We get occasionally calls from customers worrying why their CO (carbon monoxide) alarm has not gone off when their house is smelling strongly of a gas leak from the gas supply system (eg if the pilot flame in a boiler has been extinguished).
A lady rung the other day, stating that the pilot light of her LPG heater had gone out, leaving unburned flammable gas leaking into the room. She had quickly identified the smell of the gas (due to the risk of explosion from flammable gases, the gas suppliers add odorant to their gas which adds a strong smell, allowing people to detect a gas leak swiftly). Our customer was worried, as she had expected that her CO alarm would also pick up this leak. This is, of course, a misconception. CO alarms only detect the poisonous, odourless by-products from burning processes called carbon monoxide (CO). CO gas is created when flammable gas is burned without enough oxygen. CO detectors cannot detect flammable gases.
Chemically, carbon monoxide and flammable gases from a piped or bottled gas supply are very different.
Carbon monoxide consist simply of two atoms: 1x carbon and 1x oxygen.
Flammable gases (methane, propane, butane being the most common) have larger structures:
methane (1 carbon, 4x hydrogen atoms)
propane (3x carbon, 8x hydrogen)
butane (4x carbon, 10x hydrogen)
If you wish to protect yourself against leaks of flammable, unburned gases, specialist gas detectors need to be purchased.