1. Confirm the type and concentration range of the gas to be detected.
Each production department encounters different kinds of gases. When selecting a gas detector, all possible situations should be considered. If methane and other less toxic alkanes are in the majority, LEL detector is undoubtedly the most suitable. This is not only because the LEL detector is simple in principle and widely used, but also has the characteristics of convenient maintenance and calibration. If there are toxic gases such as carbon monoxide and hydrogen sulfide, we should first choose a specific gas detector to ensure the safety of workers. If there are many toxic and harmful organic gases, and considering the low concentration that may cause personnel poisoning, such as aromatic hydrocarbon, halogenated hydrocarbon, ammonia (amine), ether, alcohol, fat, etc. You should choose the photoionization detector introduced in the previous chapter, and never use LEL detector to deal with it, because it may lead to casualties.
If the gas types cover the above gases, it may get twice the result with half the effort to choose a composite gas detector.
2. Determine the occasion of use
Choose different types of gas detectors according to different industrial environments.
1) fixed gas detector
This detector is widely used in industrial equipment and production process. It can be installed at specific detection points to detect specific gas leakage. Generally, the fixed detector is two-body. The detection head composed of sensor and transmitter is installed on the detection site as a whole, and the secondary instrument composed of circuit, power supply and display alarm device is installed in a safe place as a whole, which is convenient for monitoring. Its detection principle is the same as that described in the previous section, but it is more suitable for the continuous long-term stability required by fixed detection in technology and technology. It should also be selected according to the type and concentration of on-site gas, and installed at the position where specific gas leakage is most likely to occur, such as selecting the most effective height of sensor installation according to the specific gravity of gas.
2) Portable gas detector
Because of its simple operation and small size, portable instruments can be carried to different production sites. The electrochemical detector is powered by alkaline battery and can be used continuously for 1000 hours. The new LEL detectors, PID and composite instruments use rechargeable batteries (some of them have used memoryless nickel-hydrogen or lithium-ion batteries), so they can generally work continuously for nearly 12 hours, and are widely used in various factories and health departments.
If this instrument is used as a safety alarm in an open-air place, such as an open-air workshop, a portable diffusion gas detector can be used, because it can continuously and accurately display the concentration of toxic and harmful gases on site. Some of these new instruments are equipped with vibration alarm accessories-to avoid not hearing the sound alarm in noisy environment, and computer chips are installed to record the peak value, STEL( 15 minute short-term exposure level) and TWA(8-hour statistical weighted average)-to provide specific guidance for workers' health and safety.
If people enter confined spaces, such as reaction tanks, storage tanks or containers, sewers or other underground pipelines, underground facilities, agricultural closed granaries, railway tankers, ship cargo holds, tunnels and other workplaces, they must be tested before entering, and must be tested outside the confined space. At this time, it is necessary to choose a multi-gas detector with a built-in sampling pump. Because the gas distribution and gas types in different parts (upper, middle and lower) in a closed space are very different. For example, under normal circumstances, the specific gravity of combustible gas is light, and most of it is distributed in the upper part of closed space; Carbon monoxide has the same specific gravity as air and is generally distributed in the middle of a confined space. While heavier gases such as hydrogen sulfide exist in the lower part of the enclosed space. At the same time, oxygen concentration is also one of the types that must be detected. In addition, if the possible volatilization and leakage of organic matter in the tank are considered, a detector that can detect organic gas is needed. Therefore, a complete confined space gas detector should have built-in pumping function, so that it can detect in non-contact and different parts. It has a variety of gas detection functions-detecting dangerous gases with different spatial distributions, including inorganic gases and organic gases; With oxygen detection function-to prevent hypoxia or oxygen enrichment; Portable instrument with small volume and no influence on workers' work. Only in this way can the absolute safety of the staff entering the confined space be guaranteed.
In addition, after entering the confined space, the gas composition should be continuously detected to avoid the concentration change of volatile organic compounds or other toxic and harmful gases due to personnel entry, sudden leakage and temperature change.
If it is used for emergency, leak detection and patrol, a pumping instrument with short response time, high sensitivity and resolution should be used to determine the direction of the leakage point.
In industrial hygiene inspection and health investigation, it is very convenient to use instruments with data recording and statistical calculation functions and connect computers.