An infrared temperature sensor is one of the most essential non-contact measurement devices used for temperature monitoring and control in industrial process and machinery. It is commonly used for condition monitoring applications in critical process plants, machinery, motion detection systems, electrical equipment as well as quality control in process manufacturing such as in mining, metals and food industry. The infrared temperature sensors locate hot spots in the target object without intervening in the production process. They are also capable of performing measuring in challenging applications such as measurement in area that is difficult to access, measuring moving objects, hot objects or surfaces that can get easily damaged with other contact-type sensors.
Industrial Infrared sensors are available most commonly as thermal imaging camera. The camera offers the thermal images of the objects which temperature data is color-coded. There are also other types of infrared sensors such as high-speed pyrometers or infrared thermometers which measure surface temperature by projecting a laser line to the object. Both types of sensors can be used in almost all applications, depend on the user preference and requirement. It is more crucial for the users to understand some factors that they should consider before they purchase the sensors in order to get accurate measurement.
Wavelength is a very important factor considered during temperature measurement. Selection of wavelength in an infrared sensor is directly impacted by the emissivity of the target object, which is the measure of the effectiveness of the infrared radiation emitted by the surface. At different wavelengths, emissivity of an object varies when radiated energy is monitored. Highly reflective materials such as metals tend to have low or changing emissivity. Therefore, in such cases, around 0.8 to 1 micron is considered as the optimum wavelength for choosing sensor. Similarly, depending on other types of surfaces, sensors with specified optimum wavelengths are used. To simplify this process, material groups are formed to describe the optimum wavelengths for highest object emissivity. The optimum wavelengths for metals, glass and textiles are 0.8-2.3um, 5um and 8-14um respectively. For some complex types of polymer such as polyethylene, polypropylene, nylon, polystyrene or polyester, specific infrared sensors with specialised operating wavelength are required.
Infrared sensors come with a feature of adjustable emissivity correction for getting accurate measurement of temperature at varying conditions. Therefore, when selecting an infrared temperature sensor, the wavelength band of the sensor should be known. Additionally, the values of object emissivity over temperature and wavelength range must be calculated and recorded.
Temperature measurement using non-contact infrared sensors can go below freezing point or can go up to temperatures as high as 2200°C. For instance, if the measurement need to be performed in cooling chains or laboratories, or even in hot melting materials or blast furnaces, infrared sensors specific to these environments are available. Therefore, users should be clear about the required measuring range to choose the best option for their application. A sensor with wide temperature range must be chosen if the requirement is to monitor start-up or cool-down temperatures in a process. On the other hand, for achieving better resolutions of the output signal while monitoring and controlling temperatures, infrared sensors with a narrow temperature range must be preferred. For heat-treating applications where temperature must be confined to a specific range for a long period of time, selection of sensor with the appropriate temperature range is very important.
The area that is to be measured using infrared sensor must fill the instrument’s field of view. Generally, sensors with field of view smaller than the measurement target by 50% are recommended. The reason behind this selection is the impact of sensors having field view either too large or too small than the target on temperature readings. Very large targets don’t measure temperature variation outside the measurement area. Hence, a clear line of sight between the sensor and the target is required. Based on the target size, various infrared sensors are available for targets that are small or moving in and out of the field of view.
When measuring moving or quickly heating objects, infrared sensors with fast response times are required. For these types of applications, sensors with response times selectable down to 1 millisecond are also available in the market. However, for certain applications where there is a significant thermal lag in heating a process, fast sensors may exceed the capability of existing control instruments. In such cases, a sensor with a moderate response time would be preferred.
Certain environmental factors such as dust, gases, vapours, noise, electromagnetic fields or vibrations can cause inaccuracies in measurement or even damage the lens of the sensor. Therefore, temperature sensors with features such as protective housing, air purging, or water cooling should be looked out for as they can protect the sensor against ambient environment and give accurate outputs. While choosing these accessories, the cost of prerequisite services such as air, water, and power must be considered.
These are the major considerations a user must make before purchasing an infrared temperature sensor. While non-contact infrared sensors are replacing thermocouples, various smart infrared systems supporting both analogue and digital output are available in the market.
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