The correct choice of infrared thermometer
The choice of infrared thermometer can be divided into three aspects:
(1) Performance indicators, such as temperature range, spot size, working wavelength, measurement accuracy, window, display and output, response time, protection accessories, etc.;
(2) Environmental and working conditions, such as ambient temperature, windows, display and output, protective accessories, etc.;
(3) Other selection aspects, such as ease of use, maintenance and calibration performance, and price, also have a certain impact on the choice of thermometers.
With the continuous development of technology and technology, the best design and new progress of infrared thermometers provide users with various functions and multi-purpose instruments, expanding the choice. Other selection aspects such as ease of use, repair and calibration capabilities, and price. When choosing a thermometer model, you should first determine the measurement requirements, such as the temperature of the target to be measured, the size of the target to be measured, the measurement distance, the material of the target to be measured, the environment of the target, response speed, measurement accuracy, portable or online, etc. ;In the comparison of various existing models of thermometers, choose the instrument model that can meet the above requirements; choose the best match in terms of performance, function and price among many models that can meet the above requirements.
Determine the temperature range
Determine the temperature measurement range: The temperature measurement range is the most important performance index of the thermometer. For example, Raytek (Raytek) products cover a range of -50°C - +3000°C, but this cannot be done by one type of infrared thermometer. Each type of thermometer has its own specific temperature range. Therefore, the user's measured temperature range must be considered accurately and comprehensively, neither too narrow nor too wide. According to the law of blackbody radiation, the change of radiation energy caused by temperature in the short-wave band of the spectrum will exceed the change of radiation energy caused by emissivity error. Therefore, it is better to use short-wave as much as possible when measuring temperature. Generally speaking, the narrower the temperature measurement range is, the higher the resolution of the output signal of the temperature monitoring is, and the accuracy and reliability are easy to solve. If the temperature measurement range is too wide, the temperature measurement accuracy will be reduced. For example, if the measured target temperature is 1000 degrees Celsius, first determine whether it is online or portable, and if it is portable. There are many models that meet this temperature, such as 3iLR3, 3i2M, 3i1M. If the measurement accuracy is the main thing, it is better to choose the 2M or 1M type, because if the 3iLR type is used, the temperature measurement range is very wide, and the high temperature measurement performance will be poor; For low temperature targets, we have to choose 3iLR3.
Determine target size
Infrared thermometers can be divided into single-color thermometers and two-color thermometers (radiation colorimetric thermometers) according to the principle. For a monochromatic thermometer, when measuring temperature, the area of the target to be measured should fill the field of view of the thermometer. It is recommended that the measured target size exceed 50% of the field of view. If the target size is smaller than the field of view, the background radiation energy will enter the visual and acoustic symbols of the thermometer and interfere with the temperature measurement readings, causing errors. Conversely, if the target is larger than the pyrometer's field of view, the pyrometer will not be affected by background outside the measurement area. For colorimetric thermometers, the temperature is determined by the ratio of radiant energy in two independent wavelength bands. Therefore, when the target to be measured is small, does not fill the field of view, and there are smoke, dust, and obstructions on the measurement path, which attenuate the radiation energy, it will not have a significant impact on the measurement results. For small and moving or vibrating targets, the colorimetric thermometer is the best choice. This is due to the small diameter of the light rays and their flexibility to transport light radiant energy over curved, blocked and folded channels.
For Raytek (Lei Tai) two-color thermometer, its temperature is determined by the ratio of radiant energy in two independent wavelength bands. Therefore, when the target to be measured is small, does not fill the site, and there is smoke, dust, or obstruction on the measurement path that attenuates the radiation energy, it will not affect the measurement results. Even in the case of 95% energy attenuation, the required temperature measurement accuracy can still be guaranteed. For targets that are small and moving or vibrating; sometimes move within the field of view, or may partially move out of the field of view, under these conditions, the use of a two-color thermometer is the best choice. If it is impossible to aim directly between the pyrometer and the target, and the measurement channel is bent, narrow, blocked, etc., the two-color fiber optic pyrometer is the best choice. This is due to their small diameter, flexibility, and ability to transmit optical radiant energy over curved, blocked, and folded channels, thus enabling measurement of targets that are difficult to access, in harsh conditions, or near electromagnetic fields.
