Principles and applications of toxic and harmful gas detectors

Principles and applications of toxic and harmful gas detectors

Principles and Applications of Toxic and Harmful Gas Detectors/Principles and Applications of Toxic and Harmful Gas Detectors The key component of a gas detector is the gas sensor. Gas sensors can be divided into three major categories in principle:

A) Gas sensors that utilize physical and chemical properties: such as semiconductor type (surface control type, volume control type, surface potential type), catalytic combustion type, solid thermal conductivity type, etc.

B) Gas sensors that utilize physical properties: such as thermal conduction type, optical interference type, infrared absorption type, etc.

C) Gas sensors that utilize electrochemical properties: such as constant potential electrolytic type, Galvanic cell type, diaphragm ion electrode type, fixed electrolyte type, etc.

According to the hazard, we divide toxic and harmful gases into two categories: flammable gases and toxic gases. Due to their different properties and hazards, their detection methods are also different.

Combustible gases are the most dangerous gases encountered in petrochemical and other industrial settings. They are mainly organic gases such as alkanes and certain inorganic gases: such as carbon monoxide. Certain conditions must be met for the explosion of combustible gases, that is: a certain concentration of combustible gases, a certain amount of oxygen and a source of fire with enough heat to ignite them. This is the three elements of explosion that are indispensable. That is to say, any one of the conditions is missing. Will not cause fire or explosion.

When combustible gas (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 flammable gas that explodes when encountering a fire source called the explosion concentration limit, or explosion limit for short, generally expressed in %.

In fact, this mixture will not explode at any mixing ratio but has a concentration range. An explosion will not occur when the flammable gas concentration is lower than the LEL (lower explosive limit) (insufficient flammable gas concentration) and when its concentration is higher than the UEL (upper explosive limit) (insufficient oxygen). The LEL and UEL of different combustible gases are different (see the introduction in the eighth issue). This should be paid attention to when calibrating the instrument.

For safety reasons, generally we should sound an alarm when the flammable gas concentration is between 10% and 20% of the LEL. Here, 10% LEL is called a warning alarm, and 20% LEL is called a danger alarm. This is why we call combustible gas detectors also called LEL detectors. It should be noted that the 100% displayed on the LEL detector does not mean that the concentration of the flammable gas reaches 100% of the gas volume, but reaches 100% of the LEL, which is equivalent to the lowest explosion limit of the flammable gas. If it is methane, 100 %LEL=4% volume concentration (VOL).