Different types of gas sensors exist. Sensors that work with light or resistance sensors. In general, what technologies are used to build sensors?
The most commonly used gas sensor technologies are:
- MOS (Metal Oxide)
- IR (Infra Red)
- Photoionization detection
- Ultra sons
At the end of the day, all sensors are based around something causing the electrical properties of something else to change when exposed to the thing they are trying to detect.
I think they are generally split into two broad categories: Sensors which are based around some material which undergoes the change directly and sensors which detect a variation in some other medium which is interfered with by the target.
The first group use a material that reacts to the presence of the target, which will be different depending on the gas. It may involve heating it or running a current through it, etc. Generally these will degrade over time and need re-calibration as they age.
The second group use things like laser, light, infra-red, sound, etc and detect the changes the target causes to them when they pass through it. This could be the colour of the light or a change in an interference pattern, etc.
With-in each of those categories there are a wide range of different methods. Something designed to detect one specific thing has to be fairly specialized.
Due to cost, most of your commercially mass produced sensors use methods from the first group. For high precision laboratory equipment you are generally looking at methods from the second group.