Looking at this table of altitude vs air pressure:-

http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html 

I had a few thoughts about gas detectors and how humans operate in various conditions. It also might shed some light on the apparent variability of bad air in mines.

A brief recap, if you take the total pressure of air to be Pt then Pt=Pn+Po+Pt where n=Nitrogen, o=Oxygen and t=trace gases, their "partial pressures"

Rather than concerning ourselves with the ratios of oxygen to the other stuff (our gas meter % reading) we need to consider the "partial pressure" of oxygen. The partial pressure of oxygen is the important factor for gas exchange at the lung surfaces. It also explains altitude sickness pretty well.

If you take your plane up to about 5000m, the air pressure is reduced (on average) to roughly half. This means that the partial pressures of the gases, particularly oxygen are also halved.

Here is the obvious point:-

At 5000m the partial pressure of oxygen is halved. This is equivalent to the partial pressure that would be exerted by an atmosphere containing half of the concentration of oxygen at ground level pressure. Or about 10.5% This corresponds to the level that you are in serious trouble. Either at ridiculous altitude or in an oxygen deficient atmosphere.

Armed with an oxygen meter, I've been wondering what levels of oxygen I can actually expose myself to without putting myself in serious danger. It's all very well having a number, but the actual advice about oxygen deficient atmospheres comes with some pretty hysterical warnings. Faced with possible death, elfansafety go utterly mad and the level of dogma "at 14% you feel sick and act drunk" "At 12% you can barely open your eyes" and "at 11% your head explodes" A good correlation is with altitudes. However, when one abseils/walks into an oxygen deficient atmosphere, the rate at which the "partial pressure of oxygen" changes is increased. However, there has been a load of research and results gained from a depressurisation scenario.

So, this is all well and good and probably obvious to a lot of you armed with the common sense that I lack. Here's another point.

Talking to a number of chaps about various "borderline" holes, it seems that the air can vary from one day to the next. This is very interesting an no chemical processes are likely to account for such a variation.

Taking a human lung and a gas detector "gubbins", both can be considered as chemical reactions. A number of factors influence the rates of them Temp, Concentration and Pressure. So, we'll assume that your detector has a magic gubbins to take into account of temperature changes, we'll also assume that a human is at a constant temperature.

So, it leaves pressure and concentration. (I'll avoid the gas laws)

You are stood outside on a nice day with a slight breeze, there are no fires around or anything like that. You press the "calibrate" button on your gas detector and as it has no pressure reading gubbins, it assumes the Oxygen level is 20.9% (the universal level) at that particular air pressure. It makes the assumption you are not going to be operating at different pressures, so it leaves you with something which gives you a concentration reading.

Here is the interesting bit.

In a mine with pretty low levels of oxygen, you can in fact be pretty close to having proper symptoms of hypoxia. Headaches, feeling dodgy, tunnel vision, etc. Your gas meter will give you a reading corresponding to the particular pressure and concentration of Oxygen.

The reason for this apparent variability of air quality (and reading you get on your meter) is not down to the variation of the concentration of gases in the mine, it's down to the variation of pressure.

Atmospheric pressure at ground level actually varies by a fair old amount. In the order of 10%. This means that the partial pressure of oxygen also varies by 10%. So, you could transpose it to say (pretty much) that the effective concentration of oxygen varies by 10%. 10% of 20.9% is approx 2.1% This is a fair old amount.

Put it this way, if you are down to 14% on a high pressure day, you could be down to a low of 12% on a low pressure day. Your meter will read this, as the partial pressure of oxygen (the effective concentration) is what it bases it's oxygen reading on, by the oxidation of "stuff". You will get this on your meter.

Furthermore, if you get a front go through, the quality can vary within pretty short periods of time. If it's a borderline scenario, then this could be moderately serious.

It would be interesting to note whether this is an actual performance factor on people at ground level. Clearly, it should have an effect on people's stamina/actual practical ability to do stuff.

In our case, it is functioning underground.

(feel free to tear my reasoning to bits, that's what it's there for).

Stu

Edit:- Can someone move this to general discussion? I appear to have whacked it here by accident!

Interesting reading Stu. I would be interested to know how you get on with this research. Keep me posted.

Daz. :thumbup:

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It is a legal requirement at collieries for a district deputy to read and record the barometer reading on the surface at the start and end of each of his shifts.
Many other mines follow similar checks.
If you think about a mine as a giant balloon, the changes in atmospheric pressure will result in large changes in volume of the air in the mine. This can be as simple as the mine 'breathing' in and out as the pressure varies. Or, it can be a very dangerous expansion of bad air (or methane) in the farther reaches of a mine to catch out the unwary who don't expect gas at a particular place usually.
I am not sure if it still happens (don't work in a coal mine at present) but there used to be an arrangement whereby the Met Office telephoned through a warning to mines if the barometer was expected to fall very quickly in the next few hours.
Pressure will affect gas detectors too. My detector gives a false reading and triggers an alarm when I pass through the airlocks near our main fan and give the detector a sudden change in pressure. It takes a few minutes at the new pressure to settle down again.

