Partially opened tank valve

[CrispyCritter]

Hi all,

I did a search for this but wasn't able to find anything. I am new to diving and have a basic understanding of how regulators work, but would like to have a better understanding so I have been reading up on them.

I've read a couple of threads about divers entering the water with partially opened tank valves, and running into problems with the restricted air flow when they've reached a certain depth. It got me wondering about what's happening.

Each case shares some similarities:
1. For whatever reason, the valve was only opened a little bit.
2. The pressure gauge will swing dramatically during breathing.
3. The regulator is able to meet the demand until a certain point.
4. The diver then experiences an OOA scenario, even though they have plenty of air.

It seems this problem is related to pressure; I am just not sure if it has to do with the ambient pressure, the tank pressure, or both. Anyone know what is happening from a mechanical and/or pressure perspective?

Thanks!

 

[Hatul]

It's very simple, the deeper you go the more molecules of air in each breath. So eventually the restriction cannot supply enough air.

Adam

 

[DA Aquamaster]

The flow from the tank through a restricted valve is very much tank pressure dependent and as tank pressure drops, the flow rate will also drop. That is a small part of the issue.

The other part is that the regulator normally operates at an intermediate pressure of 120-145 psi. 10-20 psi opf IP drop during inhalation is entirely normal and the flow rate is still high through the reg as the IP is still comparateively high. And the IP will quickly rise again in a fraction of a second when the inhalation stops. However, if the flow to the first stage is restricted, then the IP drops significantly and the flow rate also suffers and the flow rate through any given passage or orifice will drop at a roughly linear rate with drop in pressure. That is another small part of the problem.

Even with a partially closed valve, the ambient pressue will still be maintained above ambient with increasing depth. The total pressure is a little higher, but the "thicker" gas flowing through the reg won't start to create a noticeable problem in terms of reducing flow untill you get to extreme depths (600' or so). So that is not a part of the problem.

The major issue here is that a normal surface consumption of say .8 cfm becomes, 1.6 at 33 ft, 2.4 at 66', 3.2 at 99' and 4.0 at 132'. More importantly, if the diver makes 5 inhalations per minute, averaging maybe 4 seconds per inhalation (with a 2 second pause before and after and a 4 second exhalation) that diver is actually getting that .8 cfm at the surface at 15 times that flow rate - or in this case a peak flow rate of 12 cubic feet per minute. When you then run that number at the previous depths, you go from 12 cfm to 24 cfm to 36 cfm to 48 cfm and finally 60 cfm at 132'. That is the major problem as a partially opened valve can't supply that much gas to the first stage and the first stage operating at a very low IP cannot flow that much gas efficiently either.

And with a very low IP, the second stage has very little downstream force to assist in opening the valve to intiate and sustain the flow, so the second stage now breathes like a rock and furher increases the feeling of not gettng enough gas. That is another major part of the problem.

Finally, the diver begins to "overbreathe" the available gas being supplied and the usual response is to suck harder and attempt to inhale faster, which increases the peak flow rate and accellerates the cycle of things going wrong. The irony here is that if the diver made long slow inhalations, the reg could probably supply enough gas, but when the diver tries to get more faster, he or she aggravates the problem to the point that an immediate ascent is often not enough to get the diver ahead of the demand/supply curve again.

Understanding that can help you prevent that from occuring.

 

[CrispyCritter]

Thanks. So as the pressure increases, and the volume decreases, the 1st stage is basically wide open trying to supply air at ambient + 140 PSI to the LP part of the system?

 

[CrispyCritter]

Thanks Aquamaster, that is exactly what I was wondering. You even answered some of my follow up questions.

The major issue here is that a normal surface consumption of say .8 cfm becomes, 1.6 at 33 ft, 2.4 at 66', 3.2 at 99' and 4.0 at 132'. More importantly, if the diver makes 5 inhalations per minute, averaging maybe 4 seconds per inhalation (with a 2 second pause before and after and a 4 second exhalation) that diver is actually getting that .8 cfm at the surface at 15 times that flow rate - or in this case a peak flow rate of 12 cubic feet per minute. When you then run that number at the previous depths, you go from 12 cfm to 24 cfm to 36 cfm to 48 cfm and finally 60 cfm at 132'. That is the major problem as a partially opened valve can't supply that much gas to the first stage and the first stage operating at a very low IP cannot flow that much gas efficiently either.

I don't follow, why is the actual consumption 15 times greater?

 

[Hatul]

Thanks. So as the pressure increases, and the volume decreases, the 1st stage is basically wide open trying to supply air at ambient + 140 PSI to the LP part of the system?

As the pressure increases, the volume per breath is required to be the same by your lungs, so you need more molecules. The restriction cannot supply them. You asphyxiate.

