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:
-
Temperature Transducers
-
Liquid-in-Glass Thermometers
-
Bimetallic Thermometer
-
Pressure
Thermometers
-
Thermoelectric-Effect Instruments (Thermocouples)
-
Thermocouples Extension Leads
-
Connection of Voltage Measuring Instrument
-
Thermocouple Protection
-
Thermocouple Manufacture
-
Thermopile
-
Continuous Thermocouple
-
Resistance Thermometers
-
Thermistors
-
Quartz
Thermometers
-
Radiation Thermometers
-
Optical
Pyrometer
-
Radiation
Pyrometers
-
Unchopped Broad-Band Radiation Pyrometers
-
Chopped Broad-Band Radiation Pyrometers
-
Narrow-Band Radiation Pyrometers
-
Two-Color
Pyrometer (Ratio Pyrometer)
-
Selected-Waveband Pyrometer
-
Thermography
(Thermal Imaging)
-
Fiber Optic Temperature Sensors
-
Intelligent Temperature Measuring Instruments

Back to Principles of Measurement
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