PRESENTED BYPREM BABOOM.Sc.,B.Tech(Chemical Engineering),M.Phil, M.B.A

Fellow of Institution of Engineers IndiaAn Expert for www.ureaknowhow.com

WHAT IS TEMPERATURE?

• Qualitatively, the temperature of an object determines the sensation of warmth or coldness felt by touching it. More specifically, temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in units of degrees on a standard scale.

• Temperature is the degree of "hotness" (or "coldness"), a measure the of the heat intensity. When two objects of different temperature are in contact, the warmer object becomes colder while the colder object becomes warmer. It means that heat flows from the warmer object to the colder one.  

• Thermometer helps us determine how cold or how hot a substance is. Temperatures in science (and in most of the world) are measured and reported in degrees Celsius (oC). In the US, temperatures are commonly reported in degrees Fahrenheit (oF). On both Celsius and Fahrenheit scales, the temperature at which ice melts (water freezes) and the temperature at which water boils are used as reference points.

• On the Celsius scale, freezing point of water is defined as 0 oC, and the boiling point is defined as 100 oC. 

• On the Fahrenheit scale, water freezes at 32 oF and boils at 212 oF.

• On the Celsius scale there are 100 degrees between freezing and boiling of water, compared to 180 degrees on the Fahrenheit scale. This means that 1 oC= 1.8 oF.

• Thus the following formulas can be used to convert temperature between the two scales:

•     (1) F = 1.8 C + 32 = 9/5 C + 32    (2) C = 0.56( F -32) =5/9( F-32)

DEG F DEG C DEG RDEG K

ABSOLUTE ZERO

ICE POINT

STEAM POINT

00-273-459

491273032

671373100212

Types Of Measurement

• There are four basic types of temperature measuring devices, each of which uses a different principle: – Mechanical (liquid-in-glass thermometers,

bimetallic strips, etc.). – Thermojunctive (thermocouples). – Thermoresistive (RTDs and thermistors). – Radiative (infrared and optical pyrometers).

Mechanical temperature measuring devices

• Principle of operation: – A change in temperature causes some kind of

mechanical motion, typically due to the fact that most materials expand with a rise in temperature. Mechanical thermometers can be constructed which use liquids, solids, or even gases as the temperature-sensitive material.

– The mechanical motion is read on a physical scale to infer the temperature.

Liquid-in-glass thermometer

– The most common and well-known thermometer is the liquid-in-glass thermometer.

– As the temperature rises, the liquid expands, moving up the tube. The scale is calibrated to read temperature directly.

• Usually, mercury or some kind of alcohol is used for the liquid

Bimetallic strip thermometer

– Two dissimilar metals are bonded together into what is called a bimetallic strip, as sketched.

– Suppose metal A has a smaller coefficient of thermal expansion than does metal B.

– As temperature increases, metal B expands more than does metal A, causing the bimetallic strip to curl upwards as sketched.

Working of bimetallic strip

Pressure thermometer • considered mechanical, operates by the

expansion of a gas instead of a liquid or solid. (Note: There are also pressure thermometers which use a liquid instead of a gas.)

• Suppose the gas inside the bulb and tube can be considered an ideal gas. The ideal gas law isPV = mRT

oR is a constant. The bulb and tube are of constant volume, so V is a constant. Also, the mass, m, of gas in the sealed bulb and tube must be constant. Hence, the above equation reduces to P = constant times T.

– A pressure thermometer therefore measures temperature indirectly by measuring pressure. • The gage is a pressure gage, but is

typically calibrated in units of temperature instead.

Pressure thermometer

MERCURY IN STEEL THERMOMETER

• MERCURY - IN - STEEL thermometer works on the principle of expansion of mercury due to rise in temperature. The whole system is filled with mercury under pressure. The definite volume of mercury contained in the bulb expands under effect of temperature to be measured. The increaase in volume of confined mercury is transmitted through the capillary to the coiled burdon tube which uncoils proportionally to volume increase resulting an indication of temperature by the pointer.

