When rapid temperature changes need to be recorded, the thermistor is usually preferred as superior to ordinary thermometers or thermocouples. The thermistor comes in two varieties and finds ready usage in digital thermometers, fast response sensors and many scientific instruments that require accurate and fast measurement of temperature. The automobile industry is a heavy user of thermistors to monitor engine temperature.
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Types of Thermistors
Thermistors are special resistors that respond rapidly to small changes in temperature. They are classified as Negative Temperature Coefficient, or NTC, and Positive Temperature Coefficient, or PTC. NTC thermistors are semiconductors made from the oxides of transition metals like cobalt, copper, manganese and nickel. NTC thermistors have a wide range of operation from -200 degrees Celsius to about 1,000 degrees Celsius. PTC thermistors, usually made from barium titanate, should be used where high accuracy is required at a very narrow temperature range.
Zero Power Resistance
Zero power resistance is a measure of the resistance of the thermistors when it has not heated up. It is a useful measure where low power usage is involved in such cases of gas flow measurement and liquid level sensing.
The rate at which the zero power resistance changes when the temperature changes by 1 degree Celsius is known as the resistance-temperature coefficient (k). It is regarded as one of the most significant characteristics of thermistors. On NTC thermistors, the resistance decreases with increasing temperature, which means K is negative. For PTC thermistors, K is positive.
Thermistors, like all resistors, dissipate power.The power is usually measured in units of watts. The temperature-wattage characteristic of a thermistor describes the relation of power dissipation to variation in temperature within acceptable stability. A thermistor is stable if its specified characteristics do not vary appreciably.
Other Characteristics of Thermistors
Other characteristics of thermistors include the current-time characteristic, the dissipation constant, maximum operating temperature and maximum power rating. The current-time characteristic describes the relationship between current and time at a specific temperature. The dissipation constant measures the ratio of power dissipation over temperature change. The maximum operating temperature tells the highest temperature the thermistor can reliably measure and the maximum power rating gives the power dissipation "for an extended period of time with acceptable stability of its characteristics," as noted on the Thermometrics website.
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