Industrial measurements usually exploit resistance sensors, resistance thermometer detectors (RTDs), or thermocouples. Resistance sensors are fabricated of pure metals (platinum, nickel, and copper), of carbon, germanium, silicon, or other semiconductor materials. Metals used for temperature sensors are
characterized by large temperature coefficients of resistance, with high melting temperature, and steadiness of the thermometric characteristic. In accurate temperature measurements, platinum sensors are used, usually the Pt100 type; their nominal resistance at a temperature of 0°C amounts to 100Ω. Platinum sensors are manufactured of the sunken platinum wire in the cover of ceramic material in the rod shape. Geo- metrical sizes of Pt100 sensors are different: they are 25 to 150 mm in length, and from 2.5 to 8 mm in diameter (for the rod shaped sensors) [1, 2]. One respected manufacturer of sensors is the KFAP company in Cracow, Poland. Pt100 sensors in the form of elastic platinum foil, which is 5 mm thick, are also manufactured by JP Technologies. The resistance values of the Pt100 sensor in the function of temperature within the range of the sensor application −200°C to 850°C is defined by the international standard IEC 751 (International Electrotechnical Commission), as shown in Table 2.2. In Figure 2.1, the thermometric characteristics of the platinum sensor Pt100, nickel Ni100, and copper Cu100 are shown in the full temperature range of Cu100 (i.e., −50°C to +175°C), in accordance with the standard.
In a narrow temperature range (tens of degrees Celsius) the thermometric characteristic RT (T) of the Pt sensor is a nearly linear function. However, in a wider temperature range, the non linearity error might be considerable. The re- placement of the real thermometric characteristic with a linear function in an ex- emplary range of temperature 0°C to 300°C is related with the non linearity error equal to 3.5°C, and in the interval 0°C to 600°C, with the error of 15°C. For industrial measurements, sufficient accuracy of the reproduction of the real thermometric characteristic of the Pt sensor is assured.
For industrial measurements, almost exclusively platinum sensors are used, although standards were also established for sensors made of nickel Ni100 and copper Cu100, as shown in Figure 2.1. This situation is a result of the great advantage of platinum over nickel and copper. Platinum is a metal characterized by low chemical activity and a high melting temperature. The low chemical activity of platinum assures long usage of the platinum sensor without significant alterations of its parameters, even under hard operation conditions at high temperatures, which contribute strongly to the metal oxidation. The high melting temperature of platinum assures higher acceptable operation temperature of the platinum sensor, in comparison with the nickel or copper sensor.
Temperature sensors are placed in industrial sheaths mounted on the measuring object, as shown in Figure 2.2. Good linearity of the thermometric characteristic makes the cooperation of the platinum sensor with ADCs easier.
Taking into account the more expensive material of the platinum sensor, and the still higher costs of replacement and calibration of the sensor in the case of its consumption (oxidation) or overheating, the majority of constructors choose platinum sensors, even in the situation when the measuring range makes it possible to apply cheaper sensors (e.g., nickel or copper ones).
In industrial systems, resistance sensors for the measuring range ∆T = TH − TL (TH—upper limit, TL—lower limit), but not wider than ∆T = 300°C, are usually applied. In systems with wider measuring range, thermocouples are used much more often. A typical measuring circuit for metallic resistance sensors is the Wheatstone bridge, as shown in Figure 2.3, in which a resistor R(T) (e.g., of the Pt100 type) is the temperature sensor.
For a given temperature T0, the bridge is in the balance state when the following condition for the bridge resistances is fulfilled: R1/R(T0) = R2/R3. For the balance state of the bridge, the output voltage Vout equals 0. With an alteration of the sensor temperature by ∆T = T - T0, voltage Vout, proportional to temperature change ∆T (proportional within a nonlinearity error), appears at the bridge output. Vout corresponds to the sensor resistance alteration ∆R = R(T) − R(T0).
