The flammable gas sensor is a gas-electric converter, its function is to change the content (ie concentration) of flammable gas in the air into an electrical signal, and then the single-chip microcomputer collects signals, data processing, and concentration display for alarm control. As a sensitive element to flammable gases, the sensor is one of the cores of various types (pocket, portable, fixed) of the instrument. Therefore, the selection of the sensor is very important.

Type of gas sensor

Research and development of gas sensors began in foreign countries from the 1930s. In the past, gas sensors were mainly used for the detection and alarm of gas, liquefied petroleum gas, natural gas, and gas in mines. At present, the types of gases to be detected are expanded from original reducing gases (H2, C4H10, CH4, etc.) to toxic gases (CO , NO2, H2S, NO, NH3, PH3, etc.). There are many types of gas sensors, which can be divided into three categories in principle:

(1) Gas sensors using physical and chemical properties: such as semiconductors, catalytic combustion, etc.

(2) Gas sensors using physical properties: such as thermal conductivity, light interference, infrared absorption, etc.

(3) Gas sensors using electrochemical properties: such as current type and potential type.

The following briefly introduces several gas sensors commonly used in the industry:

(1) Semiconductor gas sensor This type of sensor mainly uses semiconductor gas-sensing materials and works on the principle that the resistance, current or voltage of the gas-sensing element changes with the concentration of the gas. Because of its many advantages such as high sensitivity, fast response, strong output signal, strong durability, simple structure, and cheap price, this type of sensor has been widely used. At present, many countries in the world have carried out research on semiconductor gas-sensitive materials, and China has also invested a lot of funds and manpower in research and achieved results. This sensor has become one of the most widely used gas sensors in the world.

(2) Solid electrolyte gas sensor This is a type of sensor whose output is second only to semiconductor gas sensors. It uses a solid electrolyte material as the gas sensing element. The principle is that gas-sensitive materials generate ions when passing through a gas, forming an electromotive force, and the amount of barium U is used to measure the gas concentration. Due to its high conductivity, good sensitivity and selectivity, this sensor has been widely used, and has almost entered various fields such as petrochemical, environmental protection, and mining. Such as measuring H2S YST-Au-WO3, NH'4CaCO3 and so on. However, this kind of sensor has high manufacturing cost, limited detection gas range, and has advantages in the field of detecting environmental pollution.

(3) Contact combustion gas sensors These sensors can be divided into direct contact combustion and catalytic contact combustion gas sensors. Its working principle is: when the gas-sensitive material is energized, flammable gas is oxidized or burned under the action of a catalyst, and the heat generated raises the heating wire, thereby changing its resistance value, and measuring the change in resistance value to measure the gas concentration . The contact combustion gas sensor is very stable at ambient temperature and can detect most of the flammable gases in the explosion F limit. It is widely used in petrochemical factories, shipyards, mine tunnels, bathrooms, kitchens, etc. And alarm. These sensors can only measure flammable gases and are not sensitive to non-flammable gases. Within the lower limit of the gas explosion, the output is linear, only proportional to the gas concentration, and changes in temperature and humidity have little effect on its working state, good selectivity, accurate reflection, high accuracy, and good reproducibility. The disadvantage is that the catalyst has a limited life. When there is a sulfur-containing substance such as hydrogen sulfide in the mixture of flammable gas and air, there may be some solid substances attached to the surface of the catalytic element and blocked when the flameless catalytic combustion is performed. The micropores of the carrier cause slow response, sluggish response or poisoning, reducing sensitivity.

(4) Polymer gas sensors Gas sensors made of polymer gas-sensitive materials have been greatly developed in recent years. When a polymer gas-sensitive material encounters a specific gas, its physical properties such as resistance, dielectric constant, surface acoustic wave propagation speed and frequency, and material weight change. D21 polymer gas-sensitive material has easy operation, simple process, and normal temperature Good selectivity, low price, easy to be combined with microstructure sensors and surface acoustic wave devices, it plays an important role in the detection of toxic gases and food freshness. The polymer gas sensor has high sensitivity to specific gas molecules, good selectivity, and simple structure. It can be used at room temperature, which can make up for the shortcomings of other gas sensors.

(5) Electrochemical sensor This type of sensor is made of membrane electrode and electrolyte. The gas concentration signal decomposes the electrolyte into positive and negative charged ions, and transmits the signal through the electrode. Its advantages are: fast response speed, accuracy, good stability, quantitative detection, but short life (greater than or equal to two years). It is mainly suitable for the detection of toxic gases. At present, most of the international gas detection uses this type of sensor.

Selection of flammable gas sensors

Combustible gas detection alarms are mainly used in petroleum, chemical, metallurgy, oil depots, liquefied gas stations, paint spraying and other places where flammable smoke leaks are likely to occur. According to the requirements for the detection of combustible gas types by alarms, contact combustion combustible gas sensors and Semiconductor flammable gas sensor.

The retardation and poisoning of probes using contact combustion sensors are unavoidable problems. The retardation is that when a sulfur-containing substance such as hydrogen sulfide is contained in the mixture of combustible gas and air, it is possible that some solid substances adhere to the surface of the catalytic element and block the micropores of the carrier while burning without flames. Slow response, slow response, and reduced sensitivity. Although it is possible to recover the retarded sensor back to the fresh air environment to some extent, if it is exposed to such environment for a long time, its sensitivity will continue to decline, resulting in the sensor eventually losing its ability to detect smoke. Poisoning is that if the ambient air contains a substance such as silane, the sensor will cause irreversible poisoning of the catalytic element, so that the sensitivity is quickly lost. When these substances are suspected in the detection environment, the probe is often calibrated. Therefore, often calibration of the sensor is a way to ensure its accuracy. Generally, the range calibration of the sensor should be performed after two months of continuous use. This regular maintenance of the sensor virtually increases the workload of the staff and increases the maintenance cost of the alarm. Semiconductor combustible gas sensors include smoke sensors made of oxide semiconductor ceramic materials as sensitive bodies and smoke sensors made of single crystal semiconductor devices. It has the advantages of high sensitivity, fast response, small size, simple structure, convenient use, and low price. Therefore, it is widely used. The performance of a semiconductor flammable gas sensor mainly depends on its sensitivity, selectivity (immunity) and stability (lifetime).

After comparing the application characteristics of the above two types of combustible gas sensors, it is found that the advantages of semiconductor combustible gas sensors are more prominent: high sensitivity, fast response, good anti-interference, easy to use, cheap, and no probe retardation and poisoning. Low maintenance costs. Therefore, this design uses a semiconductor flammable gas sensor as the core of the flammable gas information collection part of the alarm. Among many semiconductor gas sensors, the MQ-2 smoke sensor was selected for this design. This type of sensor not only has the advantages of high sensitivity, fast response, strong anti-interference ability, long life, etc. of general semiconductor smoke sensors.