Both thermistors and resistance temperature detectors (RTDs) are types of resistors with resistance values that vary predictably with changes in their temperature. Most RTDs consist of an element made of a pure metal (platinum is most commonly used) and protected within a probe or sheath or embedded into a ceramic substrate.
Thermistors are made up of composite materials, usually metal oxides such as manganese, nickel or copper, along with binding agents and stabilizers.
In recent years, thermistors have become increasingly popular due to improvements in meters and controllers. Today’s meters are flexible enough to allow users to set up a broad range of thermistors, and to interchange the probes easily.
However, unlike RTDs which offer established standards, thermistors curves vary depending on the manufacturer. A thermistor’s system electronics need to match the curve of sensor.
Whereas in RTDs there is a positive correlation between resistance and temperature (as temperature increases, resistance increases as well), in negative temperature coefficient (NTC) thermistors, the inverse relationship holds (resistance decreases as temperature increases). The relationship between temperature and resistance is linear for RTDS, but for NTC thermistors, it is exponential and can be plotted along a curve.
Both RTDs and NTC thermistors require a current or excitation source, and both are suitable for use in applications that require:
- good long-term stability
- immunity to electrical noise in the environment
If your application involves temperatures above 130°C, your only option is the RTD probe.
Cost: Thermistors are quite inexpensive compared with RTDs. If your application temperature matches the available range, thermistors are probably the best option.
However, thermistors with extended temperature range and/or interchangeability features are often more expensive than RTDs.
Sensitivity: Both thermistors and RTD react to temperature changes with predictable changes in resistance. However, thermistors change resistance by tens of ohm per degree, compared to a smaller number of ohms for RTD sensors. With the appropriate meter, the user can therefore obtain more accurate readings.
Thermistor response times are also superior to RTDs, detecting changes in temperature much faster. The sensing area of a thermistor can be as small as a pin head, delivering quicker feedback.
Accuracy: Although the best RTDs have similar accuracies to thermistors, RTDs add resistance to the system. Using long cables can alter readings outside of acceptable error levels.
The larger the thermistor, the higher the resistance value of the sensor. If you are dealing with long distances and there is no option to add a transmitter, a thermistor is the better solution.
|Temperature Range (typical)||-100 to 325°C||-200 to 650°C|
|Accuracy (typical)||0.05 to 1.5°C||0.1 to 1°C|
|Long-term stability @ 100°C||0.2°C/year||0.05°C/year|
|Power required||Constant voltage or current||Constant voltage or current|
|Response time||Fast 0.12 to 10s||Generally slow 1 to 50s|
|Susceptibility to electrical noise||Rarely susceptible high resistance only||Rarely susceptible|
|Cost||Low to moderate||High|
Conclusion:The main difference between thermistors and RTDs is the temperature range. If your application involves temperatures above 130°C, the RTD is your only option.
Below that temperature, thermistors are often preferred when accuracy is important. RTDs, on the other hand, are chosen when tolerance (i.e. resistance) is important. In short: thermistors are better for precision measurement and RTDs for temperature compensation.