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Temperature transducers!

This article was originally written in the period 1995-2000

In the majority of industrial and laboratory processes, the measurement point is usually remote from the indicating or controlling instrument. This may be due to necessity (as in the case of an adverse environment) or convenience (as with centralised data acquisition). Devices are required which convert temperature into another form of signal, usually electrical.

Thermocouples, resistance thermometers and thermnistors are in effect electrical temperature transducers and not direct- indicating thermometers, such as mercury-in-glass devices. Alternative indicating techniques for sensing and measuring temperature include optical pyrometry and other non-contact (infra red), fibre-optic and quartz oscillation systems.

The use of thermocouples, resistance thermometers and thermistors requires some form of physical contact with the medium. Such contact can be immersion or surface, depending on the sensor construction and the application.

Thermocouples Essentially comprise a thermoelement (a junction of two specified dissimilar metals) and an appropriate two-wire extension lead. A thermocouple operates on the basis of the junction located in the process producing a small voltage which increases with temperature. It does so on a reasonably stable and repeatable basis.

The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions. If the same temperature exists at the two junctions, the voltages produced at each junction cancel each other out and no current flows in the circuit. With different temperatures at each junction, different voltages are produced and current will flow in the circuit. A thermocouple can therefore only measure temperature differences between the junctions.

Thermoelectric voltages are very small and at best attain a few tens of microvolts per Cdeg. In consequence, practical thermocouples are mainly used at elevated temperatures, above, say 100degC and at depressed temperatures (below 50degC). However, with appropriate measuring instruments, they can be used at any value within their operational range.

In some applications, such as in liquid gas or a heated enclosure, the reference junction may be held at some temperature other than 0degC. In any event, the measured output will correspond to the difference in temperature between the two junctions.

The electricity conductivity of a metal depends on the movement of electrons though its crystal lattice. Due to thermal excitation, the electrical resistance of a conductor varies according to its temperature and this forms the basic principle of resistance thermometry. This effect is most commonly exhibited as an increase in resistance with increasing temperature. Such variations are very stable and precisely repeatable.

When utilising this effect for temperature measurement, a large value of temperature coefficient (the rate of change of resistance with temperature) is ideal. However, stability of the characteristic over the short and long term is also vital in practical situations. The materials most commonly utilised for resistance thermometers are platinum, copper and nickel. Platinum is the most dominant.

Platinum sensing resistors are available with alternative R0 values (for example, 10, 25 and 100 ohms. A working form of resistance thermometer is defined in IEC and DIN specifications and this forms the basis of most industrial and laboratory electrical thermometers. The platinum sensing resistor, Pt 100 to IEC 751 is dominant in Europe and in many other parts of the world. Its advantages include: Chemical stability Relative ease of manufacture The availability of wire in a highly pure form Excellent reproducibility of its electrical characteristic.

The result is a truly interchangeable sensing resistor which is widely available commercially at a reasonable cost.

Thermistors are an alternative group of temperature sensors which display a large value of temperature coefficient of resistance (usually negative, sometimes positive). They provide high sensitivity over a limited range.

The NTC thermistor is an alternative to the platinum resistance thermometer. The name derives from “thermal resistor” and defines a metallic oxide which displays a high negative temperature coefficient of resistance. This compares with the small positive coefficient of, say, platinum used for the Pt 100 sensor. The temperature resistance characteristic of the thermistor is up to 100 times greater than that of the alternative resistance thermometer and provides high sensitivity over a limited temperature range.

PTC (positive temperature coefficient) versions are also available, but their use is much less common.

High resistance thermistors (over 100kohms) are used for high temperatures (150-300degC) and devices up to 100kohms are used for the range 75-150degC. Devices below 1kohms are suitable for lower temperatures (-75 to +75degC).

Thermistors provide a low cost alternative to the Pt 100, although the temperature range is limited. Interchangeability and accuracy place then between the Pt 100 and thermocouples. Since their resistance value is extremely high, a some two-wire connection is used.

The electrical resistance of an NTC thermistor decreases non- linearly with increasing temperature. The amount of change per degree Celsius (C) is defined by either the Beta Value (material constant) or the Alpha Constant (resistance temperature coefficient).

Material resistivity is an important consideration when choosing the proper thermistor for an application. The material must be chosen such that a thermistor chip of a specified resistance value will not be too large or too small for the circumstances. Thermistor materials are available with a variety of resistivity values. The resistance of an NTC thermistor is determined by material resistivity and physical dimensions. Required resistance is usually specified at 25degC.

Added on 11 February 2009 at 3:45 pm | Read more sensors articles