My chum insists on dragging the meter everywhere and only once has it sniffed H2S in an utterly nano amount in some very dry workings with rotting wood. It was within the nose limit.
Managed to get 5% lel on the bottom of a shaft filled with rubbish. So that is 5% of 5% or F/A to be more precise.
Only time I've got CO, was when I was using my meter to get the car through an MOT.
Oxygen is the key point.
As you know, it varies hugely and the 19.5% alarm point is pretty high.
I'm personally good if I take it very easy in 13%. I've also taken it very easy and been fine prussicking in 15% (although it was hard work). These figures may also be fine for other people, they may also have adverse symptoms.
There was one mine where we were itching to go in and everytime we put a gas meter down the shaft, it would go to about 15%. This was too low and we said "no way" and waited for the air to change, low pressures, icy weeks, etc, etc. Then a couple of other people "just did it" and reported it being fine. We did it and it was also fine.
The other weekend, the same chaps who were fine in 15% went into another mine and got "the crushing chest" symptoms in a big drive with no vent. The other day, more chaps went through there right to the end and were fine.
I read a load of reports from the old days and if in doubt, they used to leave lead acetate papers for a week or so, to detect H2S with oxygen, they just used to do it and do it slowly and carefully.
There has been a bit of a culture change recently, with the advent of multiple ropes, rope access techniques, gas meters, etc, etc, etc. I think that a lot of people are keeping themselves out of places, as the meter says no.
A while ago, I was in the back of a "usual trip" in Cornwall where there is known to be bad air. We had a lighter and edged along the drive gradually, whilst paying very careful attention to ourselves and all of a sudden, there was an invisible barrier where the lighter would no longer light. This would be around the 13% mark. However, I gather that you can stroll along there and be fine. (it does go and has been metered down to about 10% which is below GET OUT time for all people)
In answer to your question, a lot of trips, anything which mined sulphides and without decent vent will probably see your meter alarming for oxygen.
You can't make any assumptions about the mine air you will be breathing and you also can't make any assumptions as to how you would fare in it.
All of the above would have been done without meters and people would have bumped into any symtoms. It's also surprising how you get phantom affects by looking at a low meter reading. You think "I'm actually gasping here" when in actual fact it's psychosomatic.
Don't try and replicate any of my figures, for god's sake. It's unlikely you would snuff it, but I don't want any blood on my hands. :surrender:
Edit:- I did a fair amount of reading as to what I should realistically expose myself to without having too much problems. Altitude training/sickness is an interesting thing to read.
Essentially, you have a pressure exerted on your lung by air, a percentage of that is oxygen, so regardless of the composition of air, your lung sees a "partial pressure" of oxygen. Going into an atmosphere with deficent oxygen lowers this "partial pressure" by reducing the significance of the oxygen component.
If you go bloody high up, you reduce the air pressure to half it's value (can't remember which height), the partial pressure of oxygen is reduced to half it's value. This roughly equates to 11% at Sea Level pressure.
Again, there is a lot of bullshit written about this sort of thing, if you consider a decent stormy low pressure of about 900mb and then a high of say 1050mb, you're looking at a 10% difference in the partial pressure of oxygen, so again, there is an actual range in which everyone operates anyway. Clearly, how this is realised in your blood is dependent on a load of other factors.
There is lots written about altitude effects, but virtually nothing about oxygen deficient atmospheres. Just a load of dogma that if you go below 17%, you'll probably act drunk and then cause an avalanche. It comes from the same people who think you need 2 ropes to do everything.