Measuring principle standard and development trend of infrared thermometer
There are many advantages to non-contact temperature measurement with an infrared thermometer, and its applications range from small or hard-to-reach objects to corrosive chemicals and sensitive surfaces. This article will discuss this advantage, give the decisiveness of the correct choice of infrared thermometer, etc. to illustrate the scope of application. Due to the movement of atoms and molecules, every object will radiate electromagnetic waves. The most important wavelength or spectral range for non-contact temperature measurement is 0.2 to 2.0 μm. Natural rays in this range are called thermal radiation or infrared rays.
A test instrument for temperature measurement by infrared rays radiated by a test object is called a radiation thermometer, a radiation thermometer or an infrared thermometer according to the German Industrial Standard DIN16160. These designations also apply to those instruments that measure temperature by visible colored radiation radiated by a body, and that derive temperature from relative spectral radiant densities.
First, the advantages of infrared thermometer temperature measurement
Non-contact temperature measurement by receiving infrared rays radiated from the object to be measured has many advantages. In this way, difficult-to-reach or moving objects can be measured without problems, such as materials with poor heat transfer properties or low heat capacity. The very short response time of the infrared thermometer enables fast and efficient regulation of the loop. Thermometers have no wearing parts, so there are no ongoing costs as there are with thermometers. Especially for small objects to be measured, such as contact measurement, there will be a large measurement error due to the thermal conductivity of the object. Here the thermometer can be used without any problems, and for aggressive chemicals or sensitive surfaces, such as on painted, paper and plastic rails. Through the long-distance remote control measurement, it can stay away from the dangerous area, so that the operator will not be in danger.
2. Principle structure of infrared thermometer
The infrared rays received from the measured object are focused on the detector through the lens through the filter. The detector generates a current or voltage signal proportional to the temperature through the integration of the radiation density of the measured object. In the electrical components connected thereafter, the temperature signal is linearized, the emissivity area is corrected, and converted into a standard output signal.
In principle, there are two types of portable thermometers and fixed thermometers. Therefore, when choosing a suitable infrared thermometer for different measurement points, the following characteristics will be the main ones:
1. Aimer
The collimator has this effect, and the measuring block or measuring point pointed by the thermometer can be seen, and the collimator can often be used for large-area measured objects. For small objects and long measuring distances, sights with instrument panel scales or laser pointing points in the form of light-transmitting lenses are recommended.
2. Lens
The lens determines the measured point of the pyrometer. For large-area objects, a pyrometer with a fixed focal length is generally sufficient. But when the measurement distance is far from the focus point, the image at the edge of the measurement point will be unclear. For this reason, it is better to use a zoom lens. Within the given zoom range, the thermometer can adjust the measurement distance. The latest thermometer has a zoomable replaceable lens. The near lens and the far lens can be rechecked without calibration. replace.
3. Sensors, i.e. spectral receivers
Temperature is inversely proportional to wavelength. At low object temperatures, sensors sensitive to long-wave spectral regions (hot-film sensors or pyroelectric sensors) are suitable, at high temperatures, short-wave sensitive sensors composed of germanium, silicon, indium-gallium, etc. will be used Photoelectric Sensors.
When selecting the spectral sensitivity, also consider the absorption bands for hydrogen and carbon dioxide. In a certain wavelength range, the so-called "atmospheric window", H2 and CO2 are almost transparent to infrared rays, so the light sensitivity of the thermometer must be within this range in order to exclude the influence of atmospheric concentration changes , when measuring thin films or glasses, it must also be considered that these materials are not easily penetrated within a certain wavelength. In order to avoid the measurement error caused by the background light, use a suitable sensor that only receives the surface temperature. Metals have this physical property, and the emissivity increases with the decrease of the wavelength. From experience, to measure the temperature of metals, generally choose * Short measurement wavelength.
3. Development trend
As in many sensing technology fields, the development trend of thermometers is also towards small, exquisite shapes, round shells with central threads are the most ideal shapes for installation on machines and equipment, and this development trend is Realization is through the continuous miniaturization of electrical components, and high calculus to make smaller and more delicate electrical components condensed in smaller and smaller spaces. Compared with the past analog technology, the precision of the linearization height of the detector signal is improved through the application of micro-controllers, thus also improving the accuracy of the instrument.
Market supply requires fast, inexpensive measurement value reception, which can directly output a temperature-proportional, linear current/voltage signal. Measurement value processing, such as leveling functions, special value storage, or boundary contacts will be placed in the intelligent On the display, regulator or SPS (program controller), the emissivity adjustment through the external cable can be adjusted outside the danger zone, even if the machine is running, and can also be adjusted by the SPS at this time. Through the use of body controls, the data bus interface can now be realized without any problems, but the network connection has not yet been realized, and the continued processing of the signal continues to use the standard analog signal of the past. In the detector section, a new material is used as a photoelectric sensor, which proves the improvement of sensitivity and even the improvement of resolution. In hot film sensors, new sensors require only shorter adjustment times, the latest developments in pyrometers with collimators, are interchangeable lenses with zoom, can be replaced without calibration rechecks, use the same basis for different measurement positions Instruments save warehouse management costs.
Fourth, the main criteria for selecting a thermometer
The use of the thermometer is mainly determined by the measurement range. Whether it is the measurement voltage or the initial value of the measurement area, it should be in line with the requirements of the measurement work. The larger the measurement voltage is, the smaller the resolution is, so the accuracy is higher. Especially when the initial value of the measurement temperature is low, the accuracy will be doubled if a large measurement voltage is selected, so it is recommended to select the smallest possible measurement voltage.
The initial value of the measurement area determines the sensitivity of the spectrum, as well as the type of detector. The measurement error is obviously smaller than that of the long-wave sensor in the short-wave sensor due to the wrong adjustment of the emissivity, so the hot-film sensor ( 8~14μm) at 800℃, the measurement error caused by the wrong adjustment of emissivity will be five times larger than that of the germanium-photodiode sensor (1,1~1,6μm). The permissible measuring range of the germanium photodiode sensor is from about 250° C.
