Common Errors in Infrared Temperature measurement

19-09-2019

The infrared temperature sensors determine the surface temperature of an object based on the infrared radiation emitted by all object at the temperature above absolute zero (-273oC). Some types of infrared temperature sensors include thermal imaging camera, infrared sensor and handheld pyrometer and camera. These modern non-contact sensor technology offers advantage over contact-type measurement in terms of reliability, accuracy, maintenance and response rate. However, there are several occasions when the IR sensor may mislead the users to erroneous results. This resource is aimed to clarify these issues to avoid such events from happening and assist the users in obtaining correct and reliable data.

Materials Emissivity

Emissivity is defined as the ability of a real object to emit thermal radiation energy in comparison to the black body at a given temperature, T. Its value lies between zero and one. As IR sensors calculate the object’s temperature based on the emitted radiation energy, failure to specify the correct value of materials emissivity will definitely produce errors in the measured value.

However, specifying the true material emissivity is a challenge. While the emissivity of the material is known at certain temperature range, the emissivity is also a dynamically changing property that largely depends on the surrounding radiant temperature and the wavelength of the measuring instrument. Understanding all parameters in the measuring instrument is crucial to make the correct temperature measurement. Currently, there are several infrared sensors specifically designed to measure at a particular wavelength to suit a specific industry, such as 7.9µm for glass and 1.1µm for hot glowing metals.

Measurement Spot Size

The accuracy of infrared sensors is also greatly influenced by the measurement spot size, especially when measuring temperature of small objects. This is often referred to as “distance-to-spot ratio” which specifies the diameter of the area being measured in response to the distance of sensor from the target. For example, for a sensor with distance-to-spot ratio of 5:1, it will measure approximately 1cm of diameter when the sensor is located 5cm away from the target.

It should be noted that the measurement spot size should be adjusted to the size of the measurement object. Using the above-mentioned sensor to measure 10cm area from 1 meter away will produce erroneous results as it will measure the temperature of the area outside of the object. Some models are designed with a laser sighting to point at the target object. However, the size of the laser beam does not always specify the true measurement spot size and users are always encourage to check the correct specifications of their purchased model before making the measurement.

Reflective and shiny objects

Most infrared temperature sensors produce unreliable data when measuring shiny and reflective objects. In general, shiny and reflective materials emit less infrared energy than normal objects, means they have less emissivity. Although using a non-reflective tape to bypass this issue may work occasionally, this is not feasible for measuring temperature of hot objects, commonly found in most of the manufacturing industry. Some IR sensors to measure at very short wavelength have been developed to reduce measurement errors due to the emissivity change.

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