Thermocouples essentially comprises of a thermo element (which is a junction of two 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.
1.3(b) RESISTANCE THERMOMETER :
Resistance thermometer utilizes a precision resistor, the resistance (Ohms) value of which increases with temperature. RTD has had positive temperature coefficient. Such variations are very stable and precisely repeatable.
1.3(c) THERMISTORS :
Thermistor is a semiconductor used as a temperature sensor. It is manufactured from a mixture of metal oxides pressed into a bead, wafer or other shape. The bead is heated under the pressure at high temperatures and then encapsulated with epoxy or glass. Beads can be very small, less than 1 mm in some cases.
The result of all these is a temperature sensing devices that displays a very distinct non linear resistance versus temperature relationship. The resistance of thermistor decreases with increase in the temperature; this is called as negative temperature coefficient of thermistor.
Thermistor exhibits a very large resistance changes for a small temperature change. This can be as large as 3 to 5% per °C. This makes them very sensitive to small temperature changes. They can detect temperature change as low as 0.1 °C or smaller. A thermistor element is significant smaller in size compared to RTDs. The sensitivity of thermistors to temperature change and their small size make it ideal for use in medical equipment.