Thermocouple, Thermistor or RTD? – Part 1
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Temperature is probably the most common parameter measured by our data loggers. Some devices like the TandD TR71A come bundled with general-purpose temperature sensors while other systems such as the dataTaker DT80 Universal Input data logger require the user to provide sensors. Now, there are quite a number of different kinds of devices that can provide an electrical output proportional to a temperature such as a thermocouple, resistance temperature detector (RTD), thermistors, and so on.

Basic Sensor Principles

In typical everyday applications, the most common sensor types are thermocouples, RTD's, and thermistors. In evaluating which to use, it is useful to understand how these devices convert temperature into an electrical signal that can be measured by a data logger. Of all of the different types of temperature sensors, thermocouples are the most common. On the surface, the thermocouple is a very simple device; it consists of wires made from 2 different metal alloy wires that are twisted, welded, or fused together creating what is known as the junction.

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There are 2 very important practical considerations of using thermocouples:
  1. The voltage that is created is very small, typically in the range of -.01 to +.06 volts
  2. The voltage is proportional to the difference in temperature between the junction and the ends of wires which requires a second independent temperature measurement at the point where the wires connect to the instrument called the reference junction temperature to derive the actual temperature at the junction.
RTDs operate on an entirely different principle, the change in resistance of a conductor (wire) with the change in temperature. For different types of metals, such as platinum, copper, or nickel, this change called the temperature coefficient of resistance is very consistent. Measurement of temperature with an RTD requires knowing the resistance of the RTD at 0°C, typically 100 ohms, measuring the resistance of the RTD, and then calculating its temperature by knowing the temperature coefficient.

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First, they have a more limited temperature measurement range than thermocouples. Second, since the resistance is relatively low, typically 100 ohms, the resistance of the lead wires between the sensor and the measuring device can have a significant impact on the accuracy of the measurement. It's for this reason that RTDs are often configured to use a 3 or 4-wire connection to improve the measurement accuracy.

The third common type of temperature sensor is thermistors which are a specialized type of electrical resistor, often a semiconducting material that has a non-linear relationship between temperature and resistance.

At CAS DataLoggers, we work every day with callers to provide them with the ideal solution for their businesses. Give us a call at 800-956-4437 or visit us at https://www.dataloggerinc.com.

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Source: CAS Datalogger
Filed Under: Industrial

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