Principle: The principle of temperature measurement with a thermistor is that its resistance changes with temperature. Most thermistors differ from normal resistors in that they have a negative coefficient of resistance, this means that the resistance decreases with an increase in temperature. Negative (NTC) thermistors are the more common although positive (PTC) are also available.
Construction, selection and sizing:A thermistor is a bulk semiconductor device, and as such can be fabricated in many forms. The more common include discs, beads and rods. Size does vary from a bead of 1mm to a disc of several centimetres in diameter and thickness. There are different types of the thermistor, most of them are differ their response to temperature changes.Thermistors are not linear, and their response curves vary for the different types. Some thermistors have a near linear temperature resistance relationship, others are available with a sharp change in slope (sensitivity) at a particular characteristic temperature. Thermistors have a much higher temperature coefficient than RTD’s, so a small temperature change is easier to detect. However, thermistors do not have the accuracy of RTD’s and this probably accounts for thermistors being limited in process instrumentation. They are available in a large range of resistivities, with varying linearity. Thermistors are available that perform temperature measurement from -73 to 316°C (-100 to 600°F). It should be noted that many have limited ranges and cannot be used above 120°C (250°F). Some thermistors have a large change in resistance to a change in temperature. Selecting these types makes for very good narrow span measurement, the temperatureversuss resistance characteristics is shown below.
- Small size
- Very high sensitivity (Select range)
- Polarity insensitive
- No cold junction compensation
- Wide selection of sensors
- Fast response
- Not easily interchangeable
- High resistance, noise problems
- Non linear
- Unstable due to drift and decalibration (especially at high temperatures)
- Narrow span