GAS FILLED DIAL THERMOMETERS

• Gas Filled Thermometers deploy Nitrogen Gas at high pressure as an expansion gas filled into a closed system comprising of a bulb (of Steel or Chrome – Moly Steel), a microbore capillary (of Steel or Stainless Steel) and a spiral or ‘C’ shaped Bourdon Tube (of MS or SS). This system when heated at bulb end the Gas in the bulb expands and a pressure is generated within which moves the spiral / ‘C’ Shaped bourden as it is the only the elastic element. This movement is transmitted to a rack and pinion movement which drives a pointer thus showing temperature on a calibrated dial.

THERMOCOUPLE

• The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan. These two wires are connected at two different junctions, one for temperature measurement and the other for reference. The temperature difference between the two junctions is detected by measuring the change in voltage (electromotive force, EMF) across the dissimilar metals at the temperature measurement junction.

CONSTRUCTION OF THERMOCOUPLE

Typical Thermocouple Circuit

CONSTRUCTION OF THERMOCOUPLE

CONSTRUCTION OF THERMOCOUPLE

GROUNDED JUNCTION

UNGROUNDED JUNCTION

EXPOSED JUNCTION

LIFE OF THERMOCOUPLE

THERMOCOUPLE

THERMOCOUPLE

TYPICAL RTD CIRCUIT

RTD ELEMENTS

LEAD EFFECTS

3 WIRE RTD

Thermistors

– A thermistor is similar to an RTD, but a semiconductor material is used instead of a metal. A thermistor is a solid state device.

– A thermistor has larger sensitivity than does an RTD, but the resistance change with temperature is nonlinear.

– Furthermore, unlike RTDs, the resistance of a thermistor decreases with increasing temperature.

– Thermistors cannot be used to measure high temperatures either, compared to RTDs. In fact, the maximum temperature of operation is sometimes only 100 or 200 oC.

From the circuit diagram, it is clear that this is a simple voltage divider. Rs is some fixed (supply) resistor. Rs and the supply voltage, Vs, can be adjusted to obtain the desired range of output voltage Vo for a given range of temperature.

Radiative temp.measuring devices (radiative pyrometry)

• Principle of operation: – Radioactive properties of an object change with

temperature. – So, radioactive properties are measured to infer the

temperature of the object. – The advantages of radioactive pyrometry are:

• There is no physical contact with the object whose temperature is being measured.

• Very high temperatures can be measured.

• The fundamental equation for radiation from a body is the Stefan-Boltzmann equation,

– where • E is the emissive power radiated per unit area (units

of W/m2). • is the emissivity, defined as the fraction of blackbody

radiation emitted by an actual surface. The emissivity must lie between 0 and 1, and is dimensionless. Its value depends greatly on the type of surface. A blackbody has an emissivity of exactly 1.

• is the Stefan-Boltzmann constant,

• T is the absolute temperature of the surface of the object (units of K).

Infrared Pyrometer

• An infrared pyrometer infers the temperature of a hot surface by measuring the temperature of a detector inside a detector chamber as shown below:

IDENTIFICATION BY COLOUR

• DARK RED :540 DEG C

• MEDIUM CHERRY RED :680 DEG C

• ORANGE :900 DEG C

• YELLOW :1010 DEG C

• WHITE :1250 DEG C

Features of Pyrometer– An optical pyrometer is useful for measuring very

high temperatures (even flames). – An optical pyrometer works by comparing a

glowing wire of known temperature to the glow (optical radiation) from a hot object.

– When the internal wire and the glow of the object are the same color, the temperatures are assumed to be equal.

• The temperature of the internal wire is controlled and known, and thus the temperature of the object can be inferred

Attribute Thermocouple RTD Thermistor

Cost Low High Low

Temperature Range

Very wide-450ºF

+4200ºF

Wide-400ºF

+1200ºF

Short to medium-100ºF+500ºF

Interchange ability

Good Excellent Poor to fair

Long-term Stability

Poor to fair Good Poor

Accuracy Medium High Medium

Repeatability Poor to fair Excellent Fair to good

Sensitivity (output)

Low Medium Very high

Response Medium to fast Medium Medium to fast

Linearity Fair Good Poor

Self Heating No Very low to low High

Point (end) Sensitive

Excellent Fair Good

Lead Effect High Medium Low

Size/Packaging Small to largeMedium to

smallSmall to medium