Assam Don Bosco University

TEMPRARTUREWhat is a Temperature?

It is a measure of total amount of thermal energy in abody. It is the measure of how fast a molecule moves inan object. Temperature of a body is a property thatdetermines whether or not a body is in thermalequilibrium with other body. Temperature is actually themacroscopic manifestation of the energy level of atomsand molecules. A lower limit of temperature at which theatoms stop moving is called absolute zero temperature. Itwas first explained by Kelvin.

Thermal energyWhat is thermal energy?

Thermal energy is the total amount of kineticenergy and potential energy of all the particles ina material or body.

HeatWhat is heat?

It is the energy that a body is absorbed orevolved. It is the total amount of molecularenergy in an object.

How do we measure temperature?LIQUID IN GLASS

THERMOMETER

THEROMCOUPLE

RESISTANCE TEMPERATURE DETECTOR(RTD)

THERMISTOR

BI-METALLIC THERMOMETER

Various units for measuring temperature°C-Degree Celsius or centigrade (0°-100°)

°F- Degree Fahrenheit (32°-212°)

K- Kelvin (0K= -273°C)

R-Rankine (0R= -459.67°F)

Relationship between different units

°C = (°F - 32)/1.8

°F = 1.8 x °C + 32

K = °C + 273.15

R = °F + 459.67

LIQUID IN GLASS THERMOMETER

A Liquid in glass thermometer is based on method ofoperation of thermal expansion of liquids. The volumeof liquid changes with temperature. To calibrate thethermometer, a bulb is made to reach thermalequilibrium. It was used by Celsius and Fahrenheit.Celsius used two fixed points in his scale, the meltingpoint and boiling point of water. He used melting pointof pure water at 0°C and boiling point at 100°C, it wasdivided into 100 grades. Similarly, Fahrenheit used 32°Fas melting point and 212°F as boiling point of alcohol, itwas divided into 180 grades. Again Kelvin used anotherscale to explain the behavior of gases, he took

0K= -273°C.

TEMPERATURE SCALES

THERMOCOUPLEIt is a type of electrical resistance thermometer.In 1821 Thomas. J. Seeback(German) and JeanCharles. A. Peltier(France), discoveredthermometric effect, which forms the basis ofthermocouple technology.

What is Seeback Effect?Seeback effect describes that a current isproduces in a circuit of two dissimilar metalshaving one junction at hot temperature andanother body at cold temperature.

Seeback effect has a relationship between voltage and temperature, it is given by-

= temperature at hot,

= temperature at cold,

a= seeback coefficient

What is Peltier effect?If emf is produced in a circuit of two dissimilarmetals then one end of the metals must be incold and another end must be in hot.

THERMOCOUPLE CIRCUIT

Two wires of different metal alloys are usedwhich converts thermal energy into electricalenergy. It requires a temperature differencebetween measuring junction and referencejunction. Here thermal emf is thethermometric property of the thermometer.

The result of such measurementon most of the thermocouple can berepresented by the relation.

Standard Thermocouple Alloy Conductor Combinations

CODE CONDUCTOR COMBINATION TYPICAL OPERATING RANGE

ºF

B Platinum-30% Rhodium / Platinum-6% Rhodium +2500 to +3100

C Tungsten-5% Rhenium / Tungsten-26% Rhenium +3000 to +4200

D Tungsten-3% Rhenium / Tungsten-25% Rhenium +2800 to +3800

E Nickel Chromium / Constantan 0 to +1650

J Iron / Constantan +0 to +1400

K Nickel Chromium / Nickel Aluminium 0 to +2300

N Nickel-Chromium-Silicon / Nickel-Silicon-

Magnesium

1200 to +2300

R Platinum-13% Rhodium / Platinum 1600 to +2600

S Platinum-10% Rhodium / Platinum 1800 to +2600

T Copper / Constantan -300 to +650

RESISTANCE TEMPERATURE DETECTOR (RTD)RTD are sensors used to measure temperatureby correlating the resistance of the elementwith temperature. Here platinum is used dueto its wide temperature range. They are slowlyreplacing the use of thermocouples in manyindustrial applications below 600°C due tohigher accuracy and repeatability.

RTD are frequently used in areas like-

Refrigeration Servicing, Furnace Servicing,Medical Research, Textile Industries etc.

Resistance temperature detector element

Wire wound element- wires arewrapped around a ceramicmandrel then inserted insidea ceramic shell.

Inner coil element- wires arecoiled and inserted into theholes of a ceramic insulator.

Thin film element- metallic ink isdeposited into a ceramicsubstrate and Lasers are usedto provide a resistance path.

For measuring the resistance of RTD Wheatstone bridge circuit is used.

In a Wheatstone bridge, electrical excitationcurrent is passed through the bridge, and thebridge output current is an indication of the RTDresistance.

A relation between temperature and resistance is given by-

for temperature from 0°C to 660°C and

for temperature from -190°C to 0°C

Resistance of platinum is 100Ω at 0°Celsius. It’s resistance is 138.5Ω at 100°Celsius. So it has a difference of 38.5Ω, i.e. 0.385Ω per 1°Celsius Rise in Temperature. There are other materials available for more unusual temperature ranges such as Germanium.

Though both are used in industrial uses there is a difference between RTD and thermocouples-

If in a process the temperature is in between -200°C to 600°C RTD ispreferred.

Whereas thermocouples have a range of -180°C to 2320°C, so for

temperature above 600°C thermocouples are preferred.

THERMISTORThermistor is a two-terminal semiconductordevice whose resistance is temperature sensitive.Thermistor is different from ResistanceTemperature Detector because the material usedin thermistor is generally a ceramic substratewhile RTD is pure metal. Resistance TemperatureDetector are useful over larger temperatureranges while thermistors typically achieve ahigher precision within a limited temperaturerange. It has very high temperature coefficient ofresistance. The resistance at any temperature T isgiven by-

where, = constant

= resistance of thermistor at temp T

= resistance of thermistor at temp

Thermistors are generally composed ofsemiconductor materials. Although positivetemperature coefficient units are available,most thermistors have a negative temperaturecoefficient (TC); that is, their resistancedecreases with increasing temperature. Thenegative T.C. can be as large as several percentper degree Celsius, allowing the thermistorcircuit to detect minute changes intemperature which could not be observedwith an RTD or thermocouple circuit.

One of the advantage of using a thermistorsensor is the large change in resistance to arelatively small change in temperature, thismakes them sensitive to a small change intemperature.

BI-METALLIC THERMOMETER

It is a temperature sensitive mechanical devicecomposed of a mixture of two or more chemicalelements like alloy. It is used to convert atemperature change into mechanical displacement.Specially used alloy in this type of thermometer isSteel and Copper or Steel and Brass alloy.

Bi-metallic thermometers are used in-

Clocks

Patio thermometers

Heat engines etc.

A bi-metallic strip contains two materials ofdifferent thermal expansion coefficients. It isarranged in such a way so as to give a largerpossible difference in their coefficient of heatexpansion. The changes in angle of twist of thebi-metallic spiral therefore dependent ontemperature.

The relation between angle of twist andtemperature is given by-

S= thickness of bi-metallic strip, l=length of bimetallic strip, a= specific thermal expansion