Gas sensors can be divided into three major categories based on their working principles:
1. Gas sensors utilizing physical and chemical properties, such as semiconductor type (surface manipulation type, volume manipulation type, surface potential type), catalytic incineration type, solid thermal conductivity type, etc.
2. Gas sensors utilizing physical properties, such as thermal conductivity, optical drying, infrared absorption, etc.
3. Gas sensors utilizing electrochemical properties, such as constant potential electrolysis, galvanic cell, barrier ion electrode, fixed electrolyte, etc.
Based on the damage, we classify toxic and harmful gases into two categories: combustible gases and toxic gases. Because their nature and damage are different, their inspection methods are also different.
Combustible gases are commonly encountered as hazardous gases in industries such as petrochemicals. They mainly consist of organic gases such as alkanes and certain inorganic gases such as carbon monoxide.
The explosion of combustible gases requires certain conditions, namely: a certain concentration of combustible gas, a certain amount of oxygen, and a source of fire that can ignite them with heat. These are the three essential elements of explosion, and none of them are indispensable. In other words, any of these conditions will not lead to fire or explosion. When combustible gases (steam, dust) and oxygen mix and reach a certain concentration, they will explode when exposed to a fire source with a certain temperature. We refer to the concentration of combustible gases that can explode when exposed to a fire source as the explosion concentration limit, abbreviated as the explosion limit, which is usually expressed in%. In practice, this mixture does not necessarily explode at any mixing ratio and requires a concentration scale.
Wholesale price of handheld toxic and harmful gas detector: Explosion will not occur when the concentration of combustible gas is below LEL (minimum explosive limit) (lack of combustible gas concentration) and when its concentration is above UEL (maximum explosive limit) (lack of oxygen). The LEL and UEL of different combustible gases are also different, which should be taken very seriously when calibrating instruments. For safety reasons, we should usually announce an alarm when the concentration of combustible gas is at 10% and 20% of the LEL, where 10% LEL is referred to. Make a warning alert, while 20% LEL is called a risk alert. This is also the element of our combustible gas detector, also known as LEL detector.
Wholesale price of handheld toxic and harmful gas detector: It is required to clarify that the 100% displayed on the LEL detector is not the concentration of combustible gas reaching 100% of the gas volume, but reaching 100% of the LEL, which is equivalent to the lowest explosive lower limit of combustible gas. If it is methane, 100% LEL=4% volume concentration (VOL). In homework, measuring with LEL method is a commonly used catalytic incineration inspection instrument. Its principle is a double bridge (usually referred to as a Wheatstone bridge) inspection unit. A catalytic incineration substance is coated on a platinum wire bridge in between. Regardless of the flammable gas, as long as it 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 flammable gas, and the concentration of the flammable 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 obtained on the market, and there are now LEL/VOL integrated detectors available. The VOL combustible detector is particularly suitable for measuring the volume (VOL) concentration of combustible gases in oxygen deficient environments.
