Temperature measurement devices include liquid thermometers, bimetal thermometers, pressure on liquid or gas expansion bulbs, thermistors, resistance temperature detectors (RTDs), infrared detectors, and crystal window tapes.
The RTD is typically used on lower, ambient-range temperatures, while thermocouples provide better reliability in higher ranges.
In addition, gas- and liquid-filled temperature sensors and thermistors are frequently used for equipment-protection and cooling systems.
For continued accurate service, operators should periodically calibrate the instruments using a standard temperature measurement device with high accuracy.
Thermocouple
The thermocouple operates on the principle that current flows in a circuit made of two different metals when the two electrical junctions between the metals are at different temperatures.
The various combinations of metals used are tabulated in most engineering handbooks, and the selection of metals is based on the maximum temperature to be measured.
Thermocouples measure as high as 980°C, with an accuracy of 1% of the full scale.
Resistance Temperature Detector (RTD)
A resistance temperature detector has a temperature-sensitive element in which electrical resistance increases repeatedly and predictably with increasing temperature.
The sensing element is typically made of small-diameter platinum, nickel or copper wire wound on a special bobbin or otherwise supported in a virtually strain-free configuration.
The detector is typically selected for high accuracy and stability.
A common application is the measurement of bearing and winding temperatures in electrical machinery