Four-gas Detection - The Inner Workings
Dangerous gases are put into three groups - toxic, suffocating, and combustible. But tracking of non-dangerous gases, such as oxygen is also necessary. Together, these are what is meant by "four-gas".
The idea behind the Four gas detector is to detect each of these. This gives the user both safety and ease of use, as they only need one monitor instead of four.
A standard four-gas detector has four sensors built in to one device.
The Electro-Chemical Sensors
An electro-chemical sensor is used to detect oxygen. The sensors work by sensing an electric discharge caused by the oxidation of a metal in the sensor. The most common metals are lead and gold.
Because these cells work by detecting oxidation, they will only work for a few years. Once the metal completely oxidizes they can no longer function.
The monitor will have two more electro-chemical cells built in to detect toxic and suffocating gases.
These cells work the same way as the oxygen sensor. Instead of metal they will have other material which when exposed to gas will cause a reaction that can be measured.
An example of this is carbon monoxide which mixes with water and releases carbon dioxide, hydrogen, and a small electric charge.
In theory you can make sensors of this variety work with any gas. Most devices on the market will detect Hydrogen Sulfide (a toxic gas) and Carbon Monoxide (an asphyxiating gas) with the second and third sensor.
Combustible Gas Sensors in a Four-gas Detector
The fourth sensor is the combustible gas sensor.
These sensors do not detect specific gases instead they can measure how flammable the gases in the air are.
The "LEL", or Lower Explosive Limit, shows combustibility. The LEL is the lowest limit at which the detected gases can explode. A 100 per cent LEL means an explosion is possible. Anything lower than 100 means there is not yet enough gas to explode.
To measure this a circuit gauges the change in electrical resistance in a circuit. This resistance will show the combustibility of the surrounding gas.
These sensors will not work in low oxygen environments, and because of this is because they require oxygen to work.
With these sensors in one package workers are able to detect major threats. In cases which need even more gas detection there are some Five Gas monitors on the market.