An infrared temperature sensor is one of the most used non-contact temperature measurement sensors used for temperature monitoring and control in industrial processes and machinery. It is commonly used for condition monitoring applications in critical process plants, machinery, motion detection systems, electrical equipment as well as for product quality control in process manufacturing such as in mining, metals and food industry. The infrared temperature sensors can locate hot spots in the target object without intervening in the production process- and obtain the true temperature of the sighted object. They are also capable of performing measurements in challenging industrial applications such as measurement in areas that are difficult to access, measuring fast-moving objects, hot objects or surfaces that can get easily damaged with thermocouples.
Industrial Infrared sensors are available as thermal imaging cameras also called IR cameras and infrared pyrometers. The camera offers thermal images of the objects whose temperature data is colour-coded. Infrared thermometers or pyrometers measure surface temperature by projecting a laser line to the object. Both types of sensors, IR Camera and infrared laser temperature sensor, can be used in almost all applications, depending on the user preference and requirement. For example, if the users need to identify hot spots in an object or detect abnormal temperature distribution, thermal imaging camera can be a better option albeit it comes with slightly higher investment. Nevertheless, it is more crucial for the users to understand some technical parameters before they purchase the sensors for their respective applications.
Wavelength
Wavelength is an essential factor that highly affects measurement of infrared temperature sensors. Selection of wavelength in an infrared sensor is directly impacted by the emissivity of the target object, which is the infrared radiation emitted by the surface. Highly reflective materials such as metals tend to have low or changing emissivity. Therefore, in such cases, choosing an infrared sensor that operates between 0.8 to 1 micron would be optimal.
In addition, emissivity also varies with changing temperature and type of surfaces, making selecting the ideal sensor challenging. To simplify this process, infrared sensor manufacturers produce guidelines that allow users to choose a specific wavelength band for material groups. For example, the optimum operating wavelengths for measurement on 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 using a temperature data logger.
Temperature range
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 needs 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. This means that users should be clear about the required measuring range to choose the best option for their respective applications. A sensor with a wide temperature range must be chosen if the requirement is to monitor start-up or cool-down temperatures in a process. This would be compensated with the resolution and accuracy of the measurement. On the other hand, if better resolutions are requireds, infrared sensors with a narrow temperature range must be used. For heat-treating applications where temperature must be maintained, it is crucial to select a sensor within the specific range and can measure reliably over long period of continuous operation.
Target size
An infrared sensor measures the average temperature of the area within the instrument’s field of view. Generally, sensors with a field of view smaller than the measurement target by 50% are recommended to minimise measurement error due to temperature variation. If the sensors are used to measure on small objects, it is recommended to choose the pyrometer with the smaller FOV. Otherwise, a thermal imager can be used to capture the temperature data of the whole body in thermal image format. More importantly, a clear line of sight between the sensor and the target is required as any objects such as dust particles, steam, gas or contaminants can influence the measurement errors.
Response time
When measuring fast-moving objects such as that in a production line environment, infrared sensors with fast response times are required. This high-speed pyrometer is also required if the users need to capture the transient thermal behaviour during the fast heating process such as during pyrolysis.
Type of application
Certain environmental factors such as dust, gases, water vapours, electromagnetic fields or vibrations can cause inaccuracies in measurement or even damage the lens of the sensor. Therefore, infrared temperature sensors are equipped with features such as protective housing, air purging, or water cooling jacket so they can protect the sensor against the ambient environment.
These are the major considerations a user must make before purchasing an infrared temperature sensor. Non-contact infrared temperature sensors are seen as alternatives to traditional thermocouples in industrial monitoring and research applications. Therefore, they are available with a wide range of selectable outputs to allow easy interface with any industrial control system.
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