Thermocouples or RTDs

Many people often ask about the difference between thermocouples and RTDs and the applications of these two devices.   So here’s what you need to know:

The thermocouple is made of two dissimilar metals joined together at two points.  The “hot junction” is in the process, and the “cold junction” is at the controller.  In theory an EMF (Electromotive Force) i.e. a millivolt current, is generated at each junction that relates to the temperature at each junction. To measure a single temperature, one of the junctions (normally the cold junction) is maintained at a known reference temperature, and the other junction (hot junction) is at the process to be sensed.  By incorporating an artificial cold junction compensator (a thermally sensitive device such as a thermistor or diode) the controller subtracts the temperature at the cold junction from the thermocouple’s signal to remove the cold junction’s error, thus giving a true temperature at the hot junction.  This is known as cold junction compensation.

Unlike the thermocouple, the RTD (Resistance Temperature Detector) is usually made with a platinum, nickel, or copper wire that is wound around a ceramic or glass core, or it can be made by plating a thin film element and sealing this element within a ceramic or glass capsule.  Since the RTD works on a change in resistance, the lead wires (from the RTD to the controller) have resistance and add an error to the signal. If the leads are long enough, then the error may be large enough to have to be corrected. 

Industrial applications use a 3 wire RTD. Two wires connect on both sides of the resistor. This measures the temperature at the resistor and the error of the lead wires. One of those two wires has another wire with it. When those two wires are measured at the controller, they give the resistance of only those two wires. This measurement is then subtracted from the resistor’s two wires to remove the lead wire resistance from it. Now the controller is only reading the temperature at the resistor.

Application Differences:

Based on the thermocouple type (which dissimilar metals are used in manufacturing) the thermocouple has a wide temperature range:  -328 to 4800 degrees F. Thermocouples have a fast response time, low initial cost, and durability for rugged industrial type applications.

RTDs have a temperature range of -328 to 1202 degrees F.  Because of the lower temperature range, the RTD is more accurate than the thermocouple, has more stable outputs over time, and is easier to calibrate.

Explanation of these controls can get quite technical, and I would like to thank David Cates of our staff for his help in keeping terms and statements in a this article simple.

Let Stromquist and Company help you with your needs for thermocouples or RTD’s by calling us at 1-800-241-9471 or contacting one of our many CGNA members.

For competitive prices and great service the HVAC and building automation control pros trust Stromquist @ Company

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