It's interesting as there is a misconception that gas meters somehow magically detect the ratios of gases without factoring in temp/pressure.

Interesting about the barometer readings with collieries.

I suppose a big disused stope could breathe out up to 1/10 of it's volume, which could fill long drives.

Interesting stuff.

You're right about an O2 detector (probably the other gas detectors too but I don't know) it measures partial pressures, calibrate it to 21% at atmosphere and at 2 X atmospheres it'll read 42%

I'd be interested to know your conclusions, for past records see if there is any corrolation between rainy days (low pressure) and bad air. Think people might remember having to trudge through a storm then having a rubbish trip underground. No good asking me I only ever seem to select the warmest, sunnest days to explore mines 😞

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The partial pressures bit is the revelation. You press the "think about it a bit" button and it calibrates the partial pressure to 20.9%, the reality is that the partial pressure varies everywhere. Your actual experience will vary between fine and hypoxia according to the air pressure.

This could also be associated (slighly) with a warm front where you get a bit of rain and then lower pressure. (It shouldn't vary that much though).

I have been in a particular borderline mine in high pressure conditions. I will continue to do so.

My mates bad experiences follow bad weather/low pressure thinking about it.

It's funny that in the summer, a lot of people stop mine exploring for nicer pastimes, when the air and water are probably more likely to be in your favour.

In order to calibrate gas detector readings with your physical response, it's probably a good idea to take a note of the air pressure, rather like the collieries.

A bit on the types of O2 sensor and the effects of transient pressure changes.

http://www.alphasense.com/pdf/AAN_009.pdf 

......and another thing.

Noting how much the effective pressure is increased (on average) the further you go underground.

At the bottom of William's Shaft (Dolcoath), the air pressure is about 14% higher, or +3" on your average barometer.

This gives you an equivalent partial pressure of nearly 24% by volume (under normal conditions).

I supposed this compromised somewhat for poor ventilation.

It would be interesting to compare the levels of work that people could do at different underground altitudes, assuming modern ventilation (and bearing in mind the huge depths of some foreign mines- we are talking +9" on the barometer and +9% effective oxygen partial pressure, it would most certainly influence human biology.

I wonder what happens to the blood levels of CO2 at that pressure. It would probably regulate to a degree.....

Anyway.....

I had a few thoughts about gas detectors and how humans operate in various conditions.

Seems very complex after wading through numerous articles on respiration.

I think a practical experiment may answer some of the questions on how all this affects you.

Select by (barometric pressure) two days at the extremes available in your area.

Set your gas meter in fresh air and then get an assistant/resusicator to seal you into a large plastic bag.

Perform aerobic excerises, taking notes of your performance versus O2 levels until you show signs of mental disfunction or collapse in a heap, analyse results if/when you recover.

Repeat for other extreme.

Post results (inc video)for our edification/amusement. 😉

Ignoring the individuals factors which govern fitness, I'd say the external factors give a pretty good rule of thumb.

A mate of mine who is a med was interested in doing some pukka medical research on it. His particular focus seemed to be about "arterial oxygen pressures".

I suppose what will happen is that there will be a varied blood oxygen demand on the actual amount of work done. Depending on the condition of the person's guts/blood, we'd see a relationship of air conc:blood conc vs time, the steepness of gradient would be down to bigness/fitness/smoking factors. Needless, to say, it would probably come down to skinny fit people being alive for slightly longer than unfit fatties.

Anyway, I don't want to have needles stuck in my arteries.

A better experiment would be to have a large diving bell and alter the pressure, rather than trying to alter the atmosphere by making an atmosphere from scratch. Burning the oxygen would clearly skew the results due to the CO2.

I imagine we'll have a go at some experiments in due course. It would definately produce some interesting results whcih we can only pontificate about.

As an oldish ex-smoker I can confirm that health/fitness is a very big variable. 🙂

I know I certainly can't swim four times faster when at 30m under the water breathing 88% partial pressure of oxygen (well twice as fast taking into viscosity), it makes no difference at all (which is a shame).

Look up Halden and his Chamber of Horrors, at 200% O2 things burn very well, in fact goats are close to explosive!

