Clearly, the measurement of temperature is of great importance in heat and mass transfer. A wide variety of techniques have been used; a good general source relating to temperature measurement is that of Jones (1985). Some of the basic principles of measurement are given in the entry on Temperature Measurement, Bases. Here, we will give a brief summary of some of the main methods, namely fluid filled thermometers, bimetallic thermometers, resistance thermometers, thermistors, thermocouples and optical pyrometers.
These can be liquid filled (operating in the range 200–670 K) or gas filled (operating in the range 4–1050 K). The best known thermometer is the mercury-in-glass thermometer, but other liquids such as alcohol may also be used in glass thermometers. The expansion of the fluid can also be transmitted to a Bourdon tube whose deflection can be recorded in the normal way. Similarly, with gas filled thermometers, the increase in pressure of the gas can be transmitted to a Bourdon-type gauge. A similar principle applies to vapor pressure thermometers where a liquid vaporizes to produce the increase in gas pressure.
The bimetallic thermometer is one of a large class of thermometers that depend on thermal expansion. In the bimetallic thermometer, two strips of metal (for instance, brass and invar) are joined together; the differential thermal expansion causes the strip to deflect when the temperature changes. The bimetallic strip can also be installed in a helical ribbon form with the temperature indication being in the form of a needle which moves circumferentially over a dial. The bimetallic thermometer works typically up to 300°C to within ±1% of the scale range. However, such thermometers have a rather slow response and, by definition, require the whole of the sensing element to be immersed in the fluid whose temperature is being measured.
The electrical resistance of many materials changes with temperature and this effect can be used as a means of determining the temperature, hence the name resistance thermometer. Typically, resistance thermometers are made of platinum and can be used over the range 200–400 K. They have a typical sensitivity in the range 0.2–20 ohms/K but their response time is normally rather slow (1–10 s).
Thermistors are semi-conductor devices operating in the range 50–400 K giving typically a 4% change in resistance per degree K. With a small enough bead, a response time of 3 ms is possible.
This is the most commonly used sensor for fluid temperature. The main types of thermocouples (together with their range of temperatures and sensitivity) are listed in Table 1. Extensive information on thermocouples is given in the books by Early (1976) and Jones (1985). Figure 1 shows the various types of thermocouple junctions used in fluid temperature measurement and also gives the approximate minimum response time for the various types.
Optical pyrometers are based on the fact that as the temperature changes, the wave length λm at the maximum intensity of radiation decreases according to the linear law:
where T is the absolute temperature (Kelvin). In the optical pyrometer, a telescope system is used to focus on the objective whose temperature is to be measured. A Tungsten filament is placed at the focal point of the objective lens of the telescope and is viewed with the eyepiece. The temperature of the filament is adjusted by increasing the voltage across it until the filament "disappears" against the image of the hot body whose temperature is to be measured. The resistance of the filament gives the temperature which matches that of the hot body. Further details of this and other radiation thermometers are given by Jones (1985).
Jones, E. P. (1985) Instrumentation Technology, Vol. 2, Measurement of temperature and chemical composition, Butterworth and Co. (Publishers) Ltd, London. ISBN: 0-408-01232-3.
Early, B. (1976) Practical Instrumentation Handbook, Scientific ERAP Publications.