The Application of Gas Detectors in Industry
Application of Toxic and Harmful Gas Detectors in Industry
In reality, many of the gases encountered in safety and health are mixtures of organic and inorganic gases. Due to various reasons, our current understanding of toxic and harmful gases is still more focused on combustible gases, gases that can cause acute poisoning (such as hydrogen sulfide and cyanuric acid), as well as some common toxic gases (such as carbon monoxide), oxygen, and other detectors. Therefore, this article will first focus on introducing these types of detectors, And provide suggestions for the application of various toxic and harmful (inorganic/organic) gas detectors based on the current situation.
The classification of toxic and harmful gas detectors and the key component of the original gas detector are gas sensors.
Gas sensors can be divided into three categories in principle:
A) Gas sensors utilizing physical and chemical properties, such as semiconductor type (surface controlled, volume controlled, surface potential type), catalytic combustion type, solid thermal conductivity type, etc.
B) Gas sensors utilizing physical properties, such as thermal conductivity, optical interference, infrared absorption, etc.
C) Gas sensors utilizing electrochemical properties, such as constant potential electrolysis, Gavanni battery, membrane ion electrode, fixed electrolyte, etc.
According to the hazards, we classify toxic and harmful gases into two categories: combustible gases and toxic gases.
Due to their different properties and hazards, their detection methods also vary.
Combustible gas is the most common hazardous gas encountered in industrial settings such as petrochemical industry. It mainly consists of organic gases such as alkanes and certain inorganic gases such as carbon monoxide. The explosion of combustible gases must meet certain conditions, that is, a certain concentration of combustible gases, a certain amount of oxygen, and sufficient heat to ignite their ignition source. These are the three essential elements of explosion, and none of them are indispensable. In other words, the absence of any of these conditions will not cause a fire or explosion. When combustible gases (steam, dust) and oxygen are mixed and reach a certain concentration, an explosion will occur when encountering a fire source with a certain temperature. We call the concentration of combustible gas that explodes when it meets the fire source as the explosion concentration limit, referred to as the Flammability limit, which is generally expressed in%. In fact, this mixture does not necessarily explode at any mixing ratio and requires a concentration range.
Explosion will not occur when the concentration of combustible gas is below LEL (* low explosion limit) (insufficient combustible gas concentration) and when its concentration is above UEL (* high explosion limit) (insufficient oxygen). The LEL and UEL of different combustible gases are different (refer to the introduction in the eighth issue), which should be taken into account when calibrating the instrument. For the sake of * *, we should generally issue an alarm when the combustible gas concentration is 10% and 20% of LEL, where 10% LEL is called. Make a warning alarm, and 20% LEL is called a danger alarm. This is why we refer to combustible gas detectors as LEL detectors.
It should be noted that the 100% displayed on the LEL detector does not mean that the concentration of combustible gases reaches 100% of the gas volume, but rather reaches 100% of the LEL, which is equivalent to the minimum explosive limit of combustible gases. If it is methane, 100% LEL=4% volume concentration (VOL). In work, detectors that measure these gases using the LEL method are common catalytic combustion detectors. Its principle is a dual bridge (commonly referred to as a Wheatstone bridge) detection unit. One of these platinum wire bridges is coated with catalytic combustion substances. As long as any flammable gas can be ignited by the electrode, the resistance of the platinum wire bridge will change due to temperature changes. This resistance change is proportional to the concentration of the combustible gas. The concentration of the combustible gas can be calculated through the instrument's circuit system and microprocessor. Thermal conductivity VOL detectors that directly measure the volume concentration of combustible gases can also be found on the market, and there are already detectors that combine LEL/VOL. The VOL combustible detector is particularly suitable for measuring the volume (VOL) concentration of combustible gases in hypoxic (oxygen deficient) environments.