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There has been some extensive medical research into this problem for a fireprotection system in the last few years. Most of the stuff is in German and it's far to much for a full translation.
I didn't check for new documents for some time since I'm no longer involved in the application of this system, but there should be a lot more available now since it is quite a fashionable system for server room and warehouses.
You'll probably find something useful if you search for

permanent hypoxia and fire protection

or

OxyReduct

There is a well known correlation between atmospheric pressure and the production of "gas" within mines - hence as has already been mentioned the requirement for collieries to make daily barometric recordings.

If you want to explore "borderline" gasey mines, do it during a period of barometric high - it pushes the gas back into the workings!

While during a "low" a mine will out-gas.

Also consider the factor that in an abandoned mine which has been left undisturbed for a long time, if various gases are being given off (rather than just oxygen depletion), the gases may have separated - methane near the roof, Co2 near the floor, with breathable air in the middle! An exciting position to find yourself in.

🙂

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even quite small local variations can be significant.

This could be clearly seen at Combe Down, a large system ( about 45 acres at its maximum extent ) at a single horizon. The multiple entrances did not differ sufficently to cause noticeable local pressure differences, but one in particular was located in a sheltered location, which had the effect of a 'cold well' and this was very much the focus of the flows through the workings.

the effects of this on flow volumes and directions could be clearly seen when this section was isolated to form a bat sanctuary.

there was another section located in a deep valley ( near the top ) where local flows reversed noticeably over quite short periods in warm weather.

an outlying section ( under the golf course ) was about the only area which had significant differences in entrance levels linked by open workings; about 20 acres in all, with a difference in entrances of around 20 metres. There was an evident flow pattern down the higher shaft and out the lower portals in this section.

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Anyone regularly going into the Admiralty area of Haggs mine in Nenthead will tell you that when the weather is high pressure - the air is fine, as soon as the pressure drops - forget trying to get up the shaft into Admiralty - your lungs will be heaving. It's a direct correlation with external weather.

Pete

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I have first hand experience of air pressure affecting air quality underground (in a poor air location) and for sure on low pressure days the 02 % is a lot less than it is on high pressure days. I haven't taken any scientific readings relating to pressure but do know there is a very big difference.

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I started doing some research some time ago (as a meteorologist in another life I was approaching it from that angle) on the link between variations in pressure and explosions in coal mines. There is also the complicating factor of seasonal variations in water vapour and coal dust.

I've collected a few papers on the subject going back to the 19th century when it first became an issue although no agreement was reached. It is a subject that I feel has been under researched. There undoubtably is a link as was fairly well established with two explosions in Illinois in the 50s in two different mines within two days affected by the same weather system. The recent Hlolbane disaster in South Africa is another example.

I need to look at some early British disasters and look at the weather synoptics at the time but that's easier said than done. It's not impossible as the data will be in the METO archives.

As far as I know the Office still issue warnings of rapid pressure variations.

EDIT:
To clarify, I'm obviously not suggesting a link between all explosions and the weather.

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Oddly enough, we went and had a look at a mine known to have marginal air the other day, had problems with the Davy lamp and decided that since there had been numerous weather fronts recently, it was good to go. The barometer was at an all year high.

Due to the lack of rain, normally saturated things had started rotting and the CO2 down there was ridiculous. It was quite a nasty experience. In hindsight from a bit of reading and the symptoms, probably about 6%

The learning outcome of this was that, "There are other factors at work".

Talking about rotting I came across this the other day when reading a paper written in the early 20s on underground ventilation at Butte.

OXIDATION OF TIMBER AND ORE.

In still or slow-moving air in timbered places there was heavy depletion of oxygen, with an increase of CO2 , but the CO2 increase was not in proportion to the loss of oxygen; the average depletion of oxygen in 18 samples of so-called timber gas was 1.03 per cent, and the average content of CO2 was 0.44 per cent, the amount of CO2
necessary to absorb the entire loss of oxygen being 1.41 per cent, instead of the 0.44 per cent found. Hence the timber probably absorbed about 70 per cent of the oxygen, and with the other 30 per cent formed the CO2 given off.
This oxidation of timber seemingly takes place at any temperature above 60° and progresses fastest when a white fungus is present. This fungus was found in greatest luxuriance where the relative humidity was 95 to 100 per cent. Where there was air in a timbered place with high relative humidity and little or no movement for a few months, the depletion of oxygen was practically complete. On the other hand the most rapid decay of timber was in main returns, with velocities of 500 to 1,000 feet per minute, temperatures above 70° F., and a relative
humidity of 100 per cent. On several occasions air currents of 5,000 cubic feet per minute coming from idle or abandoned workings were found to be much depleted of oxygen and to carry 0.50 per cent or more of CO2.

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