My partner in crime and myself use a micro 4 gas. CO, LEL, H2S and O2. Really, only the O2 is relevant and in Cornwall IS very relevant, as quite a few mines have bad air where there is no ventilation.
I've used an oil lamp myself and found that to be pretty good, if it goes out, you need to get out. Also, you get pretty good at reading the flame and calibrating whay you observe with how you feel.
I have a bit of a reservation about gas detectors and would almost rather drop a lamp down a shaft and drop it anyway if the flame stayed lit.
A case in point. There is a very interesting mine which people don't go in due to ridiculously low oxygen, it is variable though. A while ago it was deemed an idea to send me down the shaft with the meter set to alarm at 17% (hanging off a bit of string) which is pretty high really. I was going down at a comfortable rate, 20.9, 10ft later 20.8, 10 ft later 20.7. All of a sudden, flashing lights and 17.5 or whatever it was. I shat myself and thought I was in a lot of poo. As I had no idea that humans can function "when a candle will barely burn". I sweat lots, I struggled with the changeover, I didn't panic, but it was not a nice experience. The psychological impact was a lot bigger than the reality. I think I was fine at 17%
A couple of weeks later, 2 chaps I know (who are a tad less careful than me) dropped it and had to come back up as the rope was too short. We had put the detector down there on a rope either side of the date and it was about 15.5%, getting into the territory where a lamp goes out. They said the ascent was fine.
Elfnsafety says 19.5% O2 is oxygen deficient (officially) and you can probably go a fair degree lower than that. Assuming there is no CO2 present (which makes things a LOT worse) people can get right down towards the 14% mark and still function. This is where common sense kick in and say "hang on a minute, this is bloody dangerous". If you are a big bloke who needs to burn some serious oxygen ascending a rope, you may run into trouble. You cannot afford to be going giddy/semi-concious on a rope. You also need a safe margin.
I know a few chaps who use the "fackinell I can't breathe" as a gas meter, and as far as I gather, the air generally doesn't go too much below 15%.
The meter will only tell you how much you are in the grey area you are. Owning a meter will show you how many holes you thought were fine are now in the grey area. You need to do some testing on yourself to find out what you are capable of in what circumstances.
There is very little information on the internet and that which there is, is often contradictory, vague between 10% (in REAL trouble) and 17% (low). One of the chaps we "go down holes with" is a Doc and is going to do a proper study of this. Arterial O2 levels vs atmospheric oxygen levels and then correlate that to degrees of exertion. Another mate is an olympic trainer, so hopefully we can come up with a fair test and a reasonable set of parameters and caveats. That will happen in due course. We have a few guinea pigs lined up for that, in a minimum of about 15%.
A lighter can be a pretty good indicator if you look how it responds in various levels of gas. The flame starts doing very odd things at low readings. Ths snag is that it won't alarm.
I've had a Drager 4 gas which was HUGE and a bloody Neotronics "I cost more to fix than the detector cost" lump. They were both huge units suited to pros. The 4 gas small things are ideal. I'd go for a personal O2 meter myself as you are unlikely to bump into LEL, H2S (beyond smell-parts-per-million) or CO. It's almost tempting to get a 2 year one that you ditch afterwards.
Lamps do go out if you knock them over.
Best of luck and be careful :thumbup: