Since the beginning of winter, haze weather in the north has occurred frequently, people pay more and more attention to air quality, and air products are again in the rush to buy. In addition to the hot sales of masks and air purifiers, the pM2.5 detector, which leads the intelligent hardware industry in the past two years, has also continued to be popular, including handheld, box and mobile phone models, with prices ranging from hundreds to thousands of yuan. Are the results of many pM2.5 detectors accurate? Is the pM2.5 detector that costs hundreds of thousands of yuan reliable?
In fact, we have already introduced the core components of the pM2.5 detector in the article "Laser pM2.5 detector is better than infrared detector". The key to the accuracy of the detection results is which sensor is used in the core components. For the moment, the detection results of the home pM2.5 detector using laser pM2.5 sensor will be more accurate.
Of course, there is a certain distance between the domestic pM2.5 detector and the industrial pM2.5 detector in terms of data accuracy. The most authoritative detection method used in industrial pM2.5 detection is the weighing method. But this method has an obvious disadvantage: time lag. From the beginning of the measurement to the result, it will take several hours or even more than ten hours at the earliest. Obviously, the weighing method is not suitable for families, and the cost investment is huge, which is not affordable for ordinary families.
If a household air quality detector adopts this method and makes you wait for several hours or even ten hours, can you stand it? How do many household air quality detectors on the market detect particles? Today, Weiguo Technology will take you to learn how to use laser technology to measure dust.
Before introducing the sensor, we would like to thank Gustav Mi, a German physicist who discovered an optical phenomenon as early as 1908: when the diameter of particles in the atmosphere is equal to the wavelength of radiation, a scattering occurs, which is mainly caused by particles in the atmosphere, such as smoke, dust, small water droplets and aerosols, The degree of scattering is independent of wavelength, and the properties of photons will not change after scattering. This theory is called Mie scattering theory. It is this theory that provides a theoretical basis for our current use of optical methods to detect atmospheric particles.
According to Mie scattering theory, we use a 650 nm laser. Fine particles have good scattering at this wavelength, and 650 nm is in the red light region, which is visible light, easy to debug and observe the optical path. The laser emits directional light, and the diameter of the beam emitted is only a few tenths of a millimeter. After the beam enters the gas chamber, it is scattered by the dust particles in the atmosphere, and the scattered light shines in all directions. We place a photosensitive detector in the direction perpendicular to the beam to receive the scattered light. At the same time, in order to prevent the reflected light of the light beam on the inner wall of the gas chamber from causing interference, we have specially designed a light trap. This light trap is like a small black hole, which makes it difficult to shoot out after the light speed shoots in, thus greatly reducing the reflection interference of the light beam.
In fact, the intensity of the scattered light generated by each dust particle is very weak, that is, the electric pulse signal displayed on the photosensitive detector is very small. We must amplify it through a precise operational amplifier, so that the operation processing unit MCU can be easily identified.
After a large number of experiments, we found that the amplitude and width of the pulse are approximately proportional to the diameter of the dust particles, so we can judge the diameter of the dust particles according to the pulse.
The velocity of air flow in the air chamber can be designed as a fixed value, or basically considered as a fixed value, so can the number of pulses per unit time be related to the number of particles per unit volume in the air? For example, if a pulse with an amplitude of approximately 5 and a width of approximately 2 represents pm10 particles, and a pulse with an amplitude of approximately 3 and a width of approximately 1 represents pm2.5 particles, then record the number of occurrences of these two types of pulses per second. Have you calculated the concentrations of pm10 and pm2.5 in the air approximately? Clever you, believe in the following things and don't need me to elaborate.
In this era of haze, an air quality detection artifact that can detect dust in the air can help you understand the current air quality, detect the visible, and make dust nowhere to hide. Behind the elegant and low-key appearance, there is infinite potential.
As we expected, the sensor is configured for the air purifier to display the current air quality in real time. As a new means to deal with the increasingly serious haze, the visual display of the purification results is clear, which is all icing on the cake.
Finally, is the household pM2.5 detector reliable? Must be reliable! But you must ensure that you use an air radio. The intelligent air quality detector that Vigor Technology is committed to building can measure pM2.5, VOC, formaldehyde, carbon dioxide, temperature and humidity. One has the industrial gene of Hanwei Research Institute, and has focused on the research, development and manufacturing of gas sensors for 16 years. The air radio is more than just testing!
