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a branch of applied physics devoted to the development of temperature measurement methods and devices. Thermometry is also a branch of metrology, since part of its task is to provide for standardization and precision of temperature measurements through the establishment of temperature scales and standards and through the development of methods for calibrating and testing temperature measurement devices.
Temperature cannot be measured directly. Temperature variation is judged from the change in other physical properties of bodies—for example, volume, pressure, electrical resistance, electromotive force (emf), and radiation intensity—that are associated with temperature by specific principles. Therefore, temperature measurement methods are in fact methods of measuring the thermometric properties mentioned above. These properties must depend unambiguously on temperature, and their measurement must be sufficiently simple and precise. In the development of a specific method or device, it is necessary to select a thermometric substance whose appropriate property is readily reproducible and demonstrates a sufficiently pronounced change with temperature.
In order to measure temperature using any method, it is necessary to establish a temperature scale.
Various temperature measurement methods exist; they depend on the principles of operation of the devices used, the temperature ranges to be measured, the measurement conditions, and the required accuracy. They may be divided into two main groups: (1) contact methods, or thermometry proper, and (2) noncontact methods—radiation thermometry, or pyrometry.
A typical and essential feature of all contact temperature measurement methods is that every device that measures the temperature of a medium must be in thermal equilibrium with the medium (see) —that is, it must have the same temperature as the medium.
The main elements of all temperature measurement devices are a sensitive element, where the thermometric parameter is realized, and a measuring device, which is connected to the sensitive element and measures the numerical values of the property.
In gas thermometry, the thermometric parameter is the temperature dependence of gas pressure with constant volume or gas volume under constant pressure; accordingly, a distinction is made between constant-volume and constant-pressure gas thermometers. The thermometric substance in these thermometers is gas whose properties approximate those of an ideal gas. The equation of state of ideal gas, pV = RT, defines the relation of absolute temperature T to pressure p under constant volume V or of T to volume V under constant pressure. Gas thermometers are used for the measurement of thermodynamic temperature. The accuracy of such a device depends on the degree to which the gas used—usually nitrogen or helium—approximates an ideal gas.
In condensation thermometers, the thermometric parameter is the temperature dependence of the pressure of the saturated vapor of a liquid. The sensitive element—a bulb in which the liquid and its saturated vapors are in equilibrium—is connected to a manometer by means of a capillary. The thermometric substance is usually a low-boiling gas, such as oxygen, argon, neon, hydrogen, or helium. Empirical relations are used in calculating the temperature according to the measured pressure. Condensation thermometers have a limited range of application. High-precision thermometers (to 0.001 degree) are used for the establishment of reference points (seeINTERNATIONAL PRACTICAL TEMPERATURE SCALE).
In liquid-filled thermometers, the thermometric parameter is the thermal expansion of liquids, and the thermometric substance is usually mercury. The temperature is not determined by measuring the volume of the liquid; rather, the thermometer’s bore is graduated in degrees Celsius during manufacture—marks whose intervals correspond to the change in volume for a given change of temperature are applied to the thermometer. The accuracy of the thermometer depends on the precision of calibration.
Filled-system thermometers, which are used in technology and industry, use the same thermometric parameters as liquid-filled or gas-filled thermometers.
In resistance thermometers, the thermometric parameter is the temperature dependence of the electric resistance of pure metals, alloys, and semiconductors; the choice of thermometric substance depends on the range of temperature variation and the required accuracy. Empirical formulas or tables are used for the determination of temperature according to the measured electric resistance. Thermometers designed for precise measurements, which are made of platinum or alloyed germanium, are calibrated individually.
In thermoelectric thermometers, in which the sensitive element is a thermocouple, the thermometric parameter is the thermal emf of the thermocouple; the thermometric substance is selected according to the range of application and the required accuracy. Empirical formulas or tables are also used for the determination of temperature according to the measured emf. In view of the special nature of the thermoelectric thermometer, which is a differential measuring instrument, its accuracy depends on the precision of maintenance and measurement of a given temperature at one of the junctions of the thermocouple, the reference junction.
Measuring instruments that are used to determine the numerical values of thermometric parameters (manometers, potentiometers, quotient meters, bridges, millivoltmeters, and so on) are known as secondary instruments. The accuracy of temperature measurement depends on the precision of the secondary instruments. Thermometers for industrial use are not usually calibrated individually and are combined with appropriate secondary instruments whose scales are marked directly in degrees Celsius.
At low temperatures (below 90°K) and superlow temperatures (below 1°K), special temperature-measurement methods and equipment are used in addition to conventional methods (seeLOW TEMPERATURES). Among the methods are magnetic thermometry, which is used in the range from 0.006° to 30°K and is accurate to 0.001°K, and methods based on the temperature dependence of the Mössbauer effect and the anisotropy of gamma radiation (below 1°K). Noise thermometers with a Josephson-effect converter (below 1°K) are also used. A special complication of thermometry in the superlow temperature range is the establishment of thermal contact between the thermometer and the medium.
The standardization and accuracy of temperature measurements are provided by the kelvin, the State Standard Unit of Temperature, which makes it possible to reproduce the International Practical Temperature Scale in the range from 1.5° to 2800°K with the greatest accuracy attainable at the present time. By means of comparison with the standard, temperature values are transmitted to reference instruments, according to which the working instruments for measuring temperature are calibrated and checked. The reference instruments are resistance thermometers made of germanium (for 1.5°–13.8°K) and platinum (for 13.8°–903.9°K [630.7°C]), the 10 percent rhodium-platinum-platinum thermocouple (for 630.7°–1064.4°C), and the optical pyrometer (for temperatures above 1064.4°C).
REFERENCESPopov, M. M. Termometriia i kalorimetriia, 2nd ed. Moscow, 1954.
Melody izmereniia temperatury: Sb., parts 1–2. Moscow, 1954.
Temperatura i ee izmerenie: Sb. Moscow, 1960. (Translated from English.)
Sosnovskii, A. G., and N. I. Stoliarova. Izmerenie temperatur. Moscow, 1970.
D. N. ASTROV and D. I. SHAREVSKAIA