I remembered about who might be watching the forum and how excited they might get, plus, it was beyond my usual level of hyperbole!
FYI, Wellington goes down to 11% out the back and about 10.6% in the raises. There is most importantly, no CO2.
I was dragged to Hallenbeagle by some likely characters a few years ago as I used to run around with a gas meter and I had quite an exciting psychosomatic affect of the meter alarming at 17%. It is amazing what a bit of dogma can do.
I'll recall a couple of points:-
11.5%. Feeling a bit shaky, tired, sweaty at rest. Modest effort:- Feeling very shaky, face flushed. Moderate effort:- Feeling disorientated, judgement beginning to be impaired. Returning to rest:- Feeling uneasy, had to calm myself down and normal judgement returned in about 1 minute. At all times, my breathing was pretty normal.
You can attain the same level of nastiness in higher concentrations with more effort. Especially in a shaft. This is why I set my shaft concentration at an arbitrary 15%. This is what I consider and have successfully ascended carefully in 15%. I suspect there is a safety margin here and it would be possible to ascend very slowly and carefully around the 12% mark, but I wouldn't want to try it.
CO2. Those who know me will have heard the CO2 story. We dipped a shaft with my Davy Lamp (goes out at 15%- I've tested it in a bell jar with a gas meter) and it was good to go. All the factors were right, high air pressure, lots of weather fronts, had done several trips in there with no problems and so down we went. 120ft shaft, got off at 80ft into a level, felt the usual "it's about 16%ish", a bit hot and shaky but at 120ft we got off at the level and all of a sudden couldn't breathe. It was like having your head in a plastic bag. We stood still as to minimise our oxygen demand and normality returned. We progressed along the tunnel until I heard my chum (in front of me, breathing like an old dog) and thought "Enough, let's get out". As we turned around, we were panting away even more and when we got to the bottom of the shaft, we couldn't control our breathing. It was very very frightening. When the table on the link says "Exhaustion merely from the effort of breathing" they are not joking. It was the ascent from hell and we were very lucky to get out. If there is CO2, you cannot control your breathing and if the oxygen is low enough, you will black out. Thankfully, this was not the case, as the Davy lamp lit. I suspect the CO2 was around the 6% mark.
This would not have been picked up by your 4 gas H2S/LEL/CO/O2 meter, matches, lighter or Davy lamp.
CO2 is rare in Cornish mines, but not in this particular one where sawdust was dumped down shafts 80 years ago. Due to the lack of rain, the resulting chemical pathway which keeps the concentration down normally, there was an elevated concentration and we came across it.
Clearly, the usual "Progress slowly and if you start feeling "symptoms" turn around and progress out even more slowly" doesn't work as you have no choice but to breathe yourself to exhaustion in high CO2. Wherever people are breathing hard in what they might call low O2, it is due to excess CO2 in the bloodstream, which if not caused by metabolism, is caused by CO2 in the atmosphere. A few people in a very tight tunnel can put the CO2 up to a nasty level which is well documented in a certain set of experienced explorers anecdotes in the same mine. I ran the maths through, as it is possible to roughly calculate how many people you can get down a given hole before they get in trouble.
Anyway, this is off on a tangent to my bacteria-gunge question!
That gunge in the above photo is the same as Jane's deep adit, which I believe to be limonite which has been deposited from solution/suspension.
The stuff we came across was slime. Like gloopy wallpaper paste everywhere. It was like being in a film scene. Shame we didn't get any photos as I've seen nothing like it anywhere, or in any photos.