Gas sensors are small in size, large systems in function, that we encounter all over our lives. In the kitchen of our houses, in the tea shops of our workplaces, in the closed car parks of shopping malls, in the industrial facilities where we work, in short, we are in danger of gas leakage in every aspect of our lives. These gases threaten our safety of life and property with their explosive or toxic properties. For this reason, it is necessary to take the necessary measures in places where there is a danger of gas leakage. The use of gas detectors is one of the measures to be taken against gas leakage. Gas detectors detect the presence of the gas in question and activate the necessary automations. For example, when the carbon monoxide gas detector in closed car parks detects, it allows jet fans to be operated. Inside the gas detectors are sensors that perform the detection function. These sensors vary according to the characteristics and sensing principle of the gas to be detected. Frequently used gas sensors are listed below.
The structure of the catalytic sensors, which are generally used for the detection of explosive gases, includes two tiny ceramic spheres called pelistors. Inside the pelistor is platinum wire wrapped in a coil, and on it there is a layer of catalyst. When the explosive gas reaches the sensor, the catalyst reacts with the layer and oxidation occurs. As a result, the temperature of the pelistor increases. With the increase in pelistor temperature, the electrical resistance of the platinum wire coil inside increases. Only one of the pelists in the structure of catalytic sensors has a catalyst layer on it. The other pelistor acts as a reference. Thanks to the pelistors connected in the form of wheatstone bridge, the increase in ambient temperature does not lead to misconception. If the ambient temperature increases, the temperature of both pelists increases and no signal is formed on the sensor. However, when explosive gas is detected, only one pelistor heats up and the sensor generates a signal. Catalytic sensors can detect explosive gases up to 100% LEL in a healthy way. However, it is not recommended that catalytic sensors be exposed to high gas concentrations for a long time. Because in environments with high gas concentrations, oxygen levels are low and oxidation may not occur in the pelistor. In this case, the sensor can lose sensitivity and make incorrect measurements. The average lifespan of catalytic sensors under normal operating conditions is 5 years. Approximately 5% to 10% loss of sensitivity occurs each year. The life and sensitivity of the sensor may vary depending on the level and frequency of exposure to explosive gas.
Infrared (IR) Sensor
One of the most commonly used sensors for the detection of explosive gases is Infrared (IR) sensors. Infrared sensors are often used for the detection of hydrocarbon gases. Because infrared sensors perform their detection due to their structure, hydrocarbon gases use the ability to change the wavelength of Infrared rays. In general, the structure of Infrared sensors is as follows. The IR light source sends a beam to the optical receiver. The hydrocarbon gas that enters the sensor absorbs the IR beam from the light source. When the damping difference between the reference beam and the beam sent for detection reaches the threshold set as the alarm level, the sensor sends a signal. IR sensors are resistant to changes in ambient temperature, humidity, high concentration of explosive gases and blocking gases such as hydrogen sulfide. In environments with low or high oxygen concentrations, IR sensors can make reliable measurements. However, the same does not apply to catalytic sensors. In addition, catalytic sensors normally have an average lifespan of 5 years, while IR sensors have a much greater lifespan. Routine calibration of IR sensors is performed once a year on average, while catalytic sensors need to be calibrated over an average period of 6 months.
Electrochemical sensors are often used to detect toxic gases and oxygen, especially carbon monoxide, ammonia and chlorine. In short, there are two or three electrodes in the structure of electrochemical sensors. Carbon monoxide gas in the environment passes through the membrane and reaches the electrodes. With carbon monoxide gas, electrodes react chemically and electron current is formed. As the gas concentration increases, so increases the current. The detector sends an alarm signal based on the level of current generated in the sensor. Semiconductor Sensor Semiconductor sensors are often used in home applications. It is not recommended to use it in industrial plants because it can be alarmed by many different gases. In addition, semiconductor sensors that do not work consistently in fresh air can sound alarms even with water vapor. For this reason, semiconductor sensors are not preferred much today. Inside the semiconductor sensor is a surface of metal oxide heated between 300°C-400°C with heating resistance. In normal condition, oxygen and metal oxide in the air react to the surface, allowing the surface to reach a certain level of semiconductivity. When a gas leak occurs in the environment, oxygen molecules and leaking gas molecules change the semiconductivity level of the surface. The relationship between the sensor's output signal and gas concentration is logarithmic.
Sensor with Thermal Conductor
Thermal conductive sensors detect according to the principle of comparing the thermal conductivity of the gas to be detected by the air in the environment. The heated thermist or platinum wire inside the sensor acts as a reference. If the thermal conductivity of the gas in the environment is greater than the reference, the temperature of the gas decreases. If the thermal conductivity of the gas in the environment is less than the reference, the temperature of the gas rises. Heat changes change the equivalent resistance of the electrical circuit inside the sensor. In this way, the sensor detects the gas in the environment. In addition, the MQ-4 sensor, which is frequently used with arduino, has the following series;
For Flammable Gases
- MQ- 2 Gas Sensor : It is a sensor that detects methane, butane, LPG and cigarette smoke.
- MQ- 3 Gas Sensor : It is a sensor that detects alcohol, ethanol and cigarette smoke.
- MQ- 4 Gas Sensor : It is a sensor that detects methane and CNG gas.
- MQ- 5 Gas Sensor : It is a sensor that detects Natural Gas and LPG.
- MQ- 6 Gas Sensor : It is a sensor that detects LPG and Butane gas.
- MQ- 7 Gas Sensor : It is a sensor that detects carbon monoxide gas.
- MQ- 8 Gas Sensor : It's a sensor that detects hydrogen gas.
- MQ- 9 Gas Sensor : It is a sensor that detects carbon monoxide and flammable gases.
For Flammable and Normal Gases
- MQ- 131 Gas Sensor : It's a sensor that detects ozone.
- MQ- 135 Gas Sensor : It is a sensor that detects air quality.
- MQ- 136 Gas Sensor : Hydrogen is a sensor that detects sulfur gas.
- MQ- 137 Gas Sensor : It's a sensor that detects ammonia.
- MQ- 138 Gas Sensor : Benzene is a sensor that detects Toluene, Alcohol, Acetone, Propane, Formaldehyde and Hydrogen gas.
- MQ- 214 Gas Sensor : It is a sensor that detects methane and natural gas.
- MQ- 216 Gas Sensor : It is a sensor that detects natural gas and coal gas.
For Flammable Gases
- MQ- 303A Gas Sensor : It is a sensor that detects alcohol, ethanol and cigarette smoke.
- MQ- 306A Gas Sensor : LPG and cigarettes are sensors that detect butane gas.
- MQ- 307A Gas Sensor : It is a sensor that detects carbon monoxide gas.
- MQ- 309A Gas Sensor : It is a sensor that detects carbon monoxide and flammable gases.
For Carbon Dioxide
- MG 811 Gas Sensor : It's a sensor that detects carbon dioxide gas.
For Air Sensor
- AQ-104 Gas Sensor : It is a sensor that detects air quality.
- AQ-2 Gas Sensor : It is a sensor that detects flammable gases and cigarette smoke.
- AQ-3 Gas Sensor : It is a sensor that detects alcohol and gasoline.
- AQ-7 Gas Sensor : It is a sensor that detects carbon monoxide gas.
As a result, the use of gas detectors is one of the measures we can take against the destructive effects of explosive and toxic gases. Inside the gas detectors are sensors that perform the detection process. These sensors vary according to the characteristics and sensing principle of the gas to be detected. When choosing a gas detector, it should be checked whether the sensor in the detector is suitable for the gas to be detected.