 emperature is the most frequently measured process variable, and any material with a temperature sensitive characteristic can be used as a ?thermometer.? Electrical temperature sensing devices can be used for remote temperature measuring and signaling. We will compare the thermocouple, thermistor and RTD methods.
A thermocouple is a measuring device made by joining materials with different Seebeck coefficients. Two wires from dissimilar metals are joined at one end. The emf is generated when a temperature gradient exists between this wire junction and a reference junction. This measurable change of electric potential is the basis of the thermocouple method. The most common materials used in thermocouples are iron-constantan, copper-constantan, chromel-constantan, and two different platinum alloys. Table 1 shows various thermocouple types and materials.
Thermocouple junctions come in three basic forms: (1) exposed, (2) grounded and (3) ungrounded. Exposed junction was designed for faster response. Insulation is sealed beyond the exposed junction tip to prevent penetration of moisture or gas to the inner thermocouple. Grounded junction is used for high-pressure gas and liquid applications. The junction is electrically-joined to the protective sheath. This provides faster response. Ungrounded junction is used for measuring in corrosive conditions. The welded tip of the wire is physically insulated from the jacket/sheath. Thermocouples can be portable or permanent.
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A thermistor is a ?thermally sensitive resistor.? This is a semiconductor composed of metallic oxides such as manganese, nickel, cobalt, copper, iron, and titanium. Basic ceramics technology is utilized to fabricate thermistors in wafer, disk, bead, and other shapes. There are two basic types of thermistors — negative temperature coefficient (NTC) and positive temperature coefficient (PTC). NTC thermistors are much more commonly used than PTC thermistors. The resistance of NTC thermistors decreases with increasing temperature.
Thermistor applications are based on the resistance-temperature characteristic of a thermistor. NTC thermistors give a relatively large output (change of resistance) for a small temperature change. This output can be transmitted over a large distance. No compensation for ambient temperature is needed. The amount of change per °C is expressed by Beta value (material constant) or Alpha coefficient (resistance temperature coefficient). The larger Alpha or Beta the greater the change in resistance with temperature, and the temperature versus resistance curve is steeper.
The resistance versus temperature relationship is not linear. With increasing temperature the nonlinearity decreases. The Stainhart-Hart Equation expresses the relationship between resistance and temperature:
1/T = a + b + (lnR) + c(lnR)3
where T is temperature, R is resistance and a, b, c are coefficients derived from measurements. Thermistors are calibrated at three different temperatures — usually at 0°, 25°, and 70°C. This gives three different values of resistance. 
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