Difference between RTD, Thermocouple or Thermistor
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The most notable difference between a thermocouple and an RTD (Resistance Temperature Detector) is the principle of operation.
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A thermocouple operates on the principle that two dissimilar metals joined together will produce a voltage related to a temperature difference. An RTD operates on the principle that electrical resistance of certain metals changes in a predictable way depending on the rise or fall in temperature.
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Advantages of the thermocouple include a wide temperature measuring range (depending on the thermocouple type the range can be as much as from -300°F to 2300°F), fast response time (under a second in some cases), low initial cost, and durability. Overall, thermocouples are able to withstand rugged applications.
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Advantages for RTDs include stable output over a long period of time, ease of recalibration, and accurate readings over narrow temperature spans. Disadvantages, when compared to the thermocouples, are: smaller overall temperature range (-330°F to 930°F), higher initial cost and they are more fragile in rugged, industrial environments.
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Thermocouples are inexpensive, rugged, and have a fast response time but are less accurate and the least stable and sensitive. Thermocouples also read only relative temperature difference between the tip and the leads while RTD’s and Thermistors read absolute temperature. RTD’s are the best choice for repeatability, and are the most stable and accurate. However they have a slow response time and because they require a current source they do have a low amount of self heating. Thermistors have a fast output and are relatively inexpensive but are fragile and have a limited range. They also require a current source and do experience more self heating than an RTD and are nonlinear.