Explain the detection principle of gas detector in detail.
The gas detector is an instrument specially designed to detect the safe concentration of gas. Its working principle is mainly to convert the physical or chemical non-electrical signals collected by the gas sensor into electrical signals, and then rectify and filter the above electrical signals through external circuits, and control the corresponding modules through these processed signals to realize gas detection. However, the core of the gas detector is the built-in sensor components. According to the different gases detected, the detection technology principles are different, and the principles are mainly divided into the following six categories:
1) catalytic combustion principle:
The catalytic combustion sensor uses the thermal effect principle of catalytic combustion to form a measuring bridge. Under certain temperature conditions, combustible gas burns flameless on the surface of the carrier of the detection element and under the action of the catalyst, and the temperature of the carrier increases, and the resistance of the platinum wire passing through it also increases accordingly, so that the balance bridge loses balance and outputs an electrical signal proportional to the concentration of combustible gas. By measuring the resistance change of the platinum wire, the concentration of combustible gas can be known.
It is mainly used for the detection of combustible gas, with good linearity of output signal, reliable index, affordable price and no cross infection with other non-combustible gases.
2) infrared principle:
The infrared sensor continuously passes the gas to be measured through a container with a certain length and volume, and emits a beam of infrared light from the side of one of the two light-permeable end faces of the container. When the wavelength of the infrared sensor coincides with the absorption line of the gas to be measured, the infrared energy is absorbed, and the light intensity attenuation of infrared light after passing through the gas to be measured satisfies Lambert-Beer law. The greater the gas concentration, the greater the attenuation of light. At this time, the absorption of infrared rays is directly proportional to the concentration of light-absorbing substances, so the gas concentration can be measured by measuring the attenuation of infrared rays by gas.
The infrared gas sensor has the characteristics of long service life (3-5 years), high sensitivity, good stability, no toxicity, less interference from the environment and no dependence on oxygen, etc. The infrared gas sensor has high monitoring sensitivity, and can accurately distinguish even micro-PPB or low-concentration PPM gas. The measuring range is wide, generally, 100%VOL gas with high concentration can be analyzed, and 1ppb level low concentration analysis can also be performed.
3) Electrochemical principle:
Electrochemical sensor usually consists of three parts: electrode, electrolyte and semiconductor electrode are the core parts of the sensor, which are made of metal or semiconductor materials and can react chemically with gas molecules. Electrolyte is a conductive liquid, which can connect electrodes with semiconductors to form a complete circuit. Semiconductor is a special material, which can convert the current signal between electrode and electrolyte into digital signal, thus realizing the detection of gas concentration.
The working principle of electrochemical gas sensor is based on redox reaction. When gas molecules come into contact with the electrode surface, they will undergo redox reaction and generate current signals. This current signal can be transferred to the semiconductor through the electrolyte and then converted into a digital signal. The digital signal is proportional to the gas concentration, so the gas concentration can be determined by measuring the digital signal.
Mainly used for the detection of toxic gases, with high sensitivity, fast response, good reliability and long service life. It can detect a variety of gases, such as carbon monoxide, carbon dioxide, oxygen, nitrogen and so on. It is widely used in industry, medical care, environmental protection and other fields.
4)PID photoionization principle:
The principle of PID is that organic gas will ionize under the excitation of ultraviolet light source. PID uses a UV lamp, and the organic matter is ionized under the excitation of the UV lamp, and the ionized "fragments" have positive and negative charges, thus generating a current between the two electrodes. The detector amplifies the current, and the concentration of VOCs gas can be displayed through instruments and equipment.
It is mainly used in oil refining industry monitoring, emergency treatment of hazardous chemicals leakage, definition of leakage danger area, safety monitoring of oil tanks and gas stations, and monitoring of organic matter emission and purification efficiency.
5) Thermal conductivity principle:
The concentration of the measured gas is analyzed mainly by measuring the change of thermal conductivity of the mixed gas. Usually, the difference of thermal conductivity of thermal conductivity gas sensor is converted into the change of resistance through circuit. The traditional detection method is to send the gas to be measured into a gas chamber, and the center of the gas chamber is a thermistor, such as a thermistor, platinum wire or tungsten wire, which is heated to a certain temperature to convert the change of thermal conductivity of the mixed gas into the change of resistance of the thermistor, and the change of resistance can be easily and accurately measured.
6) semiconductor principle:
The semiconductor gas sensor is made by using the oxidation-reduction reaction of gas on the semiconductor surface to change the resistance value of the sensitive element. When the semiconductor device is heated to a stable state, when the gas contacts the surface of the semiconductor and is adsorbed, the adsorbed molecules first diffuse freely on the surface of the object, losing their motion energy, some molecules are evaporated, and the other remaining molecules are thermally decomposed and adsorbed on the surface of the object. When the work function of the semiconductor is less than the affinity of the adsorbed molecules, the adsorbed molecules will take away electrons from the device and become negative ion adsorption, and the surface of the semiconductor presents a charge layer.
