As a general rule, a thermistor acts as a single-purpose sensing device. The designer must relate temperature ranges, resistance/temperature characteristics, and output circuits for each application. The reward: high resolution and accuracy from a relatively inexpensive system. Thermistors offer the greatest benefit when:

• The application requires high resolution over a narrow span

• Probes are disposable, or require frequent and easy recalibration

• Low cost is a primary consideration

• The application is an OEM device produced in sufficient volume to justify the design of special linearizing circuits

• Point sensing or miniaturization is preferred for the application

The superior sensitivity and stability of resistance thermometers and thermistors, in comparison to thermocouples, gives them important advantages in low and intermediate temperature ranges. In addition, resistive devices often simplify control and readout electronics.

Resistance thermometers are specified primarily for accuracy and stability from cryogenic levels to the melting points of metals. They are accurate over a wide temperature range, may be used to sense temperature over a large area, and are highly standardized.

While normally less stable than resistance thermometers, thermistors offer lower cost and higher sensitivity over limited ranges. They are also relatively small and suitable for equipment applications where quantities of use justify the design of special readout circuits.

Finally, thermistor designs abound, so can be chosen to resist electrical noise, vibration, self-heating, leadwire resistance, and out-of-range temperature extremes.
Information courtesy of Minco, www.minco.com

A few FAQs about temperature sensing

Q: How can I choose between thermocouples, resistance temperature detectors (RTDs), thermistors, and infrared devices when measuring temperature?

A: Consider the characteristics and costs of the various sensors, as well as the available instrumentation. In addition: Thermocouples generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but not as accurate or stable as RTDs and thermistors. Conversely, RTDs are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Because they use electric current to make measurements, RTDs are subject to inaccuracies from self-heating.

Thermistors tend to be more accurate than RTDs or thermocouples, but they have a much more limited temperature range. They are also subject to self heating.

Infrared sensors can be used to measure temperatures higher than any other devices and do so without direct contact with the surfaces being measured. Using fiber optic cables, they can measure surfaces that are not within a direct line of sight. However, they are generally not as accurate, and are sensitive to surface radiation efficiency — more precisely, surface emissivity.

Q: What are the two most often overlooked considerations in selecting an infrared temperature-measuring device?

A: The surface being measured must fill the field of view, and the surface emissivity must be considered.