Thermistors are thermally sensitive resistors. An NTC (negative temperature coefficient) will exhibit very large negative temperature coefficients (4 to 5% °C). They are stable and repeatable. They can be very small devices. The chip thermistor is usually coated in epoxy. They are usually inexpensive, especially in high volumes.
Thermistor sensors usually have defined resistances at 25°C. 2252, 5,000 and 10,000 Ohms are very common.
A PTC (positive temperature coefficient) thermistor’s resistance changes proportionally to a temperature rise. Due to the larger resistance change with temperature of an NTC device, NTC devices are usually more suitable for precision measurement although the resistance change is nonlinear vs. temperature.
A PTC device exhibits a linear change in resistance for a linear change in temperature, however the resistance change of these devices is not as large as an NTC device. The highly nonlinear temperature / resistance curve can cause engineers problems with the attached circuitry.
A power source is required to provide a current to allow a resistance reading. The power should be low to ensure self heating is negligible.
The standard temperature ranges are -55 °C to 150 °C although some devices have been shown to remain long term stable up to 300 °C.
No special leads are required to connect thermistors to instruments, and as the device is operating at very high resistance compared to the leads, only 2 wires are necessary.
Thermistor elements are joined to connection leads which are them insulated and protected. There are a wide variety of designs of sensor that can be tailored to your process.