Taking the temperature - from thermocouples to thermal imaging

Instrumentation for temperature measurement covers everything from simple thermocouples to sophisticated thermal imaging systems suitable for installation in potentially hazardous areas. Jon Severn looks at some of the recent developments in this broad-based marketplace.

Temperature is one of the seven SI base units and its measurement is critical to many industrial processes for reasons of performance, quality, efficiency, safety or a combination of these. Because of the diversity of the applications for which temperature measurement is required, several alternative temperature measurement techniques have been devised, and the development process is ongoing, especially in relation to systems that measure the infrared radiation emitted by an object.

No single temperature measurement technique is dominant, as the range of applications is so vast. In some cases a simple and low-cost thermocouple is adequate, whereas other applications may call for the better accuracy offered by a platinum resistance thermometer or other type of resistance temperature detector. Infrared technologies may be more applicable if non-contact sensing or a thermal image is required. In all cases, there is a choice of portable handheld instruments or fixed units that measure the temperature on a continuous or periodic basis.

Infrared vision-based systems can also be used for applications where the surface temperature is used to indicate faults that would not normally be visible. For example, by heating along the length of a helicopter rotor blade and using an infrared camera to 'see' how the heat is absorbed and re-radiated, any changes in infrared radiation along the blade can be readily identified. Such changes could be due to pockets of air trapped between the laminations as a result of a defective production process.
Thermal imaging has also proved very useful in condition monitoring for checking electrical and mechanical components; a localised temperature rise can be an early indication of impending component failures.