Analysis of the Differences between Thermocouples and RTDs

Thermocouples are made of two dissimilar metals that are joined together. The "hot junction" is in the process and the "cold junction" is at the controller. In theory, an EMF (electromotive force) is generated at each junction, which is a millivolt current that is related to the temperature at each junction. To measure a single temperature, one of the junctions (usually the cold junction) is held at a known reference temperature, while the other junction (the hot junction) is in the process to be detected. By incorporating an artificial cold junction compensator (a thermally sensitive device such as a thermistor or diode), the controller subtracts the temperature of the cold junction from the thermocouple's signal to eliminate the error at the cold junction, thereby generating the true temperature at the hot junction. This is called cold junction compensation.

Unlike thermocouples, RTDs (resistance temperature detectors) are generally made of platinum, nickel, or copper wire wrapped around a ceramic or glass core, or by electroplating a thin film element and sealing the element in a ceramic or glass capsule. Since the operating resistance of an RTD varies, the lead wires (from the RTD to the controller) have resistance and will add error to the signal. If the lead wires are long enough, the error may be large enough and must be corrected.

Industry uses 3-wire RTDs. Two wires are connected on either side of the resistor. The temperature of the resistor and the error of the leads are measured. One of the two wires has a lead from the other. When these two leads are measured on the controller, they give the resistance of only those two leads. This measurement is then subtracted from the two leads of the resistor to eliminate the lead resistance. Now the controller only reads the temperature of the resistor.

Application Differences:

Depending on the thermocouple type (different metals used in their manufacturing), thermocouples have a wide temperature range: -328 to 4800 degrees Fahrenheit. Thermocouples have fast response times, low initial costs, and durability for harsh industrial type applications. RTDs have a temperature range of -328 to 1202 degrees Fahrenheit. Due to the lower temperature range, RTDs are more accurate than thermocouples, have a more stable output over time, and are easier to calibrate.