Choosing an infrared temperature sensor

There are a number of factors that need to be considered when selecting an infrared temperature sensor.

Infrared temperature sensors are one of the most widely used non-contact temperature measurement technologies in industrial processes and machinery. They are commonly used for condition monitoring applications in critical process plants, machinery, motion detection systems and electrical equipment, as well as for product quality control in process manufacturing such as in mining, metals and the food industry. Infrared temperature sensors can locate hot spots on a 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 difficult-to-access areas, with fast-moving objects or hot objects, or for surfaces that can get easily damaged with thermocouples.

Industrial infrared sensors are available as thermal imaging cameras (IR cameras) and infrared pyrometers. IR cameras offer thermal images of the objects in which temperature data is colour-coded. Infrared thermometers or pyrometers measure surface temperature by projecting a laser onto the object. Both types of sensors can be used in almost all applications, depending on user preference and requirements. For example, if the user needs to identify hot spots in an object or detect abnormal temperature distribution, a thermal imaging camera can be a better option, although it comes with a slightly higher investment.

Considerations

There are number of major factors that must be taken into consideration before purchasing an infrared temperature sensor.

Sensor wavelength

For infrared thermometers, the wavelength is an essential factor that highly affects the measurement. Selection of wavelength is directly dependent on the emissivity of the target object, or how infrared radiation is emitted from the surface. Highly reflective materials such as metals tend to have low or changing emissivity. In such cases, choosing an infrared sensor that operates between 0.8 and 1 micron wavelength would be optimal.

Emissivity also varies with changing temperature and the type of surface, 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 to 2.3 µm, 5 µm and 8 to 14 µm 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 also come with adjustable emissivity correction for getting accurate measurements of temperature under varying conditions. Therefore, when selecting an infrared temperature sensor, the wavelength band of the sensor needs to be known. Additionally, the values of object emissivity over the required 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 up to as high as 2200°C. For instance, if the measurement needs to be performed in cooling chains or laboratories, or for 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 application. 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, but this would come with a compromise on the resolution and accuracy of the measurement. On the other hand, if better resolution is required, infrared sensors with a narrow temperature range must be used. For heat-treating applications where temperature must be accurately maintained, it is crucial to select a sensor within the specific range that can measure reliably over a long period of continuous operation.

Target size

An infrared sensor measures the average temperature of the area within its field of view. Generally, sensors with a field of view smaller than 50% of the measurement target are recommended in order to minimise measurement error due to temperature variations. If the sensors are used to measure the temperature of small objects, it is recommended to choose a pyrometer with a smaller field of view. Otherwise, a thermal imager can be used to capture the temperature data of the whole body in a 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 cause measurement errors.

Response time

When measuring fast-moving objects such as those in a moving production line environment, infrared sensors with fast response times are required. These high-speed pyrometers are also required if the user needs to capture transient thermal behaviour during a fast heating process such as pyrolysis.

Type of application

Certain environmental factors such as dust, gases, water vapours, electromagnetic fields or vibrations can cause inaccuracies in measurements or even damage the lens of the sensor. Infrared temperature sensors are therefore equipped with features such as protective housings, air purging or a water cooling jacket so they can protect the sensor against the ambient environment.

Conclusion

Non-contact infrared temperature sensors are seen as alternatives to traditional thermocouples in industrial monitoring and research applications. They are therefore available in a wide range of types to suit the application, and with a wide range of selectable outputs to allow easy interfacing with any industrial control system.

This article was originally published on a blog at https://www.bestech.com.au/blogs/factors-to-consider-when-selecting-infrared-temperature-sensor.

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