Holding your Breath
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Well, I was trained instead performing severe breathing control, which means very deep, slow exhalations and inhalations, and practising an inspiratory pause of "proper" duration.
Most of my training was done using the ARO, a pure-oxygen closed-circuit rebreather, which was the standard here in the seventies.
With such a rebreather, you cannot "breath normally", you would suffer of severe hypercapnia doing that. So you need to learn breathing as explained above. That way of breathing, instead, minimizes CO2 retention.
At the time, it was considered good to teach students this way, also when later switching to open-circuit air, after 4-5 months of training with the ARO.
Such a breathing technique minimises CO2 retention and ensure strict control against over-exertion and dyspnea also in OC.
Of course there is the risk of lung over-expansion if you ascend without exhaling. But, again, this risk was much heavier with the rebreather, so we were trained with specific excercises to release the excess air when ascending, "feeling" it in our lungs expanding, and releasing just the excess.
I am not sure which agencies today are still teaching this kind of breathing control with the inspiratory pause. BSAC, perhaps?
In my 42 years with BSAC breath holding on scuba equipment was actively discouraged.
So no pure oxygen rebreathers at BSAC?
Perhaps that was an Italian thing, as we developed them during WW2 and they were the equipment making scuba diving popular in the fifties and in the sixties...
They were also much cheaper than the compressed air systems made by J. Costeau. Just an Aquilon or a Mistral regulator was costing more than a complete ARO...
It may be useful to go through these examples, this will make it clearer to visualise:
- how big a 1litre balloon becomes if you move from 90ft to 60ft (assume that it is going from 4bar to 3bar)
- how big a 1 litre ballon becomes if you go from 30ft to 0ft (assume we are going from 2 bar to 1bar)
Edit: I rounded the pressures, I am not familiar with imperial units and used 30ft for 10m
johndiver999
Contributor
I think a simpler way to look at it is... if you are at 10 feet and come up four feet, then you have reduced the water pressure by 40%
If you are at 100 feet and come up four feet, you have reduced that water pressure by 4%.
So it is easy to see, depth changes in shallow water cause bigger expansions than in deep water.
People who want to look at the situation more accurately, would want to involve the atmospheric pressure as well, but this only makes the situation more complicated and is not helpful in conveying a very simple conceptual model of the situation.
My experience is that different people have different ways to see it.
I think he needs to work out a few examples to see what you are already seeing: that the volume change is function of the proportional change of pressure.
I used to explain this to my student as follows.
1) If you ascend from 10m (2 bars) to the surface (1 bar) without exhaling, your lungs will double their volume and explode.
2) If you ascend from 50m (6 bars) to 40m (5 bar) your lungs will expand by a factor 6/5, which is just 1.2, which probably will not cause damage.
With metric units it is definitely simpler to explain...
1) If you ascend from 10m (2 bars) to the surface (1 bar) without exhaling, your lungs will double their volume and explode.
2) If you ascend from 50m (6 bars) to 40m (5 bar) your lungs will expand by a factor 6/5, which is just 1.2, which probably will not cause damage.
With metric units it is definitely simpler to explain...
Water pressure is 0.03037 atm for every foot (i.e. 1 atmosphere every 10 meters). So that 4 feet of depth is a difference of 0.1215 atm whether you're going 90 to 86 feet, or 4 to 0 feet.
But the relative pressure change is what matters. It's kind of like money... if a rich guy and a poor guy each find a $5 bill, they find the same amount of money but it means a lot more to the poor guy.
In the case of the pressure change, we can crank out a few simple calculations:
going from 90 to 86 ft depth:
initial pressure at 90 feet = 1 atm (air at the surface) + 2.733 atm from water =
= 3.733 atm total pressure
final pressure at 86 feet = 1 atm + 2.612 atm = 3.612 atm
the difference is 3.733 - 3.612 = 0.121 but the ratio of the initial to final pressures is
3.733 / 3.612 = 1.034
This is also the ratio of the final to initial volume, i.e. the volume expands by 3.4%
In contrast, going from 4 to 0 ft depth:
initial pressure at 4 feet = 1 atm + 0.122 = 1.121 atm
final pressure at 0 feet = 1 atm
again, the difference in pressure is 0.121 atm but the ratio of initial pressure to final pressure
1.121 / 1.00 = 1.121 so the volume expands by 12.1% !!
That difference of 0.121 atm of pressure is much more significant when the pressures are small (i.e. like poor guy finding money) than when the pressures are greater.
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