All objects with a temperature higher than absolute zero are constantly emitting infrared radiation energy to the surrounding space. Objects of infrared radiation characteristics of the size of the radiant energy and its distribution by wavelength and its surface temperature has a very close relationship. Therefore, through the measurement of the infrared energy radiated by the object itself, you can accurately determine its surface temperature, which is the objective basis for infrared radiation temperature measurement.
1. blackbody radiation law:
Blackbody is an idealized radiation body, it absorbs all wavelengths of radiant energy, there is no energy reflection and through the surface of the emissivity of 1. It should be pointed out that there is no real blackbody in nature, but in order to clarify and obtain the law of distribution of infrared radiation in theoretical studies must be selected to the appropriate model, which is Planck's quantum radiation of the body cavity. Proposed quantized vibronic model of the body cavity radiation, thus deriving Planck's law of blackbody radiation, that is, the blackbody spectral radiance expressed in wavelength, which is the starting point of all infrared radiation theory, so it is called the law of blackbody radiation.
2. The effect of object emissivity on radiometric thermometry:
Almost all actual objects existing in nature are not blackbodies. The amount of radiation of all real objects in addition to relying on the wavelength of radiation and the temperature of the object, but also with the type of material that constitutes the object, the preparation method, the thermal process, as well as the surface state and environmental conditions and other factors. Therefore, in order for the blackbody radiation law to apply to all real objects, a scaling factor, the emissivity, which is related to the nature of the material and the state of the surface, must be introduced. This coefficient expresses the closeness of the thermal radiation of the actual object to the blackbody radiation, and has a value between zero and a value less than one. According to the law of radiation, as long as we know the emissivity of the material, we know the infrared radiation characteristics of any object.
3. The main factors affecting emissivity are:
The type of material, surface roughness, physical and chemical structure, and material thickness.
When using infrared radiation thermometer to measure the temperature of the target first to measure the target in its band range of infrared radiation, and then by the thermometer to calculate the temperature of the target being measured. Single-color pyrometer and the amount of radiation in the band is proportional: two-color pyrometer and the ratio of the amount of radiation in the two bands is proportional.
4. Infrared system:
Infrared thermometer by the optical system, photodetector, signal amplifier and signal processing, display output and other components. Optical system convergence of its field of view of the target infrared radiation energy, the size of the field of view by the thermometer's optical components and their location. Infrared energy focused on the photodetector and transformed into a corresponding electrical signal. The signal through the amplifier and signal processing circuits, and in accordance with the algorithm of the instrument within the therapy and the target emissivity corrected for the temperature value of the target to be measured.
Eight, the choice of infrared thermometer can be divided into several aspects
Performance indicators, such as temperature range, spot size, working wavelength, measurement accuracy, response time, etc.; environment and working conditions, such as ambient temperature, window, display and output, protection accessories, etc.; other aspects of the choice, such as ease of use, maintenance and calibration of the performance and price, etc., but also the choice of the pyrometer Have a certain impact. With the technology and continuous development, infrared thermometer optimal design and new developments provide users with a variety of functions and multi-purpose instruments, expanding the choice.
1. Determine the temperature range:
The temperature range is one of the most important performance indicators of the pyrometer. Such as HT305 infrared thermometer product coverage for -50 ℃ ~ 1050 ℃, but this can not be completed by a model of infrared thermometer. Each type of thermometer has its own specific temperature measurement range. Therefore, the user's measured temperature range must be considered accurately and thoroughly, neither too narrow nor too wide. According to the law of black body radiation, in the short wavelengths of the spectrum caused by the temperature of the radiation energy change will exceed the emissivity error caused by the change in radiation energy, therefore, temperature measurement should try to choose the better short-wave.
2. Determine the target size:
Infrared thermometer according to the principle can be divided into monochrome pyrometer and two-color pyrometer (radiation colorimetry). For single-color pyrometer, in the temperature measurement, the measured target area should be filled with pyrometer field of view. It is recommended that the size of the target to be measured more than 50% of the size of the field of view is good. If the target size is smaller than the field of view, the background radiant energy will enter the pyrometer's visual acoustic symbols branch interference temperature readings, resulting in errors. On the contrary, if the target is larger than the pyrometer's field of view, the pyrometer will not be affected by the background outside the measurement area.
For HT305 infrared thermometer, its temperature is determined by the ratio of the radiant energy in two separate wavelength bands. Therefore, when the measured target is very small, not full of field, measurement path on the existence of smoke, dust, blocking the radiant energy attenuation, will not have an impact on the measurement results. Even in the case of energy attenuation of 95%, can still ensure the required temperature measurement accuracy. For the target is small, and in motion or vibration of the target; sometimes in the field of view of the movement, or may be partially out of the field of view of the target, under these conditions, the use of two-color pyrometer is the best choice. If the thermometer and the target can not be aimed directly between the measurement channel is curved, narrow, obstructed and so on, two-color fiber optic pyrometer is the best choice. This is because of its small diameter, flexible, can be bent, blocked and folded channel transmission of optical radiation energy, so you can measure the target is difficult to access, poor conditions or near the electromagnetic field.
