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Principles of Gas Sensors

The key component of a gas detector is a gas sensor. Gas sensors can be divided into three categories in principle:

  1. 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.
  2. Gas sensors using physical properties: such as thermal conductivity, light interference, infrared absorption, etc.
  3. Gas sensors using electrochemical properties: such as constant potential electrolytic type, Galvanic battery type, diaphragm ion electrode type, fixed electrolyte type, etc.

The principle of gas sensors: According to the hazards, we divide toxic and harmful gases into two major categories, flammable gases and toxic gases. Due to their different nature and hazards, their detection methods are also different.

  1. Combustible gas Combustible gas is the most dangerous gas encountered in industrial applications such as petrochemical industry. It is mainly organic gases such as alkanes and certain inorganic gases:

Such as carbon monoxide, there must be certain conditions for a flammable gas to explode, that is: a certain concentration of flammable gas, a certain amount of oxygen, and sufficient heat to ignite their ignition source. These are the three elements of the explosion (such as the explosion triangle shown in the left figure above). No, that is, the absence of any of these conditions will not cause fire and explosion. When combustible gas (steam, dust) and oxygen are mixed and reach a certain concentration, it will explode when it meets a fire source with a certain temperature.

We refer to the concentration of a flammable gas as it explodes when exposed to a fire source, referred to as the explosive concentration limit, referred to as the explosive limit, and is generally expressed in%. In fact, this mixture does not explode at any mixing ratio but requires a concentration range. The shaded area is shown in the figure on the right. Explosion does not occur when the concentration of flammable gas is lower than LEL (* low explosive limit) (the concentration of flammable gas is insufficient) and its concentration is higher than UEL (* high explosive limit) (low oxygen).

Different flammable gases have different LELs and UELs. This must be taken into account when calibrating the instrument. For the sake of safety, generally we should issue an alarm when the flammable gas concentration is 25% or less and 50% of the LEL. Here, 25% LEL is called the low limit alarm and 50% LEL is called the high limit alarm. This is why we call the combustible gas detector the LEL detector.

The principle of the gas sensor: It should be noted that 100% displayed on the LEL detector does not mean that the concentration of the flammable gas reaches 100% of the volume of the gas, but reaches 100% of the LEL, which is equivalent to the lowest low explosive limit of flammable gas In the case of methane, 100% LEL = 5% volume concentration (VOL). Detection of flammable gases can be made using semiconductor, catalytic combustion (anti-toxic), thermally conductive and infrared sensors.

  1. Toxic gas detection, at present, for the detection of specific toxic gases, we use the most dedicated gas sensors. There are three types of sensors for detecting poison gas: semiconductor type, electrochemical type and electrolytic cell type.

Semiconductor sensors such as the constant potential electrolytic type, Galvanic battery type, diaphragm ion electrode type, and fixed electrolytic type have high sensitivity and low resolution. Sensors of this principle have been almost eliminated, and users should be extremely careful when selecting such sensors. Electrolytic battery sensor solid electrolyte gas sensor uses solid electrolyte gas sensing material as the gas sensing element. The principle is that the gas-sensitive material generates ions when passing through the gas, thereby forming an electromotive force, and measuring the electromotive force to measure the gas concentration.

Due to its high conductivity, good sensitivity and selectivity, this sensor has been widely used, and has almost entered into various fields such as petrochemical, environmental protection, mining and other fields, second only to metal oxide semiconductor gas sensors. Such as measuring YST-Au-WO3 of H2S, measuring NH + 4CaCO3 of NH3 and so on, Electrochemical sensors are currently widely used sensors for detecting poisonous gases. It uses redox reactions to detect dozens of toxic gases through different electrolytes.

The principle of the gas sensor: According to the quality of the electrolyte, its life is generally 2 to 4 years. The composition of an electrochemical sensor is: two reaction electrodes, a working electrode and a counter electrode, and a reference electrode are placed in a specific electrolyte (as shown in the figure above), and then a sufficient voltage is applied between the reaction electrodes so that The redox reaction is performed through the gas to be measured coated with a heavy metal catalyst film, and then the current generated during the gas electrolysis is measured by the circuit system in the instrument, and then the microprocessor calculates the concentration of the gas.

At present, electrochemical sensors are widely used to detect inorganic toxic gases. Electrochemical sensors that can detect specific gases include: carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen monoxide, nitrogen dioxide, ammonia, chlorine, and cyanogen Acids, ethylene oxide, hydrogen chloride, etc.

  1. Detection of Volatile Organic Compounds There is also a photo-ionization detector (PID) for the detection of volatile organic compounds. It can measure organic toxic gases as low as ppm (and one ten thousandth) and Vapor concentration

PID can detect most volatile organic compounds (VOC). Simply put, PID can measure volatile organic compounds with carbon numbers from 1 (for example, CH2Cl2) to 10 (for example, naphthalene). PID can be used for the safety and health of various types of organic compound companies that use, produce, store, and transport. At the same time, it can also be used in various fields such as emergency accidents in the environmental protection industry, industrial health consultation, public security inspection, and chemical prevention.

  1. Oxygen detector Oxygen is also required in industrial environments, especially in closed environments. Generally, we call the oxygen content over 23.5% as excess oxygen (enriched oxygen), and it is easy to explode at this time; while the oxygen content below 19.5% is oxygen deficiency (hypoxia), at this time workers are prone to suffocation and coma. To death. The normal oxygen content should be around 20.9%. The oxygen detector is also a type of electrochemical sensor.
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