Why CO2 ?

Sorry i ment oxygen levels, not co2

They aren't cheap and while you may be able to get one on eBay, the sensor has a limited life, usually 2 years, so you could easily be buying a pup. For a monitor that has replaceable sensors, they cost about the same as a new disposable monitor.

http://www.gasmonitor-point.co.uk/ 
Very helpful and prices pretty good.

---

There are numerous gas monitor threads on AN.

For what its worth here is my twopennoth. I hasten to add that I accept no responsibility if you kill yourself acting on this advice.

If you want to be belt and braces buy a new 4 gas type monitor and get it calibrated as specified. (£350 upwards plus ongoing calibration costs).

I would strongly recommend you don't buy a used meter from (say) EBay.

You could buy a flame safety lamp (I've used ex GPO /BT Wolf and Eccles Lamps £30 - £50).

A fag lighter will do the job (will not light at 15 -16% O2).(£1).

Obviously don't use a naked flame in a potentially combustible atmosphere. More to the point don't enter any environment which you suspect might contain combustibles.

The best advice I have seen recently is to know how your body reacts to a lowered O2 environment and take it slowly.

If in doubt bail out!

I would also suggest that you read as many reputable sources you can find on the subject - knowledge is power (and might just keep you alive).

Thanks for your help, the gas monitor website looks a good idea will give them a call tomorrow, the disposable ones that just does oxygen levels are only about £100 and they aerial supposed to last for 2 years, I think that's good works out £50 a year, anyway I will be keeping away from eBay