With the frequent occurrence of haze weather, people pay more and more attention to air quality, prompting some businesses to launch various "anti haze" products. In addition to the hot sales of masks and air purifiers, various pM2.5 testers have recently appeared on the market, including handheld, box and mobile phone models, with prices ranging from several hundred yuan to tens of thousands of yuan. Are the results of many pM2.5 testers accurate? Is the pM2.5 test instrument that you bought for hundreds of thousands of yuan reliable?
1、 Different working principles
PM2.5, which is closely related to the formation of haze, has suddenly become a well-known concept, but few people know the principle and process of pM2.5 testing. There are three main methods to measure pM2.5: gravimetric method, β - ray absorption method and micro oscillation balance method. The detection of the tester should go through three procedures: cutter separation of air - "throwing off" excess particles - concentration detection. The cutter is used to separate the particles in the sampling air according to their diameters. Particles with diameters less than 2.5 μ m (0.0025 mm) will smoothly "break into" the concentration detection device, while large particles will be "thrown out".
Due to the difficulty of measurement, most professional pM2.5 testers are worth about 300000 yuan, or even more than a million yuan. At present, the price of simple testers on the market is mostly hundreds or thousands of yuan, but actually they can not accurately measure the content of pM2.5. These non professional instruments use the principle of light scattering to measure the concentration of particles. This method has not been adopted as a standard method by environmental protection departments in various countries, but there are professional instruments made according to this principle, which are also used in scientific research. The higher the concentration of particles in the air, the stronger the light scattering. The light scattering is relatively easy to measure. If you measure it, you can theoretically calculate the particle concentration.
But in practice, things are not so simple. The relationship between light scattering and particle concentration is very uncertain and is affected by many factors, such as the chemical composition, shape, specific gravity and particle size distribution of particles, which depend on the composition of pollution sources. This means that the conversion formula between light scattering and particle concentration may change at any time and anywhere, which requires instrument users to constantly use standard methods for correction. It is unlikely that amateurs without scientific training can do so.
Some researchers have made theoretical calculations: there is at least 30-40% uncertainty in measuring pM2.5 with light scattering instrument. This kind of uncertainty is inherent in this kind of instrument. This is true even for professional instruments with reliable quality, not to mention that the quality of instruments on the market is not always ideal.
2、 Test data is unreliable
Song Guangsheng, director of the National Indoor Vehicle Environment and Environmental Protection Product Quality Supervision and Inspection Center, also has doubts about the pM2.5 detector sold online. He believes that it is impossible to sell the detector for only a few hundred yuan. Generally, the detection machine used by the environmental detection center must undergo long-term inspection and comparison, and the data must be calibrated before it can be put into use, If ordinary people buy machines from the Internet, they also need to go through this process.
PM2.5 monitoring is a very delicate work, which requires professional instruments and technical means, and has a very strict quality control system. The online detector that purports to update data every 30 seconds or 5 minutes has no credibility. The concentration of pM2.5 in the air is generally at the level of one millionth (ppM), which is very low. Professional pM2.5 monitoring needs to continuously take particles in the air for one hour, so that the amount of particles accumulated is enough to facilitate accurate measurement. It is obviously impossible to achieve the amount of accurate monitoring with data of just five minutes.
Simple instruments sold on the market do not have separation devices. They can only measure one data roughly, but they are not professional testing instruments. They can only make a tentative quantity test or a trend prediction for air quality, but they are far from the concept of pM2.5.
3、 It is not meaningful for ordinary people to buy the pM2.5 tester
Is it necessary for ordinary citizens to buy pM2.5 testers with various patterns? The pM2.5 detector is a professional detector, which is of little significance to ordinary people. Because the price of professional detector is very expensive, this kind of detector appears in the market as a non professional detector. The citizen carries a non professional detector for detection, which can really tell people where there is pollution. But what the detector can detect, our nose can also detect. For example, someone has just smoked in the room, and your nose can tell you that someone has smoked as soon as you come in. For example, you can also sense that someone is sweeping the floor and blowing dust in some place, so it is not meaningful to use the pM2.5 detector.
In addition, the purpose of purchasing the pM2.5 detector is mainly to obtain the pM2.5 value. At present, there are 161 pM2.5 monitoring cities in China, and 177 more will be added by the end of this year. The Environmental Protection Bureau will update the pM2.5 data in real time every day, and ordinary people can query the accurate, open and transparent pM2.5 value through television, the Internet, mobile phones, etc., so it is not really meaningful to buy the pM2.5 detector.
