Gas detector systems are mainly used to sense the presence of gas leak in an area and then take some action, such as automatically shut down the source of the gas, or sound an alarm to inform the people in the area where the gas leak is occurring. Common gas detectors used at homes sense potentially hazardous gas leaks such as the carbon monoxide and then sound an alarm to alert the people at home that hazardous gas has been detected so that doors and windows can be opened, the source of the gas can be shut if possible, and people are alerted early enough so that they can leave the area.
There are many life threating toxic gases, the common ones being carbon monoxide, carbon dioxide, methane, sulphur and many others. Early gas detectors in the 19th century were not sensitive and were not electronic devices and they relied on everyday experiences and on the detection of some chemical reactions. For example, canaries were used to detect the presence of harmful gasses. Although the canary is a songful bird, it stops singing and dies quickly in the presence of such gasses, alarming the people around to leave the area. Similarly, chemical papers that change colour in the presence of harmful gasses were used to detect the presence of such gasses.
Nowadays there are many types of gas sensors available. Some sensors are sensitive to one particular gas, while some others are sensitive to a number of gasses. Modern gas sensors are designed using various technologies such as electrochemical, infrared, semiconductor, ultrasonic, holographic and others.
Some of the commonly used gas sensor chips are described briefly in the next section.
Semiconductor Gas Sensors
The MQ series of gas sensors are highly popular and are used in most domestic and industrial gas detection applications. These are semiconductor sensors with the SNO2 as the sensitive material. The sensor has low conductance in clean air, but its conductivity rises when subjected to the gas to be sensed. The change of the conductivity is converted to an analog output signal through a simple circuit. The detection ranges of these sensors depend upon the type of sensor used.
As shown below, MQ series of gas sensors require two voltage inputs: the heater voltage (VH), and the circuit voltage (Vcc). The heater voltage is specified as 5V and it can be AC or DC. The circuit voltage is 5V DC and should be connected as shown in the figure. In most application the same 5V voltage is used to power both the heater and the internal sensor circuitry.
The manufacturer provides sensitivity curves showing the gas concentration against the ratio of the sensor resistance to the resistance in clean air.
This is a semiconductor based carbon monoxide sensor that can be used for sensing CO concentrations in the air. The sensor has high sensitivity and fast response and can detect CO gas concentrations from 20 to 2000ppm. The sensor provides an analog output in the form of resistance. The use of the sensor is simple and it requires a heater coil to be powered with 5V and a load resistance to be connected across its output terminals and to an ADC converter. The value read from the sensor is proportional to the CO concentration.
This semiconductor sensor can be used for sensing natural gasses, mainly the methane (CH4) concentrations in the air. The gas concentrations from 200 to 10000ppm can be detected by the sensor. As with the MQ-7, an internal heater is used and the sensor’s output is an analog voltage which is proportional to the ambient gas concentration. An ADC is required to read and process the sensor’s output. MQ-4 is commonly used in domestic and industrial flammable gas detection applications.
This sensor is used to detect liquefied petroleum gas (LPG) concentrations in the air, which is mainly composed of propane and butane. The sensitive material of the sensor is SnO2 whose conductivity changes when exposed to LPG gasses. The sensor can detect gas concentrations in the range from 200 to 10000ppm. As with the other sensors, an internal heater is used and the sensor provides an analog output voltage proportional to the ambient gas concentration.
This sensor can be used to detect hydrogen gas concentrations in the range 100-10000ppm. The operational principle of the sensor is same as the others where an internal heater is used and the analog output voltage is proportional to the ambient hydrogen gas concentration.