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RTD Basics

RTDs operate by exhibiting an increase in resistivity with an increase in temperature. RTDs are most commonly made from platinum, nickel, or copper. Copper and nickel versions operate at lower temperature ranges and are less expensive than platinum. Platinum is the most versatile material because of its wide temperature range (–200°C to 850°C), excellent repeatability, stability, and resistance to chemicals and corrosion. Platinum RTDs are available in 100 (omega), 200 (omega), 500 (omega), and 1000 (omega) nominal resistance values at 0°C, of which the 100 (omega) is the most popular.

Thermistors

Thermistors (thermally sensitive resistors) are similar to RTDs in that they are electrical resistors whose resistance changes with temperature. Thermistors are manufactured from metal oxide semiconductor material which is encapsulated in a glass or epoxy bead.

Thermistors have a very high sensitivity, making them extremely responsive to changes in temperature. For example, a 2252 Ω thermistor has a sensitivity of -100 Ω/°C at room temperature. In comparison, a 100 Ω RTD has a sensitivity of 0.4 Ω/°C. Thermistors also have a low thermal mass that results in fast response times, but are limited by a small temperature range. Thermistors have either a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC).

Thermocouple Basics

A thermocouple is made up of two dissimilar metals, joined together at one end, that produce a voltage (expressed in millivolts) with a change in temperature. The junction of the two metals, called the sensing junction, is connected to extension wires. Any two dissimilar metals may be used to make a thermocouple. Of the infinite number of candidate combinations, the ISA recognizes 12. Most of these thermocouple types are known by a single-letter designation; the most common are J, K, T, and E. Measurement errors can be easily introduced with thermocouples. Since the voltage created by the thermocouple is due to the bonding of two dissimilar metals, the introduction of other junctions to the circuit results in voltage changes that are referred to as cold junction errors. If the temperature at the connections is determined, these errors can be corrected by a process called cold junction compensation. This is carried out at the receiving device, which is usually the signal conditioner.