Gas sensors can be divided into three categories in principle
Gas sensors using 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. Gas sensors using physical properties: such as heat conduction, light interference, infrared absorption, etc. Gas sensors using electrochemical properties: such as constant potential electrolysis, Galvanic battery, diaphragm ion electrode, fixed electrolyte, etc. According to the hazards, 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 gas is the most dangerous gas encountered in petrochemical and other industrial occasions. It is mainly organic gases such as alkanes and some inorganic gases such as carbon monoxide. Combustible gas explosion must meet certain conditions, that is: a certain concentration of combustible gas, a certain amount of oxygen and enough heat to ignite their fire source, humidity sensor probe, stainless steel electric heating tube, PT100 sensor, fluid solenoid valve, cast aluminum heating The device and the heating ring are the three elements of explosion (the explosion triangle shown in the left figure above), and they are indispensable, that is to say, the lack of any one of these conditions will not cause fire and 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 combustible gas that explodes when it encounters a fire source as the explosion concentration limit, referred to as the explosion limit, and is generally expressed in %.
In fact, this mixture does not explode at any mixing ratio but has a concentration range. The shaded part is shown in the above right image. No explosion will occur when the concentration of combustible gas is below the LEL (Lower Explosive Limit) (insufficient concentration of combustible gas) and when its concentration is above the UEL (Upper Explosive Limit) (insufficient oxygen). The LEL and UEL of different combustible gases are different (see the introduction of the eighth issue), which should be paid attention to when calibrating the instrument. For the sake of safety, generally we should issue an alarm when the concentration of combustible gas is 10% and 20% of LEL, here, 10%LEL means. As a warning alarm, and 20% LEL as a hazard alarm. This is why we call the combustible gas detector also known as the LEL detector. 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 100% of the LEL, which is equivalent to the lower explosion limit of the flammable gas. If it is methane, 100% LEL=4% volume concentration (VOL). In the work, the detector that measures these gases by LEL is our common catalytic combustion detector.
Its principle is a two-way bridge (commonly known as Wheatstone bridge) detection unit. One of the platinum wire bridges is coated with catalytic combustion substances. No matter what kind of flammable gas, as long as it can be ignited by the electrodes, the resistance of the platinum wire bridge will change due to temperature changes. The concentration of combustible gas is in a certain proportion, and the concentration of combustible gas can be calculated through the circuit system and microprocessor of the instrument.