Determining the distance factor (optical resolution)
The distance coefficient is determined by the ratio of D:S, that is, the ratio of the distance D between the probe of the thermometer to the target and the diameter of the target to be measured. If the thermometer must be installed far away from the target due to environmental conditions, and a small target must be measured, a thermometer with high optical resolution should be selected. The higher the optical resolution, i.e. increasing the D:S ratio, the higher the cost of the pyrometer. Raytek Infrared Thermometers D:S range from 2:1 (low distance factor) to over 300:1 (high distance factor). If the thermometer is far away from the target and the target is small, a thermometer with a high distance coefficient should be selected. For a pyrometer with a fixed focal length, the focal point of the optical system is the smallest position of the spot, and the spot near and far from the focal point will increase. There are two distance factors. Therefore, in order to accurately measure temperature at a distance close to and far from the focus, the size of the measured target should be larger than the spot size at the focus. The zoom thermometer has a minimum focus position, which can be adjusted according to the distance to the target. If D:S is increased, the received energy will decrease. 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.
4.4 Determining the wavelength range
The emissivity and surface properties of the target material determine the spectral response wavelength of the pyrometer. For high reflectivity alloy materials, there is low or varying emissivity. In the high temperature area, the best wavelength for measuring metal materials is near-infrared, and 0.8-1.0 μm can be selected. Other temperature zones can choose 1.6μm, 2.2μm and 3.9μm. Since some materials are transparent at a certain wavelength, infrared energy will penetrate these materials, and a special wavelength should be selected for this material. For example, 1.0μm, 2.2μm and 3.9μm are used to measure the internal temperature of the glass (the measured glass must be very thick, otherwise it will pass through) wavelengths; 5.0μm is used to measure the surface temperature of the glass; For example, 3.43μm is used for measuring polyethylene plastic film, 4.3μm or 7.9μm is used for polyester, and 8-14μm is used for thickness exceeding 0.4mm. For example, the narrow band 4.64μm is used to measure CO in the flame, and 4.47μm is used to measure NO2 in the flame.
4.5 Determining Response Time
The response time indicates the reaction speed of the infrared thermometer to the measured temperature change, which is defined as the time required to reach 95% of the energy of the final reading, which is related to the time constant of the photodetector, signal processing circuit and display system. The response time of Raytek's new infrared thermometer can reach 1ms. This is much faster than contact temperature measurement methods. If the moving speed of the target is very fast or when measuring a fast-heating target, a fast-response infrared thermometer should be selected, otherwise the sufficient signal response will not be achieved, and the measurement accuracy will be reduced. However, not all applications require a fast-response infrared thermometer. For static or target thermal processes where thermal inertia exists, the response time of the pyrometer can be relaxed. Therefore, the choice of the response time of the infrared thermometer should be adapted to the situation of the measured target. Determining the response time is mainly based on the moving speed of the target and the temperature change speed of the target. For static targets or target parameters in thermal inertia, or the speed of existing control equipment is limited, the response time of the thermometer can relax the requirements.
4.6 Signal processing function
In view of the difference between discrete processes (such as parts production) and continuous processes, infrared thermometers are required to have multi-signal processing functions (such as peak hold, valley hold, average value) to choose from, such as when measuring the temperature of the bottle on the conveyor belt, it is To use peak hold, the temperature output signal is sent to the controller. Otherwise the thermometer reads a lower temperature value between the bottles. If using peak hold, set the thermometer response time to be slightly longer than the time interval between bottles so that at least one bottle is always under measurement.
