At present, the sensor optical methods for measuring PM2.5 include infrared and laser methods, and the laser is divided into turbidity method and particle counting (laser cutting) method.
1. Infrared and Turbidity:
Due to insufficient light intensity, infrared light can only be measured by turbidity method. The so-called turbidity method is that one side emits light and the other side receives light. The more turbid light in the air, the greater the energy lost, and the current air turbidity is determined. In fact, this method cannot accurately measure PM2.5. Even if the emission and receiving parts of the light are covered by the electrostatically adsorbed dust, it will directly lead to inaccurate measurements.
The sensors produced by this method can only be qualitatively measured (how much can be measured), and cannot be measured quantitatively (since the values ​​will float). What's more, this method can't distinguish the particle size of the particles, so the performance of using this kind of sensor will not be good. At present, this type of sensor has Sharp (the second generation is the same), Shenrong (this is the second generation of millet purifier use this), win-win and so on.
2. Laser method and particle counting method:
A lot of related papers are laser scattering methods, which do not measure turbidity directly. The common characteristic of this type of sensor is that it cannot be separated from the fan (or pumped) because the air cannot be measured if it does not flow. The suspended particles in the air, and through the mathematical model can be roughly calculated through the size of the sensor gas directly, the air flow, etc., through a complex mathematical algorithm, eventually get a more realistic PM2.5 value, this type of sensor is laser scattering Immunity to electrostatically adsorbed dust, of course, if the sensor is blocked with dust, it is naturally impossible to measure it.
The disadvantage is that the lifetime of the laser is short. If it is continuously operated, it will be basically a lifetime of more than one year. This is the lifetime that can be achieved after the optimization algorithm of the manufacturer, but it is sufficient for most occasions, and if it does not run continuously The life of the laser can also be longer.
3. Beta ray method
The beta ray meter uses the principle of Beta ray attenuation. The ambient air is drawn into the sampling tube by the sampling pump, discharged through the filtration membrane, and the particulates are deposited on the filter membrane. When the beta ray passes through the membrane that deposits the particles, the Beta ray energy Attenuation, through the determination of the amount of attenuation can be calculated particle concentration.
Beta ray particle monitoring instrument consists of PM10 sampling head, PM2.5 cutter, sample dynamic heating system, sampling pump and instrument host. The ambient air sample with a flow rate of 1 m3/h passes through the PM10 sampling head and the PM2.5 cutter to become a particulate sample gas that meets the technical requirements. In the sample dynamic heating system, the relative humidity of the sample gas is adjusted to below 35%. After the sample enters the instrument host, the particles are collected on the filter that can be replaced automatically. A Beta ray source and a Beta ray detector were installed on both sides of the filter in the instrument. As the sample collection progresses, more and more particles are collected on the filter, and the mass of the particles increases. At this time, the Beta ray detected by the Beta Detector will weaken accordingly. Because the Beta-ray detector's output signal can directly reflect the mass change of the particles, the instrument finally calculates the particle concentration during the sampling period by analyzing the particle mass value of the Beta-ray detector and combining the sample volume collected within the same time period. When equipped with a membrane dynamics measurement system, the instrument can accurately measure particulates that are volatilized during this process, allowing the final report data to be effectively compensated, which is close to a straight real value.
4. Micro-oscillation balance method
The TEOM micro-oscillation balance method uses an oscillating hollow conical tube in the mass sensor, with a replaceable filter attached at the oscillating end. The frequency of oscillation depends on the conical tube characteristics and its mass. When the sample gas flow passes through the filter membrane, the particulate matter is deposited on the filter membrane, the change of the filter membrane quality causes the oscillation frequency to change, and the mass of the particulate matter deposited on the membrane is calculated by the change of the oscillation frequency, and then the flow rate, the ambient temperature and the The air pressure calculates the mass concentration of the particulate mark during this period.
The micro-oscillation balance particle monitoring instrument consists of a PM10 sampling head, a PM2.5 cutter, a dynamic membrane filtration system, a sampling pump, and an instrument host. The flow rate of 1m3/h ambient air sample passes through the PM10 sampling head and the PM2.5 cutter and becomes a particulate sample gas that meets the technical requirements. The sample then enters the micro-oscillation balance monitor host equipped with a membrane dynamic measurement system (FDMS). The micro-oscillating balance sensor for measuring the mass of the sample in the main unit is a hollow cone with a fixed end and a filter at the other end. The tube, sample gas flow through the filter, particulate matter is collected on the filter. In operation, the hollow conical tube is in a state of oscillating oscillating. Its oscillating frequency changes with the mass of the particulate matter collected on the membrane. The instrument obtains the mass of the collected particulate by accurately measuring the frequency change, and then collects The concentration of the sample is calculated from the sample volume collected at these particles.
5, weight method
At present, the gravimetric method is mainly used for the determination of atmospheric particles in China. The principle is to use a sampler with certain cutting characteristics to extract a fixed volume of air at a constant speed, so that the PM2.5 and PM10 in the ambient air are trapped on the known quality of the filter, according to the difference in the quality of the filter before and after sampling. Sample volume, calculate the concentration of PM2.5 and PM10. It must be noted that the volume of the denominator in the unit of ug/m3 for the measurement of particulate matter should be the volume under standard conditions (0°C, 101.3 kPa), and the volume under measured temperature and pressure should be converted to the volume under standard conditions.
Each of these methods has its own advantages and disadvantages, and it is used on different occasions.
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