3. Determine the optical resolution (distance and sensitivity)
Optical resolution is determined by the ratio of D to S, is the thermometer to the target between the distance D and the measurement of the spot diameter S ratio. If the pyrometer due to environmental conditions must be installed away from the target, but also to measure the small target, you should choose a high optical resolution of the pyrometer. The higher the optical resolution, that is, increasing the D: S ratio, the higher the cost of the pyrometer.
4. Determine the wavelength range:
The emissivity and surface properties of the target material determine the spectral response or wavelength of the pyrometer. For highly reflective alloy materials, there are low or variable emissivity. In the high-temperature region, the best wavelength to measure the metal material is near infrared, can choose 0.18-1.0μm wavelength. In other temperature zones, wavelengths of 1.6μm, 2.2μm and 3.9μm can be used. Because some materials at a certain wavelength is transparent, infrared energy will penetrate these materials, such materials should choose a special wavelength. Such as the measurement of glass internal temperature selection of 10μm, 2.2μm and 3.9μm (measured glass to be very thick, otherwise it will pass through) wavelength; measurement of glass internal temperature selection of 5.0μm wavelength; measurement of the low zone selection of 8-14μm wavelength is appropriate; and measurement of polyethylene plastic film selection of 3.43μm wavelength, polyvinyl acetate class selection of 4.3μm or 7.9μm wavelength. Thickness of more than 0.4mm choose 8-14μm wavelength; and such as measuring the flame in the C02 with a narrow band of 4.24-4.3μm wavelength, measuring the flame in the C0 with a narrow band of 4.64μm wavelength, measuring the flame in the N02 with a 4.47μm wavelength.
5. Determine the response time:
Response time indicates that the infrared thermometer on the measured temperature changes in response speed, defined as the time required to reach the final reading of 95% of the energy, which is related to the photoelectric detector, signal processing circuits and the display system time constant. Huatian electric power HT305 infrared thermometer response time of 250ms. this is much faster than the contact temperature measurement method. If the target's movement speed is very fast or measure the target of rapid heating, to choose a fast response infrared thermometer, otherwise it does not achieve sufficient signal response, will reduce the measurement accuracy. However, not all applications require fast response infrared thermometer. For the stationary or target thermal process when there is thermal inertia, the response time of the pyrometer can relax the requirements. Therefore, the choice of infrared thermometer response time and the target to be measured to adapt to the situation.
6. Signal processing capabilities:
Measurement of discrete processes (such as parts production) and continuous processes are different, the infrared thermometer is required to have signal processing capabilities (such as peak hold, valley hold, average value). Such as temperature measurement conveyor belt on the glass, we have to use the peak hold, the output signal of its temperature transmitted to the controller.
7. Consideration of environmental conditions:
The environmental conditions in which the pyrometer has a great impact on the measurement results should be considered, and appropriate solutions, otherwise it will affect the accuracy of the temperature measurement or even cause damage to the pyrometer. When the ambient temperature is too high, the presence of dust, smoke and steam conditions, you can choose the manufacturer to provide a protective cover, water cooling, air cooling system, air blowers and other accessories. These accessories can effectively solve the environmental impact and protect the pyrometer to achieve accurate temperature measurement. When identifying accessories, standardized service should be required whenever possible to reduce installation costs. When smoke, dust or other particles degrade the measured energy signal, a two-color pyrometer is the best choice. In noise, electromagnetic fields, vibration or difficult to access environmental conditions, or other harsh conditions, HT305 infrared thermometer is the best choice.
In sealed or hazardous material applications (e.g., containers or vacuum chambers), the pyrometer views through a window. The material must be strong enough to pass through the operating wavelength range of the pyrometer used. It is also necessary to determine whether the operator also needs to see through the window, so it is important to choose a suitable mounting position and window material to avoid interfering with each other. In cryogenic applications, the window is usually made of Ge or Si material, which is impervious to visible light, and the human eye cannot view the target through the window. If the operator needs to pass through the window target, should be used both infrared radiation and through the visible light through the optical material, such as should be used both infrared radiation and through the visible light through the optical material, such as ZnSe or BaF2, etc. as the window material.
8. Simple operation, easy to use:
Infrared thermometer should be intuitive, simple operation, easy to be used by the operator, which portable infrared thermometer is a set of temperature measurement and display output as one of the small, lightweight, carried by the person for the measurement of the temperature of the instrument, in the display panel can display the temperature and the output of a variety of temperature information, and some of which can be operated through the remote control or by computer software programs. Some can be operated by remote control or through a computer software program.
In the case of harsh and complex environmental conditions, you can choose to measure the temperature head and display a separate system, in order to facilitate the installation and configuration. Signal outputs can be selected to match current control equipment.
9. Infrared radiation thermometer calibration:
Infrared thermometer must be calibrated in order to make it correctly display the temperature of the target being measured. If the thermometer used is in use and the temperature measurement is out of specification, it will need to be returned to the manufacturer or service center for recalibration.
Reply by: Huatian Electric Power