocouples.

An external current must be supplied to the RTD.This current can heat the RTD, altering the results.For situations with high heat transfer coefficients,this error is small since the heat is dissipated to air.

For small diameter thermocouples and still air thiserror may become large.

When the platinum is connected to copperconnectors, a voltage difference will occur (as inthermocouples). This voltage must be subtracted off.


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ThermistorsThermistors also measure the change in resistance withtemperature.

Thermistors are very sensitive (up to 100 times more thanRTDs and 1000 times more than thermocouples) and candetect very small changes in temperature. They are alsovery fast.

Due to their speed, they are used for precision temperaturecontrol and any time very small temperature differences

must be detected.They are made of ceramic semiconductor material (metaloxides).

The change in thermistor resistance with temperature is verynon-linear.

Thermistor Advantages andDisadvantages

Advantages:Very sensitive (hasthe largest output

change from inputtemperature)

Quick response

More accurate thanRTD andThermocouples

Disadvantages:Output is a non-linear function

Limited temperaturerange.

Require a currentsource

Self heating

Fragile


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Thermistor Non-Linearity

Resistance/TemperatureConversion

Standard thermistors curves are not provided as much aswith thermocouples or RTDs. You often need a curve for aspecific batch of thermistors.

No 4-wire bridge is required as with an RTD.Thermistors do not do well at high temperatures and showinstability with time (but for the best ones, this instabilityis only a few millikelvin per year)


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Choice Between RTDs,Thermocouples, Thermisters

Cost thermocouples are cheapest by far, followed by RTDs

Accuracy RTDs or thermisters

Sensitivity thermisters

Speed - thermisters

Stability at high temperatures not thermisters

Size thermocouples and thermisters can be made quite

smallTemperature range thermocouples have the highestrange, followed by RTDs

Ruggedness thermocouples are best if your system will betaking a lot of abuse

metrology & instrumentation

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