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In Figure the normalized resistance-temperature ratio characteristic is shown for several different NTC thermistors in comparison to that of a commercial resistance-temperature detector (RTD).

A comparison of the curves will immediately show inherent advantages and disadvantages for both types of devices. The ten-fold increase in sensitivity exhibited by NTC thermistor makes it advantageous to use such devices for low cost, precision temperature measurement and control.

High sensitivity means low noise and simpler circuits compared to RTDs and thermocouples.

Another major advantage offered by thermistors is the availability of a wide range of relatively high resistance values. By using high resistance thermistors, the effects of sensor lead resistance can be minimized. The non-linearity of the thermistor resistance-temperature characteristics puts a practical limit on the temperature span over which a single thermistor can be operated in a measurement or control circuit using only analog circuits. Digital microprocessor based circuits do overcome linearity problems.

RTD’s have lower sensitivity, are more linear and can therefore be used in application where the temperature spans are very wide. Thermistors have other important advantages over RTD’s in that they are available in smaller sizes, with faster response times, at lower costs and with greater resistance to shock and vibration effects.

NTC thermistors compare very well to thermocouples over the limited temperature ranges where both sensors can be effectively used for temperature measurement and control. Of course, thermocouples will operate at much higher temperatures and over much wider spans and are available in very fine wire diameters. However, thermocouples have some notable disadvantages. First the thermal EMF values produced by thermocouples (thermo-elements) are on the order of a few microvolts per degree. Second, the electronic circuits used for thermocouple measurement and control applications must provide high gain, low noise amplification of the signal and provide compensation for the cold junction temperature.

Third, the stability and accuracy of base metal thermocouples can be degraded by environmental factors and non-homogeneities. As such, NTC thermistors provide greater sensitivity, stability and accuracy than thermocouples and can be used with less complex, less costly instrumentation.

NTC thermistors also have advantages over the solid state sensors that are finding widespread use in direct digital temperature measurement and control applications.

The solid state devices produce an output signal that is proportional to temperature over operating ranges that fall within the overall range of -55° to +150°C. The solid state devices can be incorporated into application specific integrated circuits for direct readout of temperatures. They exhibit accuracy and linearity specifications in the range of ±0.3°C (selected) up to ±4°C over their published ranges. Packaging of the devices can take any of the standard outline dimensions for solid state devices.

The NTC thermistors, by comparison, offer better sensitivity and accuracy over the operating temperature ranges, smaller sizes with faster response times and can be obtained in a wider assortment of device packages or sensor housings. Glass encapsulated NTC thermistors will also perform in much higher operating and storage temperatures than the solid state devices.