Image Credit: CCI Thermal Technologies, Inc. | Fotronic Corporation / Test Equipment Depot | Campbell Scientific, Inc.

Thermistor temperature probes sense temperature by using thermistors, devices made of semiconductor materials which exhibit a large change in resistance for a small change in temperature.

Probe Configurations

There are many different probe configurations. Most suppliers specify a thermistor temperature probe by application or intended use. Custom products are also available.

• Air probes are thermistor temperature probes for air-flow temperature measurements.
• Angled probes can be used inside pipes or angled spaces.
• Clamp probes or strap sensor probes clamp or wrap around the surface to be monitored. 
• Curved probes are used to measure temperatures on pipes or other curved surfaces.
• Drop probes are designed for well or underwater applications.
• Flat probes or ribbon probes have a flexible membrane.
• Flexible probes can be manipulated for various temperature measurements.
• Penetration probes or needle probes are used to measure internal temperatures.
• Roller probes or moving probes are used to measure temperatures on moving surfaces.
• Straight probes can be used in wells and designed to measure stationary surface temperatures.

Specifications

Specifications for thermistor temperature probes include sensed temperature, sensitive length, and diameter or width range. Sensed temperature is the temperature range for which the device is rated for sensing. Depending on the configuration, sensitive length is the height or length of the temperature-sensitive part of the probe. Suppliers specify diameter or width range as either the diameter or width of the probe, or the diameter of the object to monitor. Process connection is also an important parameter to consider when selecting thermistor temperature probes. Bayonet fittings consist of a socket with J-shaped slots on opposite sides. A bayonet base or coupling is inserted against a spring and rotated until its pins are firmly seated. Compression fittings have a tapered sleeve and are used mainly with tube fittings. Thermistor temperature probes that do not include a process connection are also available. Typically, these devices are hand held probes. 

Features and Options

Additional specifications for thermistor temperature probes include sensor termination type, features, and options. Sensor termination type is an important parameter when trying to match a probe with an instrument, transmitter or other signal conditioning equipment. Choices include: bare or insulated lead wires, metal braided leads, armor leads, flexible conduit leads, terminal blocks, conduit connectors, screw tabs or lugs, integral connection heads or thermoheads, and plugs or quick-connects. In terms of features, thermistor temperature probes may be available as an assembly, cuttable, spring-loaded, or tip-sensitive. Some products are explosion-proof or watertight. Others are rated for sanitary processing or heating, ventilation and air conditioning (HVAC) applications. Many thermistor temperature probes are configured with an analog current output, typically 4 – 20 mA.