Monitoring the concentration of combustible gases is an important means to prevent fires and explosions caused by combustible gas leaks, and to avoid a large number of casualties and property losses. At present, the main directions of technological development in China's combustible gas detection industry are: extending service life; enhancing anti-interference performance to solve the problems of false alarms and missed alarms; and improving product intelligence, which can not only monitor combustible gas leaks but also monitor the product's own performance. ### Classification and Working Principle of Combustible Gas Alarms Combustible gas alarm instruments are mainly composed of four parts: detection element, alarm system, amplifier circuit and display. Their working principle mainly relies on thermal conductors, semiconductors and infrared absorption. In most cases, the alarm principle of the alarm instrument is catalytic combustion. The key point of the alarm principle of the instrument is the balanced bridge. One arm of the bridge is a thermosensitive element with a large amount of catalyst on its surface. When it comes into contact with combustible gases in the air, relevant signals will be generated. The alarm instrument detects the concentration of combustible gases in the air based on the change of signals. Once the concentration exceeds the standard value, an alarm will be issued through signal feedback. 1. **Catalytic Combustion Type** When the relevant ambient temperature is reached, if the air contains a certain concentration of combustible gas, the surface of the sensor element will interact with the combustible gas under the action of the catalyst, resulting in flameless combustion. In this state, the internal resistance of the sensor will increase, and personnel can know and judge the content and concentration of combustible gases in the air according to the specific changes of the resistance value. For this type of alarm instrument, the sensor is equipped with a detection probe and a reference probe during use. In order to prevent the probe from rusting under long-term use or the influence of relevant factors and becoming a combustion source, it is usually necessary to select high-quality probes and install a protective cover around the probe to ensure the safety of the probe in use and effectively perform temperature compensation for the alarm instrument. In addition to the two probes, the detection circuit of this type of sensor also includes resistors, etc. Its overall structure is relatively simple, does not cost a lot of money, has strong signal output capability, and is not interfered by temperature and other types of gases. However, this type of alarm instrument also has relevant disadvantages: its detection time is short, and it is not suitable for high-concentration combustible gases indoors and outdoors. At this stage, it is mainly used to detect whether gas leaks. 2. **Optical Interference Type** Different combustible gases can present different optical paths in the air, and optical paths can be used to detect and monitor combustible gases. This type of alarm instrument has obvious characteristics, is suitable for combustible gases of different concentrations, and is easy to calibrate during use. However, the concentration value displayed during the monitoring of combustible gas concentration is not highly accurate, so personnel cannot accurately judge the specific content of combustible gases in the air. In addition, this type of alarm instrument is affected by environmental factors, air pressure, etc., and the overall production cost is high. 3. **Thermal Conductivity Type** This type of alarm instrument mainly uses thermosensitive elements to monitor combustible gases. It heats the thermosensitive elements with the help of tungsten wires, circuits and other conditions to ensure that they are at the set temperature. If the thermal conductivity of combustible gases in the air is high, the thermosensitive elements will be affected, causing heat loss. At this time, the thermosensitive resistance value decreases. After corresponding processing and adjustment, the change of resistance value is converted into voltage change to effectively monitor combustible gases. The thermal conductivity type alarm instrument has low production cost, simple design, more convenient for personnel to use and operate, and has strong stability and durability. However, it is easily affected by temperature and other factors during monitoring, and the overall detection sensitivity is low. 4. **Electrochemical Type** Electrochemical alarm instruments are mainly composed of a sensing electrode and a counter electrode. A small opening allows gas to react with the sensor, and then a hydrophobic screen is used to build a protective layer, so that the gas can fully react with the sensing electrode to generate accurate signals. This alarm instrument mainly uses the principle of oxidation and reduction reactions. Therefore, when using this alarm instrument, it is necessary to select appropriate electrode materials according to the type of combustible gas to be detected to ensure that a reaction can occur between them. ### Detection Range of Combustible Gas Alarms - **Detection range of combustible gas alarm probes**: Generally, it is between 5 meters and 10 meters, including 5 meters indoors and 10 meters outdoors. That is to say, combustible gas detectors installed indoors should be installed every 5 meters, while fixed gas alarm instruments in open outdoor places should be installed every 10 meters. - **Measurement range of combustible gas alarms**: The measurement range of combustible gas alarms is within 0~100% LEL, which is used to monitor the change of combustible gases in the surrounding air within the range of 0~100% LEL. According to national regulations, the low alarm is generally set to alarm between 15% LEL and 25% LEL, and the high alarm is generally set to 50% LEL. Most of the current combustible gas alarms have a low alarm point set to 25% LEL. Here, 100% LEL refers to the explosive limit of the gas, which means that when the combustible gas alarm shows 100% LEL, it has just reached the lower explosive limit of the combustible gas. When the gas concentration measured by the detector exceeds 100% LEL of the measurement range, the display will flash and show the highest value. When the gas concentration returns to the measurement range, the display will return to normal working status. ### Which Six Gases Can a 6-in-1 Gas Detector Detect? The 6-in-1 gas detector mainly detects the following 6 gases: 1. **Carbon Monoxide** Carbon monoxide is a colorless, odorless and toxic gas, which is a product of incomplete combustion of carbon-containing substances. Carbon monoxide can cause serious harm to the human body. If inhaled in excessive amounts, it can lead to headaches, dizziness, nausea, vomiting, palpitations and even death. Therefore, the detection of carbon monoxide is very important. 2. **Carbon Dioxide** Carbon dioxide is a colorless, odorless and non-toxic gas. It is one of the combustion products and also an important product of biological respiration. However, high concentrations of carbon dioxide can also harm the human body, causing symptoms such as headaches, dizziness and nausea. Therefore, the detection of carbon dioxide is also very necessary. 3. **Nitrogen Oxides** Nitrogen oxides are atmospheric pollutants, including a variety of gases such as nitric oxide and nitrogen dioxide. Nitrogen oxides not only harm the human body but also affect the environment, such as acid rain. Therefore, the detection of nitrogen oxides is also very important. 4. **Oxygen** Oxygen is a colorless, odorless and non-toxic gas, which is essential for the respiration of organisms on Earth. However, if the oxygen content is too high, it can cause oxygen poisoning, so the detection of oxygen is also very necessary. 5. **Hydrogen** Hydrogen is a colorless, odorless, flammable and explosive gas, which is commonly used in industrial production and chemical reactions. If hydrogen leaks or its concentration is too high, it can cause serious harm to the human body, even leading to suffocation and death. Therefore, the detection of hydrogen is also very important. 6. **Flammable Gases** Flammable gases include alkane gases and other flammable and explosive gases, which are common dangerous goods. If flammable gases leak or their concentration is too high, they can cause dangerous situations such as combustion or explosion, seriously threatening personal safety and property safety. Therefore, the detection of flammable gases is also very important. **Article sources**: Juanzong, Yingrun Environmental Protection, Yiyuntian Technology