Emissivity Analysis of Infrared Thermometer
Emissivity is the ratio of the radiant energy of an actual object to a blackbody at the same temperature under the same conditions. The so-called same conditions refer to the same geometric conditions (radiation emission area, solid angle size and direction for measuring radiant power) and spectral conditions (spectral range for measuring radiant flux). Since emissivity is dependent on the measurement conditions, there are several emissivity definitions.
Hemispheric emissivity Hemispheric emissivity is the ratio of the radiant energy flux (radiation output) emitted by the radiator per unit area to the hemispherical space and the radiation output of the black body at the same temperature, which is divided into two types: full amount and spectral amount.
normal emissivity
Normal emissivity is the emissivity measured within a small solid angle in the normal direction of the radiating surface, and it is the ratio of the radiance in the normal direction to the radiance of a black body at the same temperature. Since infrared systems detect radiant energy within a small solid angle normal to the target surface, normal emissivity is important.
For a black body, all emissivity values are equal to 1, while for real objects, all emissivity values are less than 1. The emissivity we are talking about so far is the average emissivity.
Regarding emissivity correction:
The emissivity of different object surfaces is different, in order to ensure the accuracy of temperature measurement, emissivity correction is generally required. Since the thermometer is calibrated with a black body, the surface emissivity of any object is smaller than that of a black body.
The emissivity correction method of the infrared thermometer is: 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 black body with the same temperature. the same signal. For example, if the emissivity of an object is 0.8, it is necessary to increase the magnification of the amplifier to the original 1/0.8=1.25 times. In the industrial field, it is generally difficult to determine the target emissivity parameters due to the different measurement target materials, shapes and surface states. 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. It can be adjusted according to the following steps based on experience temperature or process temperature:
For example: the measuring range of the thermometer is: 500-1400℃
The real temperature is 1200°C, the measured temperature is 1150°C,
At this point the emissivity parameter can be adjusted as:
(1150-500)÷(1200-500)=0.928≈0.93
This adjustment makes the measured value closer to the real value, and can also be adjusted by referring to the "Material Emissivity Coefficient Table". But the parameters in this table are not necessarily applicable to process needs. It must be clear that the essence introduced by the emissivity adjustment is to correct the measurement error.
