A thermometer is an instrument to measure temperature (see Temperature and Temperature Measurement, Practice). Temperature cannot be measured directly (for instance, as a length is measured by its segment). Only the measurement of another physical property being a function of temperature may provide temperature determination. A property used to measure temperature is termed a thermometric parameter and must meet the following requirements:
be independent of the influence of other factors,
be strictly reproducible,
be a continuous and monotonic function of temperature,
be a temperature coefficient of a property which is sufficiently large and its measurement sufficiently simple.
Most often used for temperature measurement are:
a volume of a liquid or a gas (liquid-in-glass and gas thermometers);
electrical resistance of metals, semiconductors (see resistance thermometers); electromotive force (see "thermocouples");
temperature measurement by radiation intensity (optical thermocouples, see pyrometers);
melting points of metals or alloys;
softening temperatures of ceramics (Seger cones);
change of color with transition temperature—heat setting inks (heat sensitive compounds of fillers and solvents coated onto technical parts);
change of color at temperature transitions to a liquid-crystalline state and an isotropic liquid of some organic compounds—liquid crystals (e.g., liquid-crystalline substances of the cholesteric type).
Each indicated property may be used as a thermometric parameter in a limited temperature range. More seldom used are such thermometric parameters as intensity of electric fluctuations (Johnson noise thermometer), magnetic susceptibility of a paramagnetic (magnetic T), sound velocity, broadening of spectral lines, etc.
Gas thermometry reduces temperature measurement (from helium temperatures to 1063°C) to measurement of pressure or a gas volume in a closed vessel (under certain conditions) followed by temperature calculation using the measurement results and the ideal gas laws. A gas thermometer is a primary instrument for determination of thermodynamic temperature. Application of exact relations requires design of complicated devices inconvenient for practical use. In practice, temperature scales are used in which a simple and convenient secondary thermometer is used and methods of transfer of thermodynamic temperature from a primary instrument to the secondary thermometer are employed (see International Temperature Scale). This requires use of precise primary instruments reproducing thermodynamic temperature, instruments for realization of the temperatures of phase equilibria of substances (for determination of the constants of the primary instruments), i.e., representing the so-called fixed points and, of course, the secondary thermometer itself together with simple and convenient methods for its calibration. The simplest thermometer is a gas thermometer which consists of a glass or metallic gas-impermeable reservoir connected with an arrangement intended for pressure measurement in the reservoir.
A schematic drawing of a gas thermometer is shown in Figure 1: reservoir 3 is immersed into a medium whose temperature is to be measured; gauge 1 is connected via capillary 2 to the reservoir; the reservoir and the capillary are filled with a working gas. A gas thermometer allows the determination of pressure p and volume V of mass m of the ideal gas with molecular weight μ converting from thermodynamic state 1 to state 2, with the gas mass m = Vpμ/TR remaining constant in both states. Depending on the character of gas transition from 1-to-2 state, three gas thermometers are distinguished: those of constant volume, constant pressure and constant temperature. A constant-volume gas thermometer is used at low temperatures (typically with helium as a working substance) and possesses the highest sensitivity. At high temperatures, when gas desorption on reservoir walls becomes pronounced and helium penetrates through the walls, gas thermometers of other design are used with nitrogen as a working substance. For precise temperature determination, corrections are made for gas nonideality, thermal expansion of the reservoir, a "harmful" volume and thermomolecular pressure as well as for hydrostatic aspect. Since the reservoir of a gas thermometer is connected with a manometer via a capillary, then there is the "harmful" gas volume of a gas above the manometer mercury and inside the capillary, whose temperature varies from the value to be measured to room temperature. With change of the bulb temperature, the amount of a gas contained in "the harmful volume" changes. A difference of the gas temperature in the bulb and in "the harmful volume" requires the appropriate corrections. Such corrections may only be determined rather approximately, therefore for low temperatures use is made of a thermometer without a "harmful volume". Such thermometer is used in metrological works. Bulb 3 (Figure 1) is partitioned with an elastic membrane 4. Smooth change of the pressure in the upper part of 3 allows 4 to be maintained in an equilibrium condition and thus pressure measured in the lower part of closed volume of 3. For technical measurements, use is made of filled-system gas thermometers working at temperatures from –150 to 600°C. At a temperature up to 600°C nitrogen is used as a working gas, while above 600°C argon is used. The scale of a filled-system gas thermometer (T = f(p)) is obtained using a knowledge of a volume of the instrument components. For this, corrections are made for the gas nonideal state, thermal expansion of the bulb and capillary, temperature variation, etc. This thermometer is inertial; it fails to measure rapid processes. A schematic drawing of its construction is given in Figure 2.
