Temperature Measurements

About temperature measurements

Temperature measurement is very important in all spheres of life and especially so in the process industries. However, temperature measurement poses particular problems, since it cannot be related to a fundamental standard of temperature in the same way that the measurement of other quantities can be related to the primary standards of mass, length and time. If two bodies of lengths l1 and l2 are connected together end to end, the result is a body of length l1 + l2. A similar relationship exists between separate masses and separate times. However, if two bodies at the same temperature are connected together, the joined body has the same temperature as each of the original bodies.

This is a root cause of the fundamental difficulties which exist in establishing an absolute standard for temperature in the form of a relationship between it and other measurable quantities for which a primary standard unit exists. In the absence of such a relationship, it is necessary to establish fixed, reproducible reference points for temperature in the form of freezing and boiling points of substances where the transition between solid, liquid and gaseous states is sharply defined. The International Practical Temperature Scale (IPTS) uses this philosophy and defines six primary fixed points for reference temperatures in terms of:

the triple point of equilibrium hydrogen: -259.34 C
the boiling point of oxygen: -182.962 C
the boiling point of water: 100.0 C
the freezing point of zinc: 419.58 C
the freezing point of silver: 961.93 C
the freezing point of gold: 1064.43 C
(all at standard atmospheric pressure)  

The freezing points of certain other metals are also used as secondary fixed points to provide additional reference points during calibration procedures. These are of particular use for calibrating instruments measuring high temperatures. Some examples are:

freezing point of tin: 231.968 C
freezing point of lead: 327.502 C
freezing point of zinc: 419.58 C
freezing point of antimony: 630.74 C
freezing point of aluminum: 660.37 C
freezing point of copper: 1084.5 C
freezing point of nickel: 1455 C
freezing point of palladium: 1554 C
freezing point of platinum: 1772 C
freezing point of rhodium: 1963 C
freezing point of iridium: 2447 C
freezing point of tungsten: 3387 C

Instruments for measuring temperature can be divided into seven separate classes according to the physical principle on which they operate. These principles are as follows:

  • Thermal expansion

  • The thermoelectric effect

  • Resistance change

  • Resonant frequency change

  • Radiative heat emission

  • Thermography

  • Acoustic thermometry

  • Fiber optic devices

  1. Temperature Transducers

  2. Liquid-in-Glass Thermometers

  3. Bimetallic Thermometer

  4. Pressure Thermometers

  5. Thermoelectric-Effect Instruments (Thermocouples)

  6. Thermocouples Extension Leads

  7. Connection of Voltage Measuring Instrument

  8. Thermocouple Protection

  9. Thermocouple Manufacture

  10. Thermopile

  11. Continuous Thermocouple

  12. Resistance Thermometers

  13. Thermistors

  14. Quartz Thermometers

  15. Radiation Thermometers

  16. Optical Pyrometer

  17. Radiation Pyrometers

  18. Unchopped Broad-Band Radiation Pyrometers

  19. Chopped Broad-Band Radiation Pyrometers

  20. Narrow-Band Radiation Pyrometers

  21. Two-Color Pyrometer (Ratio Pyrometer)

  22. Selected-Waveband Pyrometer

  23. Thermography (Thermal Imaging)

  24. Fiber Optic Temperature Sensors

  25. Intelligent Temperature Measuring Instruments

Back to Principles of Measurement

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