Tempreture measurement

Temperature Measurement, Temperature Scale, Liquid in glass thermometer, Calibration, Bimetallic thermometer, Pressure thermometer, Electrical resistance thermometer, Thermistor, Thermocouple, Radiation pyrometer, optical pyrometer

1. Temperature Measurement Prepared by : Ratnadeepsinh Jadeja

2. Introduction The temperature is a thermal state of a body which distinguishes a hot body from a cold body. The temperature of a is proportional to the stored molecular energy i.e. the average molecular kinetic energy of the molecules in a system. The temperature may also be defined as: The measure of the mean K.E. of the molecules of a substance. The degree of hotness and coldness of a body or an environment measured on a definite scale. The driving force or potential causing flow of energy as heat.

3. Introduction Instruments for measurement of ordinary temperatures are known as thermometers and those for measuring high temperatures are known as pyrometers, Thermometer Pyrometer

4. Introduction It has been found that a gas will not occupy any volume at a particular temperature. This temperature is known as absolute zero temperature . The temperatures measured with absolute zero as basis are called absolute temperatures . The point of absolute zero temperature is found to occur at 273.15 °C below the freezing temperature of water.

5. Thermometer Thermometric property Constant volume gas Pressure (p) Constant pressure gas Volume (V) Alcohole or mercury in glass Length (L) Electric resistance Resistance (R) Thermocouple Electromotive force (E) Radiation (Pyrometer) Intensity of radiation (I or J)

6. Temperature Scale For the calibration of thermometric instruments the seventh general conference on weight and measures held in 1927 formulated a convenient scale known as the International Practical Temperature Scale. It was revised at thirteenth general conference in 1968. It consists of reproducible reference temperatures or primary fixed point s defined by a number of pure substances with assigned values of temperatures. The triple point represents an equilibrium state between solid, liquid and vapour phases of a substance. Normal boiling point is the temperature at which the substance boils at standard atmospheric pressure of 760 mm Hg. Normal freezing point is the solidification or the melting point temperature of the substance at standard atmospheric pressure.

7. Temperature Scale Equilibrium state T, K T, °C Triple point of Hydrogen 13.81 -259.34 Normal Boiling point of Hydrogen 20.28 -252.87 Triple point of Oxygen 54.361 -218.789 Normal boiling point of Oxygen 90.188 182.962 Triple point of water 273.16 0.01 Normal boiling point of water 373.15 100.00 Normal freezing point of antimony (antimony point) 630.74 357.59 Normal freezing point of zinc (zinc point) 692.73 419.58 Normal freezing point of silver (silver point) 1235.08 961.93 Normal Freezing point of gold (gold point) 1337.58 1064.43

8. Temperature Scale The fixed points for these temperature scales before 1954 The use of two fixed point found unsatisfactory and later abandoned because of the following reasons: It is difficult to achieve equilibrium between pure ice and air saturated water. There is extreme sensitiveness of the steam point to the change in pressure. Temperature Celsius scale Fahrenheit scale Steam point 100 212 Ice point 32 Interval 100 180

9. Temperature Scale After 1954 It was suggested by Kelvin that a single fixed point only was necessary to establish a temperature. He pointed out that triple point of water could be used as a single point.  

10. Temperature Measuring Instruments The temperature measuring instruments are based on changes in a broad range of physical properties, among which are the following: Change in physical dimensions Change in gas pressure or vapour pressure Change in electrical properties Change in emitted thermal radiation Change in chemical phase

11. Temperature Measuring Instruments The thermometers may also be classified as follows: Expansion thermometers Liquid in glass tube thermometers Bimetallic thermometers. Pressure thermometers Vapour pressure thermometers Liquid filled thermometers Gas filled thermometers Resistance thermometers Thermocouple thermometers Radiation pyrometers Optical pyrometers

12. Liquid in Glass Thermometer The mercury or other liquid fills the glass bulb and extends into the bore of the glass stem. Mercury is the most suitable liquid and is used from -38.9 °C to about 600 °C. Liquid Boiling point °C Freezing point °C Pentane 36 -130 Ethyl alcohol 78 -100 Toluene 110 -92

13. Calibration All the portion of a glass thermometer are temperature sensitive, yet greatest sensitivity to temperature is at the bulb, where the largest volume of liquid is contained.

