There are many infrared thermometers on the market so let's be a bit dazed. So how to correctly and accurately choose infrared thermometer? When it is chosen, it not only meets actual needs, but also requires less money, while ensuring performance and quality. Nanjing Ningbo Analytical Instrument Co., Ltd., based on experience, concluded that the selection of a suitable infrared thermometer should start with the following aspects: First, it is necessary to determine the requirements of the measurement target, consider the temperature and size of the target to be measured, the measurement distance, and the material of the target to be measured. , The target's environment; Second, we must consider the instrument's response speed, measurement accuracy, whether it is on-site use or online monitoring; then select the infrared thermometer's performance, function and price, etc., to become collocation; Finally, properly consider the use of convenient Quality assurance and service issues such as sex, branding, maintenance and calibration. These requirements are as follows:

1. Measurement temperature range: The temperature measurement range is one of the most important performance indicators of infrared thermometers . Each type of thermometer has its own specific temperature measurement range. It is recommended that you select an appropriate range of infrared measurement according to the measurement requirements. Wen Yi . The measured temperature range must be considered accurate, comprehensive, neither too narrow, nor too wide, too wide temperature range, will reduce the accuracy of temperature measurement, the temperature is too high and expensive, economically a bit worthwhile; the temperature is too low can not fulfil requirements. According to the law of black body radiation, the change of radiation energy caused by temperature in the short wavelength band of the spectrum will exceed the change of radiation energy caused by the emissivity error. Therefore, it is better to use short wave as far as possible when measuring temperature. In general, the narrower the temperature measurement range is, the higher the output signal resolution of the monitoring temperature is and the accuracy and reliability are easily solved.

2 , measurement accuracy and minimum resolution: measurement accuracy and resolution are two different concepts, easy to mix. The measurement accuracy is the only indicator to ensure the accuracy of the measurement, and it is also a key indicator to determine the performance of the infrared thermometer. Resolution is the smallest measure of a particular temperature.

3 , emissivity: According to the customer's use of feedback information, when using infrared thermometers, measurement errors often occur, of which 50% of the situation, the emissivity is the culprit leading to error. Because the infrared thermometer is suitable for various occasions, the material and color of the surface of the measured object are different (especially various pipes in the HVAC system), and its ability to emit infrared energy is not the same. By adjusting the emissivity, measurement errors due to the material can be reduced. So whether the instrument has this function is very important.

4 , the target size: the spot size (spot size) , is the thermometer measuring point area. The further away you are from the target, the larger the spot size. Infrared thermometer based on the principle can be divided into monochrome thermometer and two-color thermometer ( radiometer colorimetric thermometer ) . For monochromatic thermometers, when measuring temperature, the measured target area should be full of thermometer field of view. It is recommended that the measured target size exceed 50% of the size of the field of view . If the target size is smaller than the field of view, background radiation energy will enter the thermometer to interfere with temperature readings, causing errors. Conversely, if the target is larger than the thermometer's field of view, the thermometer will not be affected by the background outside the measurement area. For colorimetric thermometers, the temperature is determined by the ratio of the radiant energy in two separate wavelength bands. Therefore, when the target to be measured is small and does not fill the field of view, smoke and dust on the measurement path, and the blockage of the radiation energy will not affect the measurement result. Even in the case of 95% energy reduction , the required accuracy of temperature measurement can still be guaranteed. For small, but moving, or vibrating targets, colorimetric thermometers are the choice because of the small diameter, flexibility, and ability to transmit optical radiation energy over curved, blocked, and folded channels. A target that is poor or near an electromagnetic field.

5, the distance coefficient ratio (D: S): the optical resolution i.e., the distance between the infrared thermometer to measure the ratio of the target and the spot diameter D of S. If you are away from measuring small diameter targets , you should choose a high ratio infrared thermometer. The distance coefficient than the higher, the higher the cost infrared thermometer. In order to obtain accurate temperature readings, the distance between the thermometer and the test target must be within a suitable range. If the pyrometer must be installed far away from the target due to environmental conditions, but also to measure a small target, you should choose a high-resolution optical thermometer. For a fixed-focus pyrometer, the minimum spot size is at the focal point of the optical system, and the spot near and far from the focal spot will increase. There are two distance coefficients. Therefore, in order to accurately measure temperature at a distance close to and away from the focus, the target size to be measured should be larger than the spot size at the focus, and the zoom thermometer has a minimum 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 is difficult to increase, which increases the cost of the instrument.

