Several Questions About Pressure

[WKenny]

I’d like to ask about the pressure of the gas that is delivered through the low pressure ports of the first stage of a scuba regulator to a wing and a dry suit during a dive to about 132 FSW.

When using the low pressure ports on the first stage of a scuba regulator, that first stage steps down the pressure of the gas in a scuba tank to an intermediate pressure of about 140 psi, give or take, above the ambient water pressure at all depths within the range that the regulator can reasonably be used, is that generally correct?

If so, since the 1st stage of the regulator routes gas from the tank to the wing and dry suit through hoses from low pressure ports, the 1st stage provides that gas to the wing and dry suit at the same intermediate pressure above ambient water pressure for all depths, (i.e. at a pressure of 140 psi above ambient pressure in this example), is that correct?

If that premise is true, is the following analysis substantially correct ? Assume a diver periodically inserts small amounts of gas into his wing as he descends for buoyancy control and small amounts of gas into his dry suit to alleviate suit squeeze. Those volumes of gas would expand instantaneously in the wing and dry suit because that gas is at a pressure of about 140 psi above ambient water pressure, until the pressure of that gas equalized with the surrounding ambient water pressure, correct? Then as the diver continued to descend and the ambient water pressure increased with depth, those volumes of that gas in the wing and dry suit would compress under the increasing water pressure with depth, and the pressure of the gas in the wing and dry suit would continually equalize with the ambient water pressure as the diver descended, is that correct?

Assume that the diver descended to a depth of 132 FSW where the ambient water pressure is 5 Atmospheres Absolute. The water pressure at that depth would be 73.5 psi., (14.7 psi times 5 atmospheres which equals 73.5 psi). If the diver inserted gas into his wing or dry suit at that depth, the pressure of that gas from the 1st stage of his regulator theoretically would be at 140 psi above the ambient water pressure of 73. 5 psi? So just a little shot of gas into the wing at that pressure would expand almost instantaneously until the pressure of the gas in the wing equalized with the surrounding ambient water pressure, is that correct? Ultimately, the pressure of all the gas that had been inserted into the wing on the descent would be blended into a single pressure at the same pressure as the ambient water pressure at the depth where the diver swimming, and the same would occur regarding the pressure of the gas in the dry suit, is that correct?

When the diver ascends, the fact that the pressure of the gas that had been inserted into the wing and dry suit had been 140 psi above the ambient water pressure at the depths where that gas was inserted is irrelevant, right? At this point with regard to maintaining a safe rate of ascent, we’re really only concerned with the difference in pressure between the higher blended pressure of the gas in the wing and in the dry suit, and the progressively lower ambient water pressure as the diver ascends, is that correct?

So if the diver’s deepest depth was 132 FSW and he inserted gas into his wing and dry suit at that depth at a pressure of 140 psi above ambient water pressure, the effect of that high pressure would be reduced to some extent because it would be blended in to an equalized pressure of the gas in the wing and dry based on the entire dive.

Does anyone have any comments about the validity of this analysis? Thank you

 

[Iknee]

PV=nRT

 

[Superlyte27]

What you're missing is that the pressure in the hose is at 140psi (pounds per square inch). For easy math, lets say the total length of the hose is 10 square inches (which it's not, but we're using easy math). So, you have 140psi spread out over 10 square inches or the length of the hose.....

How many square inches are in your drysuit? As soon as the pressure leaves the hose and enters your drysuit, you no longer have 140psi in your drysuit or your wing.

Your car tire has roughly 40psi in it right? A monster truck tire has 5psi in it. Which tire has a larger volume of gas?


Edit* and you'll never get 140psi into a wing or drysuit, because the OPV will pop before then, or in the case of a malfunction, your bladder will rupture.

 

[Doppler]

Thought process way too complicated... and in one regard, not complicated enough... but flawed regardless.

Firstly, the pressure at which gas is delivered to the wing and suit is immaterial to the process of achieving balance... between gravity and buoyant effect.

For every litre of water displaced by an expanding wing or suit, the buoyant effect is one kilo (close enough for diving maths).

To displace one litre of water at 132 feet (40 metres) the density of the gas doing the work has to be 5 times what it is on the surface (40 metres = 5 bar ambient). Therefore, five litres of gas must be added.

The pressure at which it is delivered is immaterial... however, something we might take into account on dives a little deeper than 40 metres/132 feet is the viscosity of the gas and the effect of this on the ability of the regulator to perform its function. This becomes a particular issue with regulators delivering argon as a suit inflation at depth... Argon has a molar mass of 40 grams compared to air at about 29 grams making it "tougher" for the regulator to "process."

As an aside, take a look at the cross-section of a LP hose... what fraction of a square inch is it? Divide that into the pressure in square inches and voila... guess what!

 

[DivemasterDennis]

The tank and hose pressure are closed systems that do not change with depth. So your comment 140 psi above the ambient pressure, I believe, is flawed. Gas inserted into a wing, bcd or dry suit at depth will expand as you ascend. That is certain. air inserted at 132 feet will expand more than air inserted at 66 feet. The air you added at 33 feet compressed as you descended. The expansion of the gas in these three areas is a constant once you start your ascent, and constant venting is needed, with your attention to ascent rate, to maintain a controlled and safe ascent.
DivemasterDennis

 

[Agility]

The tank and hose pressure are closed systems that do not change with depth​

I aggree the tank pressure will not change with depth (at least it will not really make any difference) but the low pressure system is held at 9.5 bar (~140 psi) above ambient pressure by the first stage of my Apeks regulator and does climb when descending. The pressure curve on the testing protocol climbes up the scale with increasing ambient pressure.

 

[dumpsterDiver]

The tank and hose pressure are closed systems that do not change with depth. So your comment 140 psi above the ambient pressure, I believe, is flawed. Gas inserted into a wing, bcd or dry suit at depth will expand as you ascend. That is certain. air inserted at 132 feet will expand more than air inserted at 66 feet. The air you added at 33 feet compressed as you descended. The expansion of the gas in these three areas is a constant once you start your ascent, and constant venting is needed, with your attention to ascent rate, to maintain a controlled and safe ascent.
DivemasterDennis

No YOU are mistaken...If you were correct then a regulator would deliver ZERO air at around 300 feet where the water pressure would about equal the intermediate pressure of the regulator at the surface.. This is one of the most basic concepts taught in a beginning scuba class???