Gas Detectors & Commonly used principles

A gas detector is a device which detects the presence of various gases within an area, usually as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down. A gas detector can also sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave the area. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.
Gas detectors can be used to detect combustable, flammable and toxic gases, and oxygen depletion.
Gas detectors come in two main types: portable devices and fixed gas detectors. The first is used to monitor the atmosphere around personnel and is worn on clothing or on a belt/harness. They can also be classified according to the operation mechanism (semiconductors, oxidation, catalytic, infrared, etc.).

Infrared spectroscopy  
Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure.
catalytic bead
The catalytic bead sensor consist of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a bridge circuit. One of the pellistors is impregnated with a special catalyst which promotes oxidation whilst the other is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation of a gas readily occurs at the catalysed bead . Passing combustible gas raises the temperature further which increases the resistance of the platinum coil in the catalysed bead, leading to an imbalance of the bridge. This output change is linear, for most gases, up to and beyond 100% LEl, response time is a few seconds to detect alarm levels (around 20% LEL), at least 12% oxygen by volume is needed for the oxidation. 

Electrochemical cell

This is a type of cell  that instead of being designed to produce power, is designed to produce a current that is precisely related to the amount of the target gas in the atmosphere. Measurement of the current gives a measure of the concentration of gas in the atmosphere. Essentially the electrochemical cell consists of a container, 2 electrodes, connection wires and an electrolyte - typically sulfuric acid. Carbon monoxide is oxidized at one electrode to carbon dioxide while oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of 5 years or greater).

Semiconductor

Thin wires of the tindioxide on an insulating ceramic base provide a sensor monitored by an integrated circuit. This sensing element needs to be heated to approximately 400 deg C in order to operate. Oxygen increases resistance of the tin dioxide, but carbon monoxide reduces resistance therefore by measurement of the resistance of the sensing element means a monitor can be made to trigger an alarm.

Metal oxide semiconductor Film