With the technology and continuous development, the design and new development of infrared thermometers provide users with various functions and multi-purpose instruments, expanding the choice. When selecting the thermometer type, you should first determine the measurement requirements, such as the measured target temperature, the measured target size, the measurement distance, the measured target material, the target environment, the response speed, the measurement accuracy, whether portable or online, etc. ; In the comparison of the existing types of thermometers, choose the instrument model that can meet the above requirements; among the many models that can meet the above requirements, choose the combination of performance, function and price. Other options, such as ease of use, maintenance and calibration performance.
1. Determine the temperature measurement range
Determine the temperature measurement range: The temperature measurement range is an important performance indicator of the thermometer. Each type of thermometer has its own specific temperature range. The product coverage is -50 ° C to + 3000 ° C, but this cannot be done with a model of infrared thermometer. Therefore, the user's measured temperature range must be considered accurate and comprehensive, neither too narrow nor too wide. According to the law of black body radiation, the change in radiant energy caused by temperature in the short wavelength band of the spectrum will exceed the change in radiant energy caused by emissivity errors. Therefore, it is better to use short waves when measuring temperature. Generally speaking, the narrower the temperature measurement range, the higher the resolution of the output temperature monitoring signal, and the easier the accuracy and reliability can be solved. The temperature measurement range is too wide, which will reduce the temperature measurement accuracy. For example, if the measured target temperature is 1000 degrees Celsius, first determine whether it is online or portable, and if it is portable.
2.Determine the target size
In order to obtain accurate temperature readings, the distance between the thermometer and the test target must be within a suitable range. The so-called "spot size" is the area of the thermometer's measurement points. The farther you are from the target, the larger the spot size.
Infrared thermometers can be divided into monochrome thermometers and dual-color thermometers (radiochromatic thermometers) according to the principle. For a monochrome thermometer, when measuring temperature, the area of the target to be measured should be filled with the field of view of the thermometer. It is recommended that the size of the target to be measured exceeds 50% of the field of view. If the size of the target is smaller than the field of view, the background radiant energy will enter the audiovisual notes of the thermometer and interfere with the temperature reading, causing errors. Conversely, if the target is larger than the field of view of the thermometer, the thermometer will not be affected by the background outside the measurement area. For a colorimeter, its temperature is determined by the ratio of the radiant energy in two independent wavelength bands. Therefore, when the target to be measured is small, not full of field of view, and the presence of smoke, dust, or obstruction on the measurement path attenuates the radiant energy, it will not affect the measurement result. Even in the case of energy attenuation of 95%, the required temperature measurement accuracy can still be guaranteed. For small targets that are in motion or vibration, a colorimeter is the choice. This is due to the small diameter and flexibility of the light, which can transmit light radiation energy on curved, blocked and folded channels, so it can measure targets that are difficult to access, harsh conditions, or near electromagnetic fields.
3. Determine the distance coefficient (optical resolution)
The distance coefficient is determined by the ratio of D: S, which is the ratio of the distance D from the thermometer probe to the target to the diameter of the target to be measured. The higher the optical resolution, that is, increasing the D: S ratio, the higher the cost of the thermometer. If the thermometer must be installed away from the target due to environmental conditions, and you need to measure a small target, you should choose a thermometer with high optical resolution. For a fixed focal length thermometer, the spot position is at the focal point of the optical system, and the spot size increases both near and far from the focal position. There are two distance coefficients. Therefore, in order to accurately measure temperature at distances close to and away from the focus, the size of the measured target should be larger than the size of the spot at the focus. The zoom thermometer has a focus position that can be adjusted according to the distance to the target. Increasing D: S will reduce the received energy. If the receiving aperture is not increased, the distance coefficient D: S will be difficult to increase, which will increase the cost of the instrument.
4.Determine the wavelength range
The emissivity and surface characteristics of the target material determine the corresponding wavelength of the thermometer's spectrum. For high reflectivity alloy materials, there is a low or varying emissivity. In the high-temperature region, the wavelength of measuring metal materials is near infrared, and 0.8-1.0 μm can be selected. Other temperature zones can be selected from 1.6μm, 2.2μm and 3.9μm. Since some materials are transparent at a certain wavelength, infrared energy can penetrate these materials, and a special wavelength should be selected for this material. For example, the internal temperature of the glass should be 1.0 μm, 2.2 μm, and 3.9 μm (the measured glass must be thick, otherwise it will transmit). The surface temperature of the glass should be 5.0 μm. For example, the measurement of polyethylene plastic film is 3.43μm, polyester is 4.3μm or 7.9μm, and thickness of more than 0.4mm is 8-14μm. For example, a narrow band of 4.64 μm is used to measure CO in the flame, and 4.47 μm is used to measure NO 2 in the flame.
5.Determine the response time
The response time is defined as the time required to reach 95% of the reading energy. It represents the reaction speed of the infrared thermometer to the measured temperature change. It is related to the time constant of the photodetector, signal processing circuit and display system. The choice of the response time of the infrared thermometer should be adapted to the situation of the measured target, and the determination of the response time is mainly based on the speed of the target's movement and the temperature of the target. If the target moves quickly or measures a rapidly heated target, a fast-responding infrared thermometer should be selected; otherwise, a sufficient signal response cannot be achieved, which will reduce the measurement accuracy. However, not all applications require fast response infrared thermometers. When there is thermal inertia for stationary or target thermal processes, the response time can be relaxed.
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