Interesting stuff. It reminded me of Ham Jenks' take on Great Beam mine (Bugle).

Without talking about localities too much (as it is a sensitive one) the place we encountered CO2 had a sawmill nearby and they used to chuck the sawdust down shafts. This usually behaves itself but was clearly responsible for an epic amount of CO2 buildup.

I haven't thought about the chemistry too much, but there is probably a chemical pathway which results in the production or a carbonate as a a spanner in the works of the usual sulphide oxidation mechanism. It's pretty complicated to start with!

Bumping this thread as came across:

http://digital.library.unt.edu/ark:/67531/metadc12312/m1/1/ 

makes very interesting reading. Basically confirms what stuey is saying, that the most concerning effect of low barometric pressure is on how efficiently blood is oxygenated at different pressures.

Type "barometric" into the search box. Probably other copies on the net to suit your book reader.

Fantastic, thanks!

I have also realised that the CO2/low O2 thing is a lot more complicated than I initially thought. Hence the different symptoms at different concentrations and the huge variability.

given the wind direction also effects pressure would this have to be taken into account before entering workings. just thinking that if it can effect tidal range so might well effect the mines breathing as well

Plus how all of those things have interacted over time

(e.g. wind blowing for a certain direction over several days while pressure is rising)

Put it all together and you've got an incredibly complex problem that I don't think theres ever an easy answer to.

Other than always take your gas meter.

Low o2 is one thing, low o2 with variable amount of CO2 is another. It all adds up to a hugely variable problem.

A little knowledge is a dangerous thing. We've been in a hole with lowish air and a bit of CO2 several times and one day, we decided that there had been many fronts go across, the pressure was high and it would probably be good to go... It turned out we nearly died due to an alarming build up of CO2. Since we had no means of directly measuring CO2, it would remain a mystery as to what specific atmosphere to avoid.

That text is hugely informative and interesting and I am going to go after a hard copy. It really is the best information I have read on the matter (and I've read a fair bit). Again, there are some contradictions.

I've come to the conclusion that the only way to do it is by actual empirical data gained from direct experimentation.

My mate who is a doc and often accompanies me on our various dices with death showed an interest in doing some formal research into various atmospheres and blood oxygen saturation. That's the real deal. I gather it involves arterial blood sampling, which is meant to be quite uncomfortable.

I have no interest in nearly snuffing it again, however I am quite prepared to go up to the line with a mate with BA and a BA set myself with a diving mouthpiece. (you don't really want to be mucking about with a facemask when you are about to black out).

I am aware a few people are reading this thinking "this mad *&^&%% is going to kill himself and bring the whole hobby into disrepute". Believe me, this is not the case. I have just ignored detailed comments about the methods, equipment and how carefully we proceed in these conditions.

Anyway, it's a fascinating text about something which does have scope to make more light about borderline hypoxic atmospheres.

As with anything, it is important to know as much as you can in order to conserve as much energy as possible and retain a safe margin in which to act. My mission is to use as little effort with ropework so that if I get in a pickle, I have the most energy left in which to extract myself. With low air, it's important to be absolutely clear about what the get out point is with a safe margin. The idea being that the process which results in this information will involve heavy breathing apparatus gear and all kinds of meters, whereas the object of the exercise is to travel as light as possible in the mine with nothing more than a meter.

Here are some measurements taken from a disused colliery gas vent. (Not measured by me I have to add).

To clarify, these measurements are of what is coming out of the vent, they aren't taken in the mine. Nevertheless, interesting to see that a direct correlation seems to be there.

🔗Personal-Album-2571-Image-86456[linkphoto]Personal-Album-2571-Image-86456[/linkphoto][/link]