How to Determine the Three Basic Data of a Thermometer
1. Determine distance coefficient (optical resolution)
The distance coefficient is determined by the ratio of D: S, which is the ratio of the distance D between the thermometer probe and the target to the diameter of the measured target. If the thermometer must be installed away from the target due to environmental conditions, and to measure small targets, a high optical resolution thermometer should be selected. The higher the optical resolution, i.e. increasing the D: S ratio, the higher the cost of the thermometer. The Raytek infrared thermometer D: S ranges from 2:1 (low distance coefficient) to over 300:1 (high distance coefficient). If the thermometer is far from the target and the target is small, a thermometer with a high distance coefficient should be selected. For a fixed focal length thermometer, the focal point of the optical system is a small spot, and the spot near and far from the focal point will increase. There are two distance coefficients. Therefore, in order to accurately measure temperature at distances close to and far from the focal point, the size of the measured target should be greater than the size of the spot at the focal point. The zoom thermometer has a small focal point position that can be adjusted based on the distance to the target. Increasing D: S reduces the received energy. Without increasing the receiving aperture, it is difficult to increase the distance coefficient D: S, which increases the instrument cost.
2. Determine wavelength range
The emissivity and surface characteristics of the target material determine the spectral corresponding wavelength of the thermometer. For high reflectivity alloy materials, there is a low or varying emissivity. In the high-temperature zone, the optimal wavelength for measuring metal materials is near-infrared, which can be selected from 0.8 to 1.0 μ M. Other temperature zones can be selected as 1.6 μ m. 2.2 μ M and 3.9 μ M. Due to some materials being transparent at a certain wavelength, infrared energy can penetrate these materials, and special wavelengths should be selected for this type of material. If measuring the internal temperature of glass, choose 1.0 μ m. 2.2 μ M and 3.9 μ M (the measured glass must be very thick, otherwise it will penetrate) wavelength; Choose 5.0 for measuring the surface temperature of glass μ M; Select 8-14 for low-temperature measurement area μ M is appropriate. If measuring polyethylene plastic film, choose 3.43 μ m. Polyester selection 4.3 μ M or 7.9 μ m. Select 8-14 for thicknesses exceeding 0.4mm μ M. Narrow band 4.64 is used to measure CO in flames μ m. Measure NO2 in flames using 4.47 μ M.
3. Determine 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 final reading energy. It is related to the time constant of the photodetector, signal processing circuit, and display system. Raytek's new infrared thermometer has a response time of up to 1ms. This is much faster than the contact temperature measurement method. If the target's movement speed is very fast or when measuring rapidly heated targets, a fast response infrared thermometer should be selected, otherwise insufficient signal response will be achieved, which will reduce measurement accuracy. However, 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. The determination of response time is mainly based on the movement speed of the target and the temperature change speed of the target. For stationary targets or targets involved in thermal inertia, or if the speed of existing control equipment is limited, the response time of the thermometer can be relaxed.
