Thermocouples
Thermocouples are made of two dissimilar metal wires joined at their measuring end forming the "measuring junction" also known as the "hot junction". A small voltage, known as the Seeback voltage, is created at a junction of dissimilar metal alloys. This voltage changes as a function of temperature, See Figure 1. The control or monitor measures this small voltage and converts it to a temperature signal. Modern instrumentation also measures the temperature where the thermocouple is connected to the instrument. This is the reference junction. See Figure 2. Any temperature effects near the instrument can be canceled out leaving an accurate reading of the process to be measured.

A thermocouple may be directly connected to a control or monitor. Extension wires, if used, must be of the same materials as the thermocouple wires. Thermocouples designed with their measuring junctions in contact with new surfaces are known as grounded junction thermocouples. These are the most common, generally have the fastest response times, and are the most economical. Ungrounded junction thermocouples offer the advantage of electrical isolation. Dwyer manufactures both types of thermocouples. Thermocouples are generally more rugged and less expensive than other sensor types. All Dwyer thermocouples are manufactured using industry standard alloys and meet astringent ANSI standards. This assures interchangeability with other standard thermocouples without special instrument recalibration. Dwyer thermocouples are used in all types of applications, can measure wide temperature ranges, and are offered in a large variety of standard configurations.

RTDs
RTDs are usually platinum wire wound on a glass or ceramic bobbin and sealed with a coating of ceramic or glass. They can also be made by depositing platinum as a film on a substitute and then encapsulating it. The electrical resistance of the RTD changes as a function of temperature. Circuitry similar to a Wheatstone bridge is built into controls designed for use with RTDs. Constant current into the bridge produces an output voltage that varies with temperature. Lead wire resistance can significantly affect the RTD measurement. This is typically corrected using a third (compensating) lead wire. See Figures 3 and 4.

Extension wires used with RTDs may be plain copper wire. RTDs are generally more accurate and more stable over time than thermocouples. Dwyer RTDs are built to rigorous DIN (most common) or NIST standards and are offered in a wide variety of standard configurations.

The third wire in the 3-wire RTD compensates for the wire lead resistance and its temperature change.

Thermistors
Thermistors have a semiconductor material which changes its electrical resistance as a function of temperature. Extension wires used with thermistors can be plain copper wire. Thermistors offer accuracy similar to RTDs within narrow temperature ranges near ambient temperature. They also generally offer faster response times. Since thermistor standards vary, care must be taken to match the instrumentation to the sensor.