Thermistor Thermometer | Thermistor Temperature Sensor | Construction and Principle
Thermistor is a temperature sensitive device. If the temperature varies, then the resistance of the thermistor either increases or decreases. By using this property, we can use it as a temperature sensor. Thermistor Thermometer is a resistor type thermometers. But, it differs from RTD or Resistance Temperature Detector. In thermistors, the semiconductor materials are used, while RTD has pure metals.
The semiconductor materials are prepared from the oxides of chromium, cobalt, nickel, manganese, and sulphides of iron, aluminium or copper. Because of semiconductor, resistance of the thermistor varies significantly with temperature, more than the normal resistance. Thermistor thermometers have high sensitivity but it has nonlinear characteristics. This can be understood from the following example; for a typical 2000 Ω thermistor, the change in temperature at 25oC is 80Ω/oC, whereas for a 2000 Ω platinum RTD, the change in temperature at 25oC is 7Ω/oC.
Types of ThermistorThermistors are classified into two types. They are, Negative Temperature Co-efficient Thermistors, Positive Temperature Co-efficient Thermistors. The fig.1(a) and fig.1(b) shows their resistance-temperature characteristics curve.
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The characteristics of NTC thermistor is more common, which is shown in fig. 1(a). In NTC thermistor, the resistance decreases as the temperature increases, according to the following expression.
Where, RT is the resistance at temperature T (K) R0 is the resistance at temperature T0 (K) T0 is the reference temperature, normally 25oC β is a constant, its value is decided by the characteristics of the material, the nominal value is taken as 4000. The people are always required that the β should be higher. If the value of β is high, then the resistor – temperature relationship will be very good. Then the sensitivity of the thermistor also increases. That means for the same raise of temperature, we can get the higher variations in the resistance. This is very good property to make any instrument with greater sensitivity.
From the expression (1), we can obtain the resistance temperature co-efficient. This is nothing but the expression for sensitivity. It clearly shows that the αT has negative sign. That indicates the negative resistance-temperature characteristics of the NTC thermistor. If β = 4000 K and T = 298 K, then the αT = - 0.0045/oK. This is much higher than the sensitivity of platinum RTD. So, it helps to measure the very small difference if the temperature. However, alternative forms of heavily doped thermistors are now available (at high cost) that have a positive temperature co-efficient. The expression (1) is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistors is very definitely a non-linear sensor.