Right folks. I have a question, it is relating to an interesting trip and I'm trying to make sense out of observations.

A while ago, a couple of us got into trouble in a local mine where CO2 had built up. It had been concentrated to a degree which left us exhausted from breathing (around the 6% mark). This had been caused by the usual water facilitated chemical pathway not resulting in the removal of the gas as it had been dry.

I have wanted to have a go at Wheal Busy mine for a long time and there had been only a weekend of trips in there since the place closed. The last of the prior-to-us trip resulted in the last man up having a very close one with marginal air.

There is limited ventilation on the level coming from one open shaft, a sulphide lode and a lot of long tunnels.

I was humourously told not to take a gas meter or a Davy lamp as it would cause alarm, I gathered that it was not possible to light a lighter at the bottom of the shaft on the last trip.

I stayed up the top and the chaps went down with a gas meter and some air cylinders. Oddly enough, the oxygen reading was pretty high at 20.something% and ******! I didn't have my kit on me.

So, they went for a poke around (very small tunnels) and came back up. I gathered that the O2 had dropped to what read as 17% (and perhaps marginally lower) and the last man out was having energy problems on the rope.

I'm wondering why this was.

I did a few calculations based on tunnels of 60cm^2 and exhaled air being -4% O2 +4% CO2 which is a roughish figure. The results concluded that with 3 chaps going along the tunnel once and back, it would cause the general air do drop 1% O2 and increase 1% CO2. This isn't actually record breaking stuff and most certainly not sufficient to cause any real excitement.

NB:- The air pressure was steadily dropping through the day (only a nadge though).

So, I thought "what is responsible for the depletion of breathable air at the bottom of the shaft after a trip?" and I came to the next conclusion. CO2.

Since CO2 is a nadge heavier than air, it sinks. Clearly, where there is any ventilation or turbulance, it will either get exhausted, or kept diffuse. So, the squillion dollar question is "what is the rate of separation of CO2 from still air?" Is it significant?

I don't have any reference to this in any of my library and due to climatologists, if you type anything remotely scientific about CO2 and Sinks into Google, you get the global warming hysteria from the usual hand wringers.

What I'd like to do is answer the following question:-

"If you enter a shaft which is the only shaft into a mine with no ventilation which has good air in, does your breathed out air separate in the shaft, causing potential problems at the bottom of the shaft?"

I assume there is some maths related to this (possibly in a similar vein to radon) which excludes all the fluid flow stuff (static air here) and would give an indication about whether to rush in the shaft, or to consider timescales a bit more.

Over to you lot.

"If you enter a shaft which is the only shaft into a mine with no ventilation which has good air in, does your breathed out air separate in the shaft, causing potential problems at the bottom of the shaft?"

As CO2 is heavier than air and respiration is continually increasing the CO2 level at the expense of O2 I would perceive a problem, eventually.

Edit: The human lung is a CO2 generator, an unventilated shaft being somewhat like a sealed room?

If you took 3 people and put them down a shaft with 2m^2 cross section, if they are taking it steady, they will breathe out a fair amount. If you took 2 of them and put them up the same shaft, they'll breathe out a lot more. This could add up to quite a significant amount of CO2. (Won't somebody think of the children?)

I'm not sure whether a mixture of exhaled gas will separate (I suspect not). I expect that CO2 produced on the floor by rotting stuff will stay there. If you have a glass of CO2 (you can do this by mucking about with a cola bottle) you can pour it onto a candle and make it go out. However, this is already separated and the short distance means there are no fluid flow issues to diffuse the gas.

The situation at the bottom of the shaft is:-

1. Due to the fact that people have been sat at the bottom breathing like hell and have depleted the air to a significant degree and the problem continues upwards a tad due to diffusion.

2. The above, plus CO2 which has come down the shaft.

The more I think about it, the more it seems exceedingly unlikely that any separation does occur in the short term, but perhaps it may over a longer period of time. Given the attention to detail in the subject matter of science, it would follow that it's in an arcane fat book somewhere, or one of the nerds on here knows about it.

I suppose brownian motion is significant enough to keep it diffused to a degree.

The second to last man up could partially remedy the situation by having a weed sprayer filled with aqueous alkali!

If CO2 was present as a thin concentrated layer on the floor before entry it is quite possible that this could have been disturbed by the party's movements. We were taught to take special care in workings that may contain "blackdamp" - continuous sampling and take care not to stir up low lying pools or layers.

At 17% O2 the ability to think clearly can be affected without realising so. It was considered (in M & Q Act) that 1.25% CO2 in the general body rendered the ventilation inadequate. So far as a falling barometer goes it is unlikely to have been the cause of your troubles. We had an arrangement with the Met. Office to advise of falls steeper than 3mb/hr as rapid falls were the cause of outflows of gases from wastes/old workings rather than any separations from the general body.

Our close one with CO2 was due to the rotting of sawdust. It built up as a layer close to the floor. We felt fine after the initial panic and proceeded off along a tunnel, where there was more CO2 which we stirred up. As we went along the tunnel (down slight gradient) it got worse until we turned around and were in a really exciting level of it.

I gather the CO2 sensor in your body is around the aorta and if you drop into some high CO2 and breathe quickly (as a response to feeling like there is no air) you breathing runs away, however, if you take it steady for a moment, the CO2 will be dispersed around your blood stream which will return your breathing to more-normal, until the high CO2 causes your blood to further saturate until the detector goes off the scale again. (This is how I understand it). So, it's possible to drop into high CO2 and with a few breaths start hyperventilating, then getting it back under control before it goes off the scale again.

Needless to say, this mine with sawdust is going to shortly be sealed up, but the one in the topic of the conversation is merely poorly ventilated with this "phenomena" in the shaft.

I'm not particularly worried about low oxygen (apart from when I'm on a rope) as we've tested ourselves down to very low levels indeed and it's manageable. High CO2 is to be avoided at all costs.

I'd like to have a go at this same shaft in the near future (after a bit of time for it to air out-6 months or so) and I'd like to know more about CO2 in shafts, so we can minimise any likely close ones.