Analysis of emissivity of infrared thermometer
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 same 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). Since emissivity is related to measurement conditions, there are several definitions of emissivity.
Hemispheric Emissivity Hemispheric Emissivity is the ratio of radiant energy flux (emittance) per unit area of radiator to the emittance of blackbody at the same temperature, which is divided into total amount and spectral amount.
Normal emissivity
Normal emissivity is the emissivity measured in a small solid angle along the normal direction of the radiation surface, which is the ratio of the radiation brightness along the normal direction to the radiation brightness of the blackbody at the same temperature. Because all infrared systems detect the radiation energy in a small solid angle along the normal direction of the target surface, the normal emissivity is very important.
For blackbody, all kinds of emissivity are equal to 1, while for actual objects, all kinds of emissivity are less than 1. What we are talking about at present is the average emissivity.
On emissivity correction:
The emissivity of different surfaces is different. In order to ensure the accuracy of temperature measurement, emissivity correction is generally required. Because the thermometer is calibrated in blackbody, the emissivity of any object surface is smaller than that of blackbody.
The method of emissivity correction of infrared thermometer is to adjust the magnification of the amplifier according to 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 that generated by a blackbody with the same temperature. For example, if the emissivity of an object is 0.8, the magnification of the amplifier needs to be increased to 1/0.8=1.25 times. However, in the industrial field, it is generally difficult to determine the target emissivity parameters because of the different materials, shapes and surface States of the measured targets. Measurement errors caused by other factors will cause the difference between the measured value and the real value. The introduction of emissivity parameter adjustment can solve this problem well without affecting the measurement linearity. Can be adjusted according to the following steps based on the empirical temperature or process temperature:
For example, the measuring range of the thermometer is 500-1400℃
The real temperature is 1200℃ and the measured temperature is 1150℃.
At this time, the emissivity parameter can be adjusted as follows:
(1150-500)÷(1200-500)=0.928≈0.93
After such adjustment, the measured value is closer to the real value, and it can also be adjusted with reference to the Table of Material Emissivity Coefficient. However, the parameters in this table may not be suitable for the process needs. It must be clear that the essence of emissivity adjustment is to correct the measurement error.