Remember the ideal gas equation PV=nRT where n=# of molecules

V is the volume of each breath stays constant, P increases so n must increase. Each breath at depth requires more molecules.

This problem is independent of the type of regulator or whether there is an intermediate pressure or not. The restriction simply cannot supply enough air for you.

Adam

 

[diver 85]

Hi all,

I did a search for this but wasn't able to find anything. I am new to diving and have a basic understanding of how regulators work, but would like to have a better understanding so I have been reading up on them.

I've read a couple of threads about divers entering the water with partially opened tank valves, and running into problems with the restricted air flow when they've reached a certain depth. It got me wondering about what's happening.

Each case shares some similarities:
1. For whatever reason, the valve was only opened a little bit.
2. The pressure gauge will swing dramatically during breathing.
3. The regulator is able to meet the demand until a certain point.
4. The diver then experiences an OOA scenario, even though they have plenty of air.

It seems this problem is related to pressure; I am just not sure if it has to do with the ambient pressure, the tank pressure, or both. Anyone know what is happening from a mechanical and/or pressure perspective?

Thanks!

I think I've probably made every dive in my life with a partially opened tank valve & have never had a problem-------> I ALWAYS back it down a quarter turn after fully opening it....

 

[DA Aquamaster]

I don't follow, why is the actual consumption 15 times greater?

The techicnal term is "brain fart".

.8 cfm for your gas consumption would assume a constant flow rate, but it does not work that way in real life. The idea went like this when the brain fart struck - if there are 60 seconds in a minute and you make 5 inhalations, each 4 seconds long (12 seconds per cycle with 4 second inhaling 2 seconds pausing, 4 seconds exhaling and 2 more seconds pausing), then you are only inhaling 4 out of 60 seconds.

Factor out the brain fart and it is obvous that you are of course ignoring 4 of the 5 breaths, so in fact once you factor out brain farts and related math errors, you are inhaling the .8 scfm over a 20 second period of time, not a 4 second period of time. So its a factor of 3 not 15. But at 132' you are still needing to get 12 cfm of flow rather than .8 cfm and that is where the factor of 15 came in but got misapplied.

I used to get erasers thrown at me in algebra class for doing **** in my head and not showing my work. And the teacher was right to make me do the work as if you don't sometimes your brain gets ahead of itself and stupid errors like this one occur.

And the error should have been obvious as even the highest performing second stages out there only flow betwen 65-71 cfm, so the last sub 200' dive I did would not have been possible s the answer did not pass the captain obvious smell test.

 

[Bob DBF]

Thanks. So as the pressure increases, and the volume decreases, the 1st stage is basically wide open trying to supply air at ambient + 140 PSI to the LP part of the system?

Because the valve is partially open the volume of air it can deliver is the same regardless of the depth (quibbling aside). The ammount of air you demand as you go deeper increases due to the pressure. At a certain point you may demand more air than the valve can deliver and the games begin.


Bob
--------------------------
I may be old, but I’m not dead yet.

 

[DA Aquamaster]

I think I've probably made every dive in my life with a partially opened tank valve & have never had a problem-------> I ALWAYS back it down a quarter turn after fully opening it....

On older valves that was important as some of them could stick open if left open hard against the stop. However with modern valves that is not an issue.

However the time honored "roll it back 1/4 turn closed" is a practice that has persisted since the dawn of scuba, despite the need to do that being absent for the last 30 years or so.

In a perfect world it would do no harm to close the valve 1/4 turn as most valves are for all intents and purposes open enough after the first few turns and the last couple are icing on the cake.

However where the practice goes horribly wrong is when the valve gets reset by mistake so that it is now only 1/4 turn open. This can happen when a diver or well meaning boat crew gets confused about lefty/loosey righty tighty and then closes a valve all but 1/4" turn when they thought they were helping you by opening a valve they thought you forgot to open.

From that perspective you are ALWAYS far better off to leave your valve all the way open or all the way closed, that way there is not a possibility of a 1/4 turn open valve delivering enough air to convince you it is open at the surface. It will either be open or closed and will deliver gas or not.

----

As a general rule I do not like boat crew messing with my valves, and if I feel someone messing with them just before leaving the boat, I'll recheck the regs with an eye on the SPG to ensure the needle is solid.

This saved my butt on a technical dive where we were smart bombing and dropping into a strong current with all the air deflated from the wing and dry suit to let us get down quickly. Had the valve been off when I went in, I would not have been able to add any gas to the suit or wing and after the suit squeezed me on the uncontrolled descent it would have been very hard to turn the gas back on. Little mistakes like that kill people.

The mate and I had a freindly, "you ever touch my valves again and I'll kill you" kind of discussion and the captain to his credit agreed to help.