Making sense of heat measurement sensors
16 Mar 2010
It is important to consider some key parameters before finalising the selection of infrared temperature sensors, writes Chris Jones
Plant and maintenance engineers use infrared temperature measurement devices, both hand-held and fixed/online versions, as non-contact, relatively low cost, preventive maintenance tools. These devices accurately monitor, control and manage process temperatures and help to locate ’hot spots’ on critical process plant, machinery and electrical connections.
When selecting the most suitable temperature measurement device for the application, engineers need to carefully consider their measurement requirements. Infrared thermometers measure the temperature of an object without touching it. It is, therefore, possible to perform fast, reliable temperature measurements of moving, hot or difficult-to-access objects.
Non-contact sensors
While contact temperature sensors or probes can influence the temperature of the target object, sometimes even damage the product itself, the non-contact method ensures precision measurement without damaging the target object. IR sensors can also measure very high temperatures, whereas a contact sensor would either be destroyed or would have a short service life.
Not only are IR devices now relatively inexpensive, they also offer a raft of technical benefits and a variety of options for users, including open connectivity to fieldbus systems and options for hazardous environments. For accurate temperature measurement using IR sensors, users must carefully consider two key parameters: emissivity and wavelength.
All bodies above absolute Kelvin (-273ºC) emit IR radiation in three ways, via a combination of emitted radiation, radiation reflected from the surroundings, and by transmitting the radiation through itself. How these factors interact depends on the object material. However, for non-contact IR temperature measurements, only the emitted radiation element is important.
While contact temperature sensors or probes can influence the temperature of the target object, sometimes even damage the product itself, the non-contact method ensures precision measurements without damaging the target object
The relationship of the emission types to each other is best described in the following way. If it is considered that at any given temperature, the sum of the radiation of the three emission types is equal to one, and it is assumed that solid bodies transmit negligible radiation, the transmitted element can be treated as zero. Therefore, the heat energy coming from an object only comprises emitted and reflected radiation.
It is now easier to understand why objects such as polished and shiny metals can only have a low emission, or emissivity, as radiation from the surrounding environment is strongly reflected (and so proportionally high) by these surfaces. For example, the typical emissivity for freshly milled steel at 20ºC is 0.2; so its reflected energy would be 0.8. This means 80% of the emitted heat energy from the object would be reflected ’heat energy’ from surrounding objects. However, at a much higher temperature of 1,100ºC, the same material will have a typical emissivity of 0.6.
In contrast, objects such as textiles or matt black surfaces reflect very little and therefore emit a high proportion of the heat energy. Emissivity of a black, matt paint at 100ºC is typically 0.97 and so is much more suited to non-contact temperature measurement.
Many low-cost devices have fixed emissivity correction of 0.95, which makes them unusable for almost all accurate temperature measurement tasks. Some of the more advanced temperature sensors have adjustable emissivity correction.
Wavelength
The previous description of emissivity is rather simplistic when it comes to explaining the relationship between the three radiated energy components. However, it should be noted that the emissivity of an object will vary when monitoring the radiated heat energy at different wavelengths. Therefore, developing sensors that measure temperature at specific wavelengths can significantly increase measurement stability.
Put simply, material groups can helpfully be used to describe the optimum wavelengths for the highest object emissivities and, therefore, the most stable results. For metals, 0.8 to 2.3µm, glass 5µm, textiles and most matt surfaces 8-14µm. Plastics are more complex, requiring specific wavelength sensors for different polymers.
