BSL Physics Glossary - smoke/gas sensor - definition
When there is a fire a lot of gas particles are produced. The sensor detects these and turns the electricity supply on. It can be used to detect the presence of carbon monoxide gas (CO). Smoke alarms or gas alarms work on the same principle. In a fire many gas particles are created and trigger the gas sensor so the fire alarm sounds. In your central heating system there may be a fault on the gas pipe or in the boiler, so that carbon monoxide gas is escaping. The sensor detects the gas and the alarm is set off.