A variety of thermistor based testers are available. They include versions with a folding probe, reinforced handle for insertion into semi-solids, and ones that can mount on a refrigerator.
Accessories include the calibration keys used with thermistor thermometers and shockproof rubber boots to add additional protection to the meters. Also include are the handles and extension cables used with probes that do not have a handle.
Multiple temperature data loggers are available. These include versions with built-in and external thermistor probes.
The thermistor is a semi-conductor device whose resistivity (r) varies as a function of temperature (T):
- R = Ro [1 + a (T-To)] where,
- R = resistance of temp. at T
- T = temp at the end of measurement
- Ro = resistance of temp. at To
- To = temp at the beginning of measurement
Temperature resistance coefficient is the parameter that determines if the resistivity variation is positive (as with the Positive Temperature Coefficient, or PTC sensors) or negative (as with the Negative Temperature Coefficient, or NTC thermistors). It is possible to determine the temperature by applying a potential difference and measuring the resistance.
Thermistor sensors are suitable for a temperature range of -50 to 150oC (-58 to 302oF). Higher temperatures may damage the semi-conductor sensor. Accurate temperature measurements are possible (tenths of degree) due to the high sensitivity of the sensor.
The thermocouple consists of the junction of two wires of different metals. At a given temperature, a potential difference results at the opposite extremes of the two wires (Seebeck effect), with the respective variations linearly related within small intervals. It is therefore possible to determine the temperature given the potential difference and characteristics of the two metals. The measurement end of the thermocouple probe is called the hot junction, while the connection of the thermocouple to the meter is the cold junction. An error is introduced as the cold junction is exposed to the ambient temperature. This error can be eliminated by physically putting the cold junction into an ice bath and forcing a reference temperature of 0oC, or by electronically compensating for the cold junction temperature effect. There are various types of thermocouples, identified by an ANSI code using a letter of the alphabet. The K type is the most commonly used thermocouple.
Available are K-type, T-type, thermistor and pt100 temperature probes. These probes include many different styles from penetration, liquids, air/gas, and wire probes.