How to select an infrared temperature sensor

Plant and maintenance engineers use infrared temperature measurement devices, both handheld 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 without having to interrupt production.

Using an online infrared sensor is beneficial in applications where the temperature of an object, material, surface or liquid is critical to the production process. When selecting the most suitable temperature measurement device for the application, engineers need to consider carefully 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. 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 are obtained without interfering with the target object. Infrared sensors can also measure very high temperatures, whereas a contact sensor would either be destroyed or have a short service life.

Not only are infrared devices now relatively inexpensive, they also offer technical benefits and a variety of options for users, including handheld or inline process control, open connectivity to fieldbus systems and options for hazardous environments.

For accurate temperature measurement using infrared sensors, users must carefully consider two key parameters: emissivity and wavelength.

Emissivity

All bodies above absolute zero (-273degC) emit infrared radiation 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 material of the measurement object. However, for non-contact infrared temperature measurements, only the emitted radiation element is important.

The relationships between the emission types 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 20degC is 0.2 (reflected energy would be 0.8). This means 80 per cent of the emitted heat energy from the object would be reflected 'heat energy' from surrounding objects! However, at a much higher temperature of 1100degC, 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 100degC 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. All Micro-Epsilon temperature sensors have adjustable emissivity correction.

Wavelength

The previous description of emissivity is rather simplistic in order to explain 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. Developing sensors that measure temperature at specific wavelengths can therefore increase measurement stability significantly.

Put simply, material groups can be used to describe the optimum wavelengths for highest object emissivities and therefore the most stable results. For metals, 0.8-2.3um, glass 5um, textiles and most matt surfaces 8-14um. Plastics are more complex, requiring specific wavelength sensors to be developed for: polyethylene, polypropylene, Nylon and polystyrene (3.43um); polyester, polyurethane, Teflon, FEP and polyamide (7.9um); and thicker, pigmented films (8-14um).

To summarise, when selecting an infrared temperature sensor, it is crucial to know the wavelength band over which the sensor measures and to ensure that the correct wavelength band is used for the object to be measured. In addition, the object emissivity values over this wavelength and the temperature range to be measured must also be known or calculated.

For a free guide to The Basics of Non-Contact Temperature Measurement go to www.micro-epsilon.co.uk or email [email protected].