A resistance thermometer is a type of a thermometer in which the variation of electrical resistance of pure metals, alloys and semiconductors with the temperature R = f (T) is measured. Thermometers which use pure metals, and primarily platinum, find particularly wide application.
The platinum resistance thermometer is a typical example of a secondary thermometer; Figure 1 depicts the construction of sensing elements of a standard resistance thermometer for low (a) and high (b) temperatures. The sensing element of a platinum resistance thermometer consists of wire or strip wound (or sometimes printed) on a rigid carcass (made from quartz, ceramics, mica) enclosed in an envelope (made from metal quartz, glass) through which terminals run to the measuring devices. Platinum resistance thermometers are used in the range from –263 to 1000°C. Thermometers of this type work with devices reading temperatures in °C or K in correspondence with tables R(T) for such thermometers. Platinum resistance thermometers have wide application in very precise temperature measurements (for instance, in metrological studies). The International Temperature Scale ITS-90 was introduced in 1990 (see International Temperature Scale). In the temperature range from the triple point of equilibrium hydrogen (13.8033 K) to the silver point (961.78°C), the ITS-90 offers a specification for the platinum resistance thermometer. The sensing element must be manufactured from pure tension-free platinum.
A consumer receives from the standards body a platinum resistance thermometer calibrated over the ITS-90 range and a detailed table T90(R(T90)/R(213.16 K)).
Other metals are also used to manufacture resistance thermometers, for instance Copper resistance thermometers are for industrial application and work in a temperature range of –150 to 600°C. Resistance thermometers may be manufactured from manganin and constantan alloys; however, such thermometers suffer from a number of drawbacks, in particular, narrow temperature measurement intervals and low response. Resistance thermometers manufactured from the rhodium-iron alloy (Rh + 0.5% Fe) may be employed in a wide range from 0.5 to 300 K. These resistance thermometers have high, prolonged stability, sufficiently high voltage sensitivity and possess a comparatively high (compared to a platinum resistance thermometer) specific resistance at low temperatures. This allows higher, again in comparison to a platinum resistance thermometers, voltage to be used subject to the avoidance of heating effects.
Carbon thermometers are used to measure low temperatures in superconducting magnetic systems because of their high sensitivity and small dependence of R on the presence of a magnetic field. Glass-carbon and composite sintered carbon resistance thermometers are most widely used.
Semiconductor thermometers include germanium resistance thermometers used at a temperature below 20 K. They possess higher reproducibility and prolonged stability compared to carbon thermometers and are suitable for precise measurements. Usually pure monocrystalline germanium is not employed, but germanium doped with different impurities. Control of the quantity of the main and doping components allows the control of the R(T) form of the curve (most often antimony or arsenic is used as a dopant). Figure 2 shows the bridge-type circuits used (a) and the construction of miniature (b) germanium resistance thermometers. For highly precise measurements, four leads (two current and two potential) are used. In the model shown, a sensitive element is wrapped into a fluoroplastic film and placed into a metal sleeve. Mass-produced thermometers are equipped with the tables graded at a large number of points. Multicomponent doping of a resistance thermometer allows R(T) to be obtained for a large temperature range (3–90 K). Figures 3a and 3b show a comparison of R(T) and dR/dT for three thermometers: carbon (1) germanium (2) platinum (3).
Figure 2. Germanium resistance thermometry: (a) Bridge circuit used, (b) Detail of construction of miniature thermometer.
Semiconductor resistance thermometers have found wide application in industry. They include thermoresistor thermometers which, in turn, include thermistors and posistors. Thermistors are manufactured from the materials with a negative temperature resistance coefficient R/R0(T); posistors are manufactured from the materials having a positive temperature coefficient of resistance which, for the majority of thermoresistor semiconductors, is much higher than for other materials. Thermoresistors are used in the temperature range 170–750 K. They are made from oxide materials. To obtain a required R value, such stabilizing substances as nickel oxide are used. When plastic substances are used, mixtures are pressed to obtain necessary shape and sintered. Miniature bead-type resistance thermometers are available for measurement of living organisms, biological objects, in particular, plants. These resistance thermometers are designated for small temperature intervals. Mass produced resistance thermometers do not possess high stability of characteristics when operated for the first (2–5) × 103 h. At longer times, their properties change only slightly.
Posistor resistance thermometers are made from ferroelectric ceramics based on titanates, zirconates, etc., plumbum, boron. They work within a narrow temperature range from 20 to 100°C and find application in automatic control and protection systems.