Temperature measurement basics

Tips from Omega Engineering Inc., Stamford, Conn.

Temperature can be measured using a diverse array of sensors, all of which infer temperature by sensing some change in a physical characteristic. Six types that engineers are likely to encounter include thermocouples, resistive temperature devices (RTDs and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. Here we briefly define each type:

Thermocouple temperature measurement sensors

Thermocouples consist essentially of two strips or wires made of different metals and joined at one end. Changes in the temperature at that juncture induce a change in electromotive force (emf) between the other ends. As temperature goes up, the output emf of the thermocouple rises, though not necessarily linearly.

Resistive temperature devices (RTDs)

Resistive temperature devices capitalize on the fact that the electrical resistance of materials changes with temperature. Two key types are metallic devices (commonly referred to as RTDs), and thermistors. As their name indicates, RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature. Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise.

Infrared devices

Infrared sensors are non-contacting devices. They infer temperature by measuring the thermal radiation emitted by a material.

Bimetallic devices

Bimetallic devices take advantage of the difference in rate of thermal expansion between different metals. Strips of two metals are bonded together. When heated, one side expands more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer. These devices are portable and they do not require a power supply, but they are usually not as accurate as thermocouples or RTDs, and they don't readily lend themselves to temperature recording.

Fluid-expansion devices

Fluid-expansion devices, typified by the household thermometer, are generally one of two types — mercury or organic liquid. Versions employing gas instead of liquid are also available. Mercury is considered an environmental hazard, so there are regulations governing the shipment of devices that contain it. Fluid-expansion sensors do not require electric power, do not pose explosion hazards, and are stable even after repeated cycling. On the other hand, they do not generate data that is easily recorded or transmitted, and they cannot make spot or point measurements.