Temperature sensors are used in diverse applications such as food processing, HVAC environmental control, medical devices, chemical handling and automotive under the hood monitoring (e.g., coolant, air intake, cylinder head temperatures, etc.). Temperature sensors tend to measure heat to ensure that a process is either; staying within a certain range, providing safe use of that application, or meeting a mandatory condition when dealing with extreme heat, hazards, or inaccessible measuring points.
There are two main flavors: contact and noncontact temperature sensors. Contact sensors include thermocouples and thermistors that touch the object they are to measure, and noncontact sensors measure the thermal radiation a heat source releases to determine its temperature. The latter group measures temperature from a distance and often are used in hazardous environments.
Thermistors, like thermocouples, are also inexpensive, readily available, easy to use, and adaptable temperature sensors. They are used, however, to take simple temperature measurements rather than for high temperature applications. They are made of semiconductor material with a resistivity that is especially sensitive to temperature. The resistance of a thermistor decreases with increasing temperature so that when temperature changes, the resistance change is predictable. They are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that (1) the material used for RTDs is pure metal and (2) the temperature response of the two is different. Thermistors can be classified into two types; depending on the sign of k (this function refers to the Steinhart-Hart Thermistor Equation to convert thermistor resistance totemperature in degrees Kelvin). If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.
As an example of NTC thermistors, we will examine the GE Type MA series thermistor assemblies designed for intermittent or continual patient temperature monitoring. This application demands repeatability and fast response, especially when used with the care of infants and during general anesthesia.