- A thermistor is a temperature-sensitive resistor whose resistance changes with temperature.
- Most temperature-sensitive resistors are constructed from a semiconductor material (carefully chosen metal oxides) and the resistance increases with a fall in temperature (they have a negative temperature coefficient)
- So they are known as negative thermal conductivity (NTC) thermistors.
- A Wheatstone bridge circuit is used to measure the resistance accurately.
- The main disadvantage of thermistors is the non-linear resistance versus temperature characteristic, although this can be compensated for using an appropriate calibration equation programmed into an electronic measurement system.
- Thermistors remain highly popular due to their cost, miniature size and convenience.
- Thermistor probes are commonly placed in the nasopharynx, oesophagus, rectum or bladder (integrated with a urinary catheter).
- They have excellent accuracy and their small mass means that there is a quick response to variations in temperature.
- But they ‘age’ and their resistance changes with time. They also exhibit hysteresis.
- True or False? ‘A thermistor comprises a junction of dissimilar metals’
- Answer: False. Dissimilar junctional metals are thermocouples
- True or false: ‘A thermistor demonstrates the Seebeck effect’
- Answer: False. The Seebeck effect applies to themocouple
The Lay Medical Man 