Discussion on the Selection Method of Infrared thermometer

Infrared temperature measurement technology has played an important role in product quality control and monitoring, equipment online fault diagnosis, safety protection and energy saving. In recent years, non-contact infrared thermometers have developed rapidly in technology, their performance has been continuously improved, their scope of application has also been continuously expanded, and their market share has increased year by year. Compared with contact temperature measurement methods, infrared temperature measurement has the advantages of fast response time, non-contact, safe use and long service life.  

Infrared Thermometer

working principle  

All objects with a temperature higher than absolute zero are constantly emitting infrared radiation energy to the surrounding space. The infrared radiation characteristics of an object—the magnitude of the radiation energy and its distribution by wavelength—has a very close relationship with its surface temperature. Therefore, by measuring the infrared energy radiated by the object itself, its surface temperature can be accurately determined, which is the basis for infrared radiation temperature measurement.  

When using an infrared radiation thermometer to measure the temperature of a target, it is first necessary to measure the infrared radiation of the target within its band range, and then the temperature of the measured target is calculated by the thermometer. Monochromatic pyrometers are proportional to the amount of radiation within a band; dual-color pyrometers are proportional to the ratio of the amount of radiation in the two bands.  

infrared system  

The infrared thermometer is composed of optical system, photoelectric Detector, signal amplifier, signal processing, display output and other parts. The optical system gathers the target infrared radiation energy in its field of view, and the size of the field of view is determined by the optical parts of the thermometer and its position. Infrared energy is focused on a photoDetector and converted into a corresponding electrical signal. The signal passes through the amplifier and signal processing circuit, and is converted into the temperature value of the measured target after being corrected according to the algorithm of the internal treatment of the instrument and the emissivity of the target.  

The selection of infrared thermometer can be divided into three aspects:  

(1) Performance indicators such as temperature range, spot size, working wavelength, measurement accuracy, response time, etc.; environmental and working conditions, such as ambient temperature, window, display and output, protection accessories, etc.; other options, such as ease of use , maintenance and calibration performance, and price, etc., also have a certain impact on the choice of thermometer.  

(2) Determine the temperature measurement range The temperature measurement range is an important performance index of the 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.  

(3) Determine the 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.  

Determining Optical Resolution (Distance Sensitive)  

The optical resolution is determined by the ratio D to S, which is the ratio of the distance D between the pyrometer to the target and the diameter S of the measurement spot. If the thermometer needs to be installed far away from the target due to environmental conditions, and a small target needs to 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.  

Determine the wavelength range  

The emissivity and surface properties of the target material determine the spectral response or 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 the wavelength of 0.18-1.0mm can be selected. Other temperature zones can choose 1.6mm, 2.2mm and 3.9mm wavelength. 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, the wavelengths of 1.0mm, 2.2mm and 3.9mm are used to measure the internal temperature of the glass (the glass to be tested must be very thick, otherwise it will pass through) wavelengths; the wavelength of 5.0mm is used to measure the internal temperature of the glass; For example, the wavelength of 3.43mm is used for measuring polyethylene plastic film, and the wavelength of 4.3mm or 7.9mm is used for polyester. If the thickness exceeds 0.4mm, the wavelength of 8-14mm is used; another example is to measure the CO2 in the flame with a narrow-band 4.24-4.3mm wavelength, measure the CO in the flame with a narrow-band 4.64mm wavelength, and measure the NO2 in the flame with a 4.47mm wavelength.  

determine 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. 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. But 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.  

environmental considerations  

The environmental conditions of the thermometer have a great influence on the measurement results, which should be considered and properly resolved, otherwise it will affect the temperature measurement accuracy and even cause damage to the thermometer. When the ambient temperature is too high and there is dust, smoke and steam, you can choose the protective cover, water cooling, air cooling system, air blower and other accessories provided by the manufacturer. These accessories can effectively address environmental influences and protect the thermometer for accurate temperature measurement. When specifying accessories, standardize service should be requested as much as possible to reduce installation costs. Two-color thermometers are a good choice when smoke, dust, or other particles reduce the measured energy signal. Under noise, electromagnetic field, vibration or inaccessible environmental conditions, or other harsh conditions, the fiber optic two-color thermometer is the best choice.  

In the case of harsh and complicated environmental conditions, a system with a separate temperature measuring head and display can be selected for easy installation and configuration. The signal output form matching the current control equipment can be selected.  

Calibration of Infrared Radiation Thermometer  

Infrared thermometers need to be calibrated to make it correctly display the temperature of the measured object. If the temperature measurement of the thermometer used is out of tolerance during use, it needs to be returned to the manufacturer or the repair center for re-calibration.