Factors Contributing to Measurement Errors of Infrared Thermometers
1. Radiation rate
Radiance rate is a physical quantity that measures the relative radiation ability of an object to a blackbody. It is not only related to the material shape, surface roughness, and concavity of the object, but also to the direction of the test. If the object has a smooth surface, its directionality is more sensitive. The emissivity of different substances is different, and the amount of radiation energy received by an infrared thermometer from an object is proportional to its emissivity.
(1) The setting of emissivity is based on Kirchhoff's theorem: the hemispherical monochromatic emissivity (ε) of an object's surface is equal to its hemispherical monochromatic absorptivity (α), where ε=α. Under thermal equilibrium conditions, the radiated power of an object is equal to its absorbed power, that is, the sum of absorption rate (α), reflectance (ρ), and transmittance (γ) is 1, that is, α+ρ+γ=1. For opaque (or thick) objects, the transmittance can be seen as γ=0, with only radiation and reflection (α+ρ=1). As the emissivity of the object increases, the reflectance decreases, and the influence of background and reflection decreases, resulting in higher testing accuracy; On the contrary, the higher the background temperature or reflectivity, the greater the impact on the test. From this, it can be seen that in the actual detection process, attention must be paid to the corresponding emissivity of different objects and thermometers, and the setting of emissivity should be as accurate as possible to reduce the error of the measured temperature.
(2) Test angle
The emissivity is related to the testing direction, and the larger the testing angle, the greater the testing error. This is easily overlooked when using infrared for temperature measurement. Generally speaking, the testing angle is best within 30 ° C and should not exceed 45 ° C. If it is necessary to conduct the test at a temperature greater than 45 ° C, the emissivity can be appropriately lowered for correction. If the temperature measurement data of two identical objects need to be analyzed and judged, the testing angle must be the same during testing in order to be more comparable.
2. Distance coefficient
The distance coefficient (K=S: D) is the ratio of the distance S between the thermometer and the target to the diameter D of the temperature measuring target. It has a significant impact on the accuracy of infrared thermometers, with higher K values resulting in higher resolution. Therefore, if the thermometer must be installed far away from the target due to environmental conditions and needs to measure small targets, a high optical resolution thermometer should be selected to reduce measurement errors. In practical use, many people overlook the optical resolution of thermometers. Regardless of the diameter D of the measured target point, turn on the laser beam and aim it at the measurement target to test. In fact, they overlooked the requirement of S: D value for the thermometer, which may result in a certain degree of error in the measured temperature.
3. Target size
The accuracy of the instrument measurement is determined by the measured object and the field of view of the thermometer. When using an infrared thermometer for temperature measurement, it can generally only measure the average value of the determined area on the surface of the target being measured. There are generally three situations during testing:
(1) When the measured target is larger than the test field of view, the thermometer will not be affected by the background outside the measurement area, and can display the true temperature of the measured object within the determined area of the optical target. At this time, the testing effect is the best.
(2) When the target being tested is equal to the test field of view, the background temperature has been affected, but it is still relatively small, and the test effect is average.
(3) When the target being measured is smaller than the test field of view, the background radiation energy will enter the visual and acoustic symbols of the thermometer and interfere with the temperature reading, causing errors. The instrument only displays the weighted average of the measured object and background temperature.
4. Response time
The response time represents the reaction speed of an infrared thermometer to changes in the measured temperature, defined as the time required to reach 95% of the energy at the final reading. It is related to the time constants of the photodetector, signal processing circuit, and display system. If the target is moving at a fast speed or when measuring rapidly heated targets, a fast response infrared thermometer should be used, otherwise it will not achieve sufficient signal response and will reduce measurement accuracy. But not all applications require fast response infrared thermometers. When there is thermal inertia in a stationary or target thermal process, the response time of the thermometer can be relaxed. Therefore, the selection of response time for infrared thermometers should be adapted to the situation of the target being measured.
