Analysis of the Emissivity of Infrared Thermometers Using Wuxi Slosen Principle
Emissivity is the ratio of radiation energy between an actual object and a blackbody at the same temperature under the same conditions. The so-called identical conditions refer to the same geometric conditions (emission radiation area, solid angle size and direction for measuring radiation power) and spectral conditions (spectral range for measuring radiation flux). Due to the correlation between emissivity and measurement conditions, there are several definitions of emissivity.
Hemisphere emissivity Hemisphere emissivity is the ratio of the radiant energy flux (radiance) emitted by a radiator into hemispherical space per unit area to the radiance of a blackbody at the same temperature, which is further divided into total and spectral quantities.
Normal emissivity
Normal emissivity is the emissivity measured within a small solid angle in the normal direction of the radiation surface. It is the ratio of the radiation brightness in the normal direction to the radiation brightness of a blackbody at the same temperature. Due to the fact that infrared systems detect radiation energy within a small solid angle in the normal direction of the target surface, the normal emissivity is crucial.
For blackbody, all emissivities are equal to 1, while for actual objects, the values of all emissivities are less than 1. The emissivity we are currently referring to is the average emissivity.
Regarding emissivity correction:
The emissivity of different object surfaces varies, and in order to ensure the accuracy of temperature measurement, emissivity correction is generally required. Due to the fact that the thermometer is calibrated in blackbody, the surface emissivity of any object is lower than that of blackbody.
The method for correcting the emissivity of an infrared thermometer is to adjust the amplification factor of the amplifier based on the emissivity of different objects, so that the signal generated by the radiation of an actual object with a certain temperature in the system is the same as the signal generated by a blackbody with the same temperature. For example, if the emissivity of an object is 0.8, it is necessary to increase the amplification factor of the amplifier to the original 1/0.8=1.25 times. However, in industrial sites, it is generally difficult to determine the target emissivity parameters due to the varying materials, shapes, and surface states of the measured targets. Measurement errors caused by other factors can cause differences between measured values and true values. The introduction of emissivity parameter adjustment can effectively solve this problem without affecting the measurement linearity. Based on experience temperature or process temperature, adjust according to the following steps:
For example, the range of the thermometer is: 500-1400 ℃
The actual temperature is 1200 ℃, and the measured temperature is 1150 ℃,
At this point, the emissivity parameter can be adjusted to:
(1150-500) ÷ (1200-500)=0.928 ≈ 0.93
After such adjustment, the measured values are closer to the true values, which can also be adjusted according to the "Material Emissivity Coefficient Table". However, the parameters in this table may not necessarily be applicable to the process requirements. It must be clarified that the essence of introducing emissivity adjustment is to correct measurement errors.
