Selecting an infrared temperature sensor is mainly considered in terms of performance indicators and environmental and operating conditions. The performance indicators include temperature range, spot size, working wavelength, measurement accuracy, response time, etc.
Environmental and working conditions include ambient temperature, windows, displays and outputs, protective accessories, and more. Others You can also make comprehensive comparisons of factors such as ease of use, maintenance and calibration, and price. With the development of infrared technology and continuous development, users have many options for infrared sensors.
First, determine the temperature range
The temperature measurement range is one of the most important performance indicators of the sensor. Each type of sensor has its own specific temperature measurement range. 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 of the radiant energy caused by temperature in the short wavelength band of the spectrum will exceed the change of the radiant energy caused by the emissivity error. Therefore, it is better to use the short wave as far as possible when measuring temperature.
Second, determine the target size
Infrared temperature sensors can be divided into monochrome temperature sensors and two-color temperature sensors according to the principle. For monochromatic temperature sensors, when measuring temperature, the measured target area should fill the sensor 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, the background radiation energy will enter the sensor's visual symbol and interfere with the temperature readings, causing errors. On the contrary, 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.
The two-color temperature sensor is determined by the ratio of the radiant energy in two separate wavelength bands. Therefore, when the measured target is small and there is no full scene, smoke and dust on the measurement path, and the attenuation of the radiation energy will not affect the measurement result, and some even in the case of 95% energy attenuation, Can still guarantee the required accuracy of temperature measurement. Targets that are small and are in motion or vibration; targets that are sometimes moving within the field of view, or that may be partially out of the field of view. In this condition, using a two-color temperature sensor is the best choice. If it is impossible to aim directly between the thermometer and the target, the two-color optical fiber temperature sensor is the best choice when the measurement channel is bent, narrow, or obstructed. This is due to its small diameter, flexibility, and ability to transmit optical radiation energy over bent, blocked, and folded channels, and therefore can measure targets that are difficult to access, have harsh conditions, or are near an electromagnetic field.
Third, determine the optical resolution
The optical resolution is determined by the ratio of D to S, which is the ratio of the distance D from the sensor to the target and the diameter S of the measurement spot. If the sensor must be installed far away from the target due to environmental conditions, and a small target is to be measured, a sensor with a high optical resolution should be selected. The higher the optical resolution, ie the increase in the D:S ratio, the higher the cost of the pyrometer.
Fourth, 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 are low or varying emissivities. In the high temperature region, the best wavelength for measuring metallic materials is near-infrared, and wavelengths of 0.18-1.0 μm can be selected. Other temperature zones are available with wavelengths of 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. Such as measuring the internal temperature of the glass 10μm, 2.2μm and 3.9μm (tested glass to be very thick, otherwise it will be through) wavelength; measuring the internal temperature of the glass selection of 5.0μm wavelength; low-zone selection of 8-14μm wavelength is appropriate; The wavelength of 3.43 μm is used for the measurement of polyethylene plastic film, and the wavelength of 4.3 μm or 7.9 μm is used for polyesters. Thickness of more than 0.4mm choose 8-14μm wavelength; Another example is to measure the flame in the C02 with a narrow band 4.24-4.3μm wavelength, measuring the flame in the C0 with a narrow band 4.64μm wavelength, measuring flame in the N02 with a wavelength of 4.47μm.
Fifth, determine the response time
The response time indicates the response speed of the infrared temperature sensor to the measured temperature change and is defined as the time required to reach 95% of the final reading. It is related to the time constant of the photodetector, the signal processing circuit and the display system. New infrared temperature sensor response time up to 1ms. This is much faster than contact temperature measurement. If the target is moving fast or when measuring fast heating targets, use a fast-response infrared temperature sensor. Otherwise, a sufficient signal response cannot be achieved, which will reduce the measurement accuracy. However, not all applications require a fast-response infrared temperature sensor. When there is thermal inertia for a stationary or target thermal process, the response time of the thermometer can be relaxed. Therefore, the selection of the response time of the infrared temperature sensor should be compatible with the situation of the target being measured.
Sixth, signal processing function
Different from discrete processes (such as part production) and continuous processes, infrared temperature sensors are required to have signal processing functions (such as peak hold, valley hold, and average). When measuring the temperature of the glass on the conveyor belt, peak hold is used, and the temperature output signal is sent to the controller.
VII. Consideration of environmental conditions
The environmental conditions of the temperature sensor have a great influence on the measurement result and should be taken into consideration and properly solved. Otherwise, the accuracy of the temperature measurement may even affect the damage of the thermometer. When the ambient temperature is too 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. Investigate smoke, dust, or other particles to lower the measured energy signal. A two-color temperature sensor is the best choice. Fiber optic two-color temperature sensors are the best choice for noise, electromagnetic fields, vibration or inaccessible environmental conditions, or other harsh conditions.
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