Gas sensor

Gas Sensor is a sensor device used for the detection and presence of a certain gas in an area and is mostly used for safety concern. This sensor, when detecting a gas, sends a signal to indicate the presence and concentration of the gas. When sensor response is beyond a certain set level, it activates the alarm to inform the users. Gas sensor is among the top priority in the industry as some gases are harmful and can cause dangerous effects on human bodies.

There are various types of gas sensors available on the market based on different technologies. Some frequently used sensors include infrared sensor, metal-oxide-semiconductor sensor, electrochemical sensor, catalytic sensor, chemresistor, acoustic method based sensor and sensor using photoionization method.

Infrared gas sensors use transmitters and receivers to detect combustible gases such as hydrocarbons vapors. The transmitters are light sources and receivers are light detectors. If a gas is present in an optical path, it will interfere with the light transmission between the transmitter and receiver and this interference results in detection of a gas. The most important example of infrared sensor is NDIR that stands for non-dispersive infrared and is used for the detection of CO2 gas. Non-dispersive is a method that all the light from light source passes through the gas sample in a chamber and is only filtered immediately before the light reaches the detector. In NDIR sensor, IR lamps send light source through the air to be detected at the IR detector at the other end. The difference between the light radiated by the lamp and the light absorbed by the detector is measured and the measured difference is directly proportional to the CO2 molecules in air inside the tube.

Electrochemical gas sensor is a sensor device to detect the type and concentration of a gas target by oxidizing or reducing the gas at the electrodes of electrochemical sensor. Electrochemical gas sensors detect gases like carbon monoxide, chlorine, nitric oxide and nitrogen dioxides etc. The electrodes are where the chemical reaction occurs and the current generated is correlated with the concentration of the target gas. These sensors are highly sensitive and specific devices.

Metal oxide semiconductors sensors are usually used for toxic gases and industrial gas detection. The sensor is fabricated with a gas sensitive film consisting of metal oxide, such as tin or tungsten oxides etc. The metal oxide film reacts with existing target gases and the resistance or conductance of the sensor changes upon detection. These sensors are usually low cost but not very sensitive compared to other methods such as electrochemical method. The detection limit of metal-oxide-semiconductor sensor is at part-per-million (ppm) level most of the time.

Catalytic sensor detects combustible gases mostly hydrocarbons and works by catalytic oxidation. These sensors are constructed with a platinum treated wire coil. When a combustible gas comes into contact with the catalytic surface, the target gas is oxidized changing the resistance of the metal wire. The resistance change is proportional to the concentration of target combustible gas. Catalytic sensor is also a low cost sensor and has been used widely in industrial application.

Acoustic sensor has also been used for commercial propose for a long time. Acoustic wave has the feature to propagate through or on the surface of a material, so any change to the characteristics of the propagation path of the waves affects the velocity or amplitude of the wave. The changes in velocity can be detected by measuring the frequency or phase characteristics of the sensor and can then be compared with the original status to see the difference, and if any difference is present in the recorded frequency it shows the detection of a gas.

Photoionization sensor is another widely used sensor for gas detection used in industrial applications. In this method, high-energy photons, which are typically in the vacuum ultraviolet (VUV) range, break molecules of the target gas into positively charged ions. The ions then produce an electric current used as a signal for the detector, and if signal beyond certain level an alarm sound could be triggered to notify the gas detection in the plant.

Chemresistor is basically a resistor showing response to certain gases. A typical example of chemiresistor is the use of polymer materials doped with carbon black to detect volatile organic compounds (VOCs). When the sensor is placed in an environment with target gases, the material on the sensor swells after absorbing target gases and this process triggers a change in electrical resistance. The resistance change reflects the concentration level of existing gases. Most time, a chemresistor is not enough to recognize target gases due to the low specificity. Sensor arrays of chemresistors could help solve the problem but it has to be optimized well and requires the help of advanced algorithms.

The gas sensors have wide applications starting from gas detection and hazardous gas monitoring in the industry to gas detection and monitoring in small commercial parking or service garages, to pharmaceutical and petrochemical facilities. When using a gas sensor, its calibration with the standard instrument is the most important thing. Calibration is basically a safety procedure that is performed to ensure that sensors are measuring the correct level of gas and its concentration. In addition, the life span of gas sensors also mostly depends on the amount of gas vapors to which they are exposed. Contaminated sensors may not register hazardous gas levels and this is why frequent calibration is very essential and to maintain the safety levels.

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