14. Bimetallic Thermometers In a bimetallic thermometer differential expansion of bimetallic strips is used to indicate the temperature. The bimetallic thermometers are made with metals which have widely different thermal expansion coefficients. In order to protect the bimetallic thermometers against wear and corrosion, they are usually mounted in wells. Invar (an alloy of nickel and iron), Nickel-iron alloys with chromium and manganese added are often used for thermal expansion material

15. Helical type Bimetallic Thermometers

16. Spiral type Bimetallic Thermometers

17. Pressure Thermometers In pressure thermometers liquids, gases and vapours can all be used. The fluid is confined in a closed system. In this case pressure is a function of the temperature, so that when the fluid is heated, the pressure will rise. And the temperature can be indicated by bourdon type pressure gauge. The following pressure thermometers can be used. Vapour pressure thermometer. Liquid filled thermometer Gas filled thermometer. These devices are commonly known as Filled systems.

18. Pressure Thermometers

19. Static Errors in Filled Systems The sources of static errors in filled systems are: The ambient temperature effect. The head effect. The barometric effect. The immersion effect.

20. Electrical Resistance Thermometer or RTD Electrical methods of temperature measurement are very convenient due to the following reasons: They furnish signal that is easily detected, amplified or used for control purpose. They are usually quite accurate when properly calibrated and compensated.. Range of temperature from -200 to 650 °C. If the electrical resistance of a wire of known and calibrated material is measured, the temperature of the wire can be determined. The resistance element should be of a suitable metal having high temperature coefficient, reproducibility, stability and ability to resist corrosion and oxidation.

21. Thermistors A thermistor (Thermal Resistor) is a thermally sensitive variable resistor made of a ceramic like semi conducting material. Thermistors, unlike metals, respond negatively to temperature; as the temperature rises, the thermal resistance decreases. Thermistors are often composed of oxides of manganese, nickel and cobalt formulations having resistivities of 100 to 450000 Ω in cryogenic applications, doped germanium and carbon-impregnated glass are used. The electrical characteristics of thermistors are controlled by varying the type of oxide used and physical size and configuration of the thermistor. The platinum resistance thermometer: 300 °C to 400 °C Copper resistance thermometer: 0 °C to 180 °C.

22. Thermocouple Thermometer

23. Thermocouple Thermometer For higher range of temperatures i.e. above 650 °C, filled thermometers are unsuitable. For higher range of temperatures, thermocouples and pyrometers are used. Seebeck effect: When the two junctions of a pair of dissimilar metals are maintained at different temperatures, e.m.f. (electromotive force) is generated. The phenomenon of generation of e.m.f is called Seebeck effect. Peltier effect: If direct current is passed through a pair of dissimilar metals, there us a heating at one junction, cooling at the other depending upon the material combinations.

24. Thermocouple Thermometer Materia and Range of thermocouple. Platinum vs platinum-rhodium : 400 to 1450 °C. Chromel vs alumel : -200 to 1200 °C. Iron vs constantan : -200 to 750 °C. Copper vs constantan : -200 to 350 °C.

25. Radiation Pyromoters Radiation heat transfer is defined as “the transfer of energy across a system boundary by means of an electromagnetic mechanism which is caused solely by a temperature difference.” Where as the heat transfer by conduction and convection takes place only in the presence of medium, radiation heat transfer does not require a medium.

26. Radiation Pyromoters The principle, that the radiant energy emitted from a body increases with temperature, is used in measuring temperatures particularly in the higher ranges. A device which measures the total intensity of radiation emitted from a body is called total radiation pyrometer.

27. Optical Pyromoters An optical pyrometer works on the principle that matters glow above 480 °C and the colour of visible radiation is proportional to the temperature of the glowing matter.

28. Optical Pyromoters