6. Wavelength range: The emissivity and surface characteristics of the target material determine the corresponding wavelength of the spectrometer's spectrum. For high reflectivity alloy materials, there is a low or varying emissivity. In the high-temperature area, the wavelength of the metal material is measured in the near-infrared range and can be selected from 0.8 to 1.0 μm . Other temperature zones can use 1.6μm , 2.2μm and 3.9μm . Since some materials are transparent at a certain wavelength, infrared energy penetrates these materials, and special wavelengths should be selected for this material. If the internal temperature of the glass is measured, select 1μm , 2.2μm and 3.9μm (the measured glass should be very thick, otherwise it will pass through ) ; the glass surface temperature should be 5μm ; the low temperature area should be 8 ～ 14μm . The measurement polyethylene plastic film selected 3.43μm, 4.3μm or polyester-based selection of 7.9μm, a thickness of more than 0.4mm selection of 8-14μm. For example, if the CO in the flame is measured with a narrow band of 4.64 μm , the NO2 in the flame is measured to be 4.47 μm .

7. Response time: The response time is the time required for the infrared thermometer to reach 95% of the final reading , indicating the response speed of the infrared thermometer to the measured temperature change. It is also related to the photodetector, signal processing circuit and display system. The time constant relates. The choice of the response time of the infrared thermometer must be compatible with the situation of the target to be measured. The response time is determined mainly based on the target's movement speed and the target's temperature change speed. If the target's moving speed is fast or the target of rapid heating is measured, a fast-response infrared thermometer should be selected, otherwise, a sufficient signal response cannot be achieved and the measurement accuracy will be reduced. However, not all applications require a fast-response infrared thermometer. When there is thermal inertia for a stationary or target thermal process, the response time can be relaxed.

8 , signal processing function: In view of the discrete process ( such as part production ) and the continuous process is different, so the infrared thermometer requires multiple signal processing functions ( such as peak hold, minimum hold, average ) available for selection, such as temperature measuring conveyor belt When the bottle is on, the peak hold is used and the temperature output signal is sent to the controller. Otherwise, the thermometer reads the lower temperature between the bottles. If you use the peak hold, set the thermometer response time slightly longer than the interval between bottles, so that at least one bottle is in the measurement.

9. Environmental conditions: The environmental conditions in which the thermometer is located have a great influence on the measurement results and should be considered and properly addressed. Otherwise, the accuracy of the temperature measurement may even be affected. When the ambient temperature is high, dust, smoke, and steam are present, accessories such as protective covers, water cooling, air cooling systems, and air purifiers can be used. These accessories can effectively solve the environmental impact and protect the thermometer to achieve accurate temperature measurement. When determining attachments, standardized services should be required as much as possible to reduce installation costs. Light colorimetric thermometers are the choice when smoke, dust, or other particles reduce the energy signal under noise, electromagnetic fields, vibration or inaccessible environmental conditions, or other harsh conditions.

In sealed or hazardous material applications ( such as containers or vacuum boxes ) , the pyrometer observes through the window; the material must have sufficient strength and pass the working wavelength range of the thermometer used; also determine if the operator is also Need to observe through the window, so choose the appropriate installation location and window material to avoid mutual influence. In cryogenic measurement applications, Ge or Si material is usually used as a window, not visible light, and the human eye cannot observe the target through the window. If the operator needs to pass through the window target, an optical material that transmits both infrared radiation and visible light, such as ZnSe or BaF2 , should be used as the window material. When the pyrometer is present in the working environment, an intrinsically safe infrared thermometer can be used to safely measure and monitor in a certain concentration of environment. In the case of harsh and complex environmental conditions, it is possible to select a system in which the temperature measuring head and the display are separated to facilitate installation and configuration.

Nanjing Ningbo Analysis Instrument Co., Ltd.
2016.09.22

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