The liquid-in-glass thermometer is intended for temperature measurement using thermal expansion of a liquid as a suitable property. It is employed in technology, laboratory practice and medicine to measure temperatures from –200 to 750°C. A liquid-in-glass thermometer consists of a glass (sometimes quartz) bulb with a glass capillary joined to it. A thermometer liquid occupies the whole bulb and partly the capillary. Depending on the measured temperature range, it may be filled with kerosene (from –20 to 300°C), mercury (from –35 to 750°C), pentane (from –200 to 20°C), ethyl alcohol (from –80 to 70°C), etc. A liquid-in-glass thermometer is subjected to special heat treatment ("aging") eliminating the zero drift on its scale caused by repeated heating and cooling. For precise measurements, corrections should be made for the zero drift. The accuracy of measurement depends on the depth of immersion into a medium to be measured; immersion should be continued to correspond to a specific scale division or to a specially made line on the scale. If this is impossible, correction needs to be made for the protruding column, dependent on the measured temperature, the column temperature and its height. The main drawbacks of this type of instrument are its inevitable thermal lag and its dimensions which are often inconvenient for application. Amongst such types, mercury-in-glass thermometers are most often used. Mercury is easily purified, it does not wet glass and vapor pressure in the capillary above mercury is small. Liquid-in-glass thermometers are unsuitable for automatic recording of results and often replaced by resistance thermometers or other instruments. Special constructions are metrological, clinical thermometers and others. Clinical mercury thermometers are graduated from 34 to 42°C with a division of 0.1°C and are maximum thermometers, i.e., the capillary column remains on the level of a maximum rise on heating and falls down when shaken.
In liquid-in-steel thermometers, the system is filled with liquid and changes in temperature of the liquid in the sensing bulb cause changes in pressure which are measured by pressure gauges in which a spiral, a sylphon membrane, etc. are used as sensitive elements. The construction may be identical to that shown in Figure 2 and the device provides automatic recording of temperature. A working temperature range covers from –150 to 330°C. As working substances, use is made of propyl alcohol, methaxylol, xylol, mercury, etc.
Temperature dependence of the pressure of saturated liquid vapors is also used as a thermometric parameter. The sensitive element is a reservoir partially filled with a liquid or a solid phase being in thermodynamic equilibrium with its vapor. Such a device is used to measure low temperatures. A thermometer scale T = f(P) is determined by the vapor pressure vs. temperature curve. For a vapor-pressure thermometer to work, it is necessary for the thermometer reservoir to be at the lowest temperature with respect to the temperature of its other parts. The accuracy of measurement depends on the gas purity, superheating in the volume of a liquid occupying the bath in which temperature is measured, and on hydrostatic pressure measurement. Table 1 lists the sensitivity of vapor-pressure thermometers using different working substances.
If pressure is measured by a spring-element pressure gauge, then the measured temperature ranges may be extended from the boiling point to the critical temperature of working substance. The accuracy of measurement is worse and is determined by the pressure gauge precision. Its construction is identical to that depicted in Fig. 2. A measurement range comprises temperatures from –30 to 300°C. For filling the system, use is made of the substances indicated in table and also of Freon-22, propylene, methyl chloride, acetone, ethyl benzene, ethyl alcohol, etc. The scale of such thermometers is graduated in degrees of temperature; it is nonlinear, if reservoir dimensions are small.