At which point is the air coming out from the cylinder subjected to ambient pressure?

[garywong007]

During the OWD course, students are often told that the reason air is depleted faster at greater depth is because the volume of air is compressed with increasing depth. For example, a body of air with 10 litre volume is only 5 litre at 10 meters so a diver taking one breath (say, 3 litres) at the surface is equivalent to taking 6 litres of air at 10 meters with the same breath.

But since the air tank is a concealed space and made of solid material that won't deform under water pressure (within a certain depth), therefore the pressure and volume of the air inside is not affected by depth (i.e. if the air inside the tank is at 200 bars and 12 litres on the surface, it should remain the same at 20 meters). My question is then: when the air comes out from the tank, going into the HP chamber of the 1st stage, then the IP chamber, the LP hose, the inlet tube, the housing and finally into the lung of the diver, at which point in time during this route its volume becomes subject to the influence of the ambient pressure?

If one litre of air is released at the surface (i.e. 1 atmospheric pressure) from an air tank with air inside compressed to 200 bars, its volume should expand to 200 litres, but what would the volume of that same 1 litre of air be if it is released at 10 meters (2 atmospheric pressure), 20 meters (3 atmospheric pressure)... and so on and so forth?

Thank you.

 

[RobPNW]

To me it seems that the air itself wouldn't be subject to ambient pressure until it exhausts from the 2nd stage where it hits ambient pressure. Along the route from the tank through the hoses and regulators, the air pressure is controlled based on what the diaphragms "tell" the equipment to make the pressure but it's the diaphragms that are exposed to ambient pressure along the way.

 

[inquis]

So, to be able to remove the necessary amount of carbon dioxide, the lungs need to inflate fully

Your CO2 production rate in your tissues is consistent across depths for a given level of exertion. The transfer away ultimately is inversely tied to the concentration of CO2 molecules in the lungs (molecules per lung volume, so also independent of depth). A larger breath increases the volume, and therefore the elimination rate. However, if the rate was already sufficient to keep up with metabolism, then a larger breath is wasteful. That's why you need only inhale about half a liter rather than full lungs while sitting on the couch.

What is important, however, is exhaling fully. You won't get rid of all the CO2 molecules, but the more you do, the faster the initial transfer rate on your next breath. This maximizes the transfer for a given amount of inhalation (i.e., the consumption rate).

 

[rsingler]

To me it seems that the air itself wouldn't be subject to ambient pressure until it exhausts from the 2nd stage

Almost.
200x in the tank
9x-13x in the hose to the second stage, depending upon recreational depth.
And as it exits the second stage valve it expands 9x to 4x to ambient depending on recreational depth.
You inhale your usual volume at that ambient pressure, which contains 1x to 4x the number of surface molecules per unit volume, based on ambient pressure.
The number of molecules sucked determines the drop in tank pressure. The volume exhausted is the same as the post-expansion volume drawn from the last valve.

 

[Rogerdd]

1. Yes, mostly. CO2 diffuses more rapidly than Oxygen, so a full breath at the surface is not quite as necessary. However, at depth, all those extra molecules slow diffusion a bit, so a full breath now becomes more important (see "lung dead space").
2. Unfortunately, no.
The answer is merely that PV=PV, roughly.

for 2. Maybe a way to better understand is this.
1 at the surface, 2 at depth;
For the same quantity of air P1V1 = P2V2 so V2 = V1 * (P1/P2)

However to function correctly the human metabolism doesn’t need a fixed quantity of air (P*V) but a fixed volume (V).

So at the surface the quantity of air used is P1*V and at depth it’s P2*V or [amount of air at the surface]*(P2/P1)

 

[OMyMyOHellYes]

Perhaps starting with tech divers who don't understand bar-litres?

Say you go to the bar ... say you have a litre ...

That's what? Somewhere between four and five pints?

Should probably wait a good while before diving again.

 

[MarkA]

Say you go to the bar ... say you have a litre ...

That's what? Somewhere between four and five pints?

Should probably wait a good while before diving again.

1 imperial pint = 0.568 litres

Along with other liquid measures included in the 1824 act like the firkin and kilderkin should be consigned to the scrap heap of history, and metric be used.

 

[lairdb]

1 imperial pint = 0.568 litres

Along with other liquid measures included in the 1824 act like the firkin and kilderkin should be consigned to the scrap heap of history, and metric be used.

I usually dive with a tank that contains 53.6 firkin of breathing gas, and I am 1.04 smoots tall.