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.

 

[boulderjohn]

I may be misreading the intent of the question, but if I am reading it correctly, then maybe others are doing the misreading.

As I understand it, his wording is actually a misunderstanding of why the volume of air at the surface has an apparent lesser volume of air at depth. The answer is that it is all about the lungs. The P1V1=P2V2 equation tells us what would happen to lungs subjected to ambient pressure at depth, not the tank. Left to their own devices, lungs at 20 meters/66 feet would be 1/3 the size they were at the surface. But on scuba, they are not left to their own devices. We fill them with air from the tank. and it takes 3 times as much air from the tank to fill those lungs as it took at the surface. Consequently, being at 20 meters makes it seem as if the tank has only 1/3 as much air, but it actually has just as much.

 

[garywong007]

Appreciate all for your inputs and insights. I can see there are different interpretations to my question here. That is probably because I did not make myself very clear the way I phrased the question Maybe I could make my question clearer with an example and actual numbers:

Tank size : 12L,
Tank pressure : 200 bars

If the diver's lung volume is 4 litres, when he takes a breath at sea level (i.e. 1 ATM), the tank would have to release just 0.02 litre of air (compressed to 200 bar) from the tank to fill the lung volume. (i.e. 0.02 litre x 200 bar/1 ATM = 4 litres). At 10 meters, the volume of air that needs to be released from the tank to fill the lung volume would be 0.04 litre (i.e. 0.04 litre x 200 bar/2 ATM = 4 litre), and at 20 meters, 0.06 litre (i.e. 0.06 litre x 200 bar/ 3 ATM) = 4 litres) ...so on and so forth....

In other words, that 0.02 litre of air that will expand to 4 litres at sea level will only expand to a 2 litres at a depth of 10 meters and 1.333 litres at a depth of 20 meters....

If so far I am correct, then my question is : Since the air in the tank is NOT affected by changes in depth in terms of its pressure and volume (means not affected by the ambient pressure at any particular depth and remains constant), then, at which point during the air travel route this 0.02 litre (or 0.04, 0.06... for that matter) of air released from the tank will start to expand its volume and adjusts itself to fill the diver's lung (i.e. 4 litres) at different depths? My understanding is at the moment when the air from the LP hose passes the downstream valve and enters the 2nd-stage housing. That means the air, starting from the tank, into the HP chamber, then the IP chamber, the LP hose all the way up to the sealed orifice, is NOT affected by ambient pressure. Only when the air is released into the 2nd-stage housing it is subject to the ambient pressure. At 10 meters, it will expands to only 1/2 of its volume at sea level and at 20 meters, 1/3 of its volume at sea level.

Hope I have asked my question in a clearer way this time and your inputs are very much appreciated.

 

[Sumo888]

Appreciate all for your inputs and insights. I can see there are different interpretations to my question here. That is probably because I did not make myself very clear the way I phrased the question Maybe I could make my question clearer with an example and actual numbers:

Tank size : 12L,
Tank pressure : 200 bars

If the diver's lung volume is 4 litres, when he takes a breath at sea level (i.e. 1 ATM), the tank would have to release just 0.02 litre of air (compressed to 200 bar) from the tank to fill the lung volume. (i.e. 0.02 litre x 200 bar/1 ATM = 4 litres). At 10 meters, the volume of air that needs to be released from the tank to fill the lung volume would be 0.04 litre (i.e. 0.04 litre x 200 bar/2 ATM = 4 litre), and at 20 meters, 0.06 litre (i.e. 0.06 litre x 200 bar/ 3 ATM) = 4 litres) ...so on and so forth....

In other words, that 0.02 litre of air that will expand to 4 litres at sea level will only expand to a 2 litres at a depth of 10 meters and 1.333 litres at a depth of 20 meters....

If so far I am correct, then my question is : Since the air in the tank is NOT affected by changes in depth in terms of its pressure and volume (means not affected by the ambient pressure at any particular depth and remains constant), then, at which point during the air travel route this 0.02 litre (or 0.04, 0.06... for that matter) of air released from the tank will start to expand its volume and adjusts itself to fill the diver's lung (i.e. 4 litres) at different depths? My understanding is at the moment when the air from the LP hose passes the downstream valve and enters the 2nd-stage housing. That means the air, starting from the tank, into the HP chamber, then the IP chamber, the LP hose all the way up to the sealed orifice, is NOT affected by ambient pressure. Only when the air is released into the 2nd-stage housing it is subject to the ambient pressure. At 10 meters, it will expands to only 1/2 of its volume at sea level and at 20 meters, 1/3 of its volume at sea level.

Hope I have asked my question in a clearer way this time and your inputs are very much appreciated.

Yeah, I think you're looking at this the wrong way.

If your lungs are 4 litres, then it will draw 4 litres of air from the tank regardless of depth. It's just that at 10meters, that 4 litres is the equivalent of 8 litres at the surface.

I would argue the air in the tank is subject to changes in depth. If you were to submerge to a depth such that the ambient pressure is equal to or exceeds the tank pressure, you're not getting any air from that tank.

 

[inquis]

That means the air, starting from the tank, into the HP chamber, then the IP chamber, the LP hose all the way up to the sealed orifice, is NOT affected by ambient pressure.

See post #2. As soon as the air passes the HP seat into the intermediate pressure chamber of the first stage regulator, it has been impacted by ambient pressure (and a spring). That air is now at ambient+9.5 bar or thereabout. After passing the seat in the second stage regulator, it will be at ambient.

 

[justinthedeeps]

Now that the question is resolved and explained--the gas in a SCUBA cylinder is already massively over-compressed way beyond ambient pressure into a much smaller, dense volume, and actually expands three times:

1) at the first stage of the regulator, typically to 8-10 bar above ambient pressure,
2) at the second stage of the regulator, expanding inversely proportional to ambient pressure (which increases with depth), and
3) upon ascent to surface, if underwater

Can we begin the "wrong answers only" phase of this thread?

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