Personally id go for the BW Technologies "Gas Alert Microclip". Its a 4 gas (Oxygen, Carbon Monoxide, Hydrogen Sulphide, and combustibles). Ive used loads of detectors, made by loads of companies and over all that one is the best. Its not the cheapest out there but its reliable and its fairly light and only about the size of a 20 pack of fags (maybe even smaller). due to its size and weight it sits nicely on your collar (where these should really be worn - your not breathing the air at your waist). If you cant get that one I would definately go for a BW, ive used BW, Crowcon, Zellwegger, MSA, RAE, Drager and others that I cant think of right now. All of the BW ones I have used have out performed all others in my opinion