Partially opened tank valve

[CrispyCritter]

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

Wow, I have to admit it has been a long time since I have seen that equation! I understand this part, I was curious what happens inside the regulator when the demand exceeds the supply capacity.

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.

Ok, this might be a silly question, but doesn't CFM stand for cubic feet / minute? I would assume that this already factors time into it and you wouldn't need to adjust for the number of breaths you were taking per minute.

 

[DA Aquamaster]

Ok, this might be a silly question, but doesn't CFM stand for cubic feet / minute? I would assume that this already factors time into it and you wouldn't need to adjust for the number of breaths you were taking per minute.

Let's say you have a balloon with an inflated volume of .8 cu ft and you fill it in one minute.

You can fill it at a slow, constant rate over the entire 60 seconds, or you can fill it in five 4 second bursts (20 seconds total.) To get the same volume of gas is the same 1 minute, you have to have a flow rate that is three times higher using the five 4 second intervals. But averaged over a minute the flow from both examples is still .8 cfm.

 

[ianr33]

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....

The problem comes when someone (buddy,divemaster,yourself when distracted) "opens" the valve a second time then cracks it back a touch. That leaves it only barely open. It will breathe on the surface but not at depth.

This happened to a buddy of mine that was going through a divorce.We got down to 60 feet and he became rather agitated!

I open my valves all the way(gently) and leave them there. If it gets messed up and is really closed it soon obvious after 2 or 3 breaths on the surface.

 

[halocline]

The problem comes when someone (buddy,divemaster,yourself when distracted) "opens" the valve a second time then cracks it back a touch. That leaves it only barely open. It will breathe on the surface but not at depth.

This happened to a buddy of mine that was going through a divorce.We got down to 60 feet and he became rather agitated!

It wasn't the ex-wife that "opened" the valve, was it?

 

[CrispyCritter]

Let's say you have a balloon with an inflated volume of .8 cu ft and you fill it in one minute.

You can fill it at a slow, constant rate over the entire 60 seconds, or you can fill it in five 4 second bursts (20 seconds total.) To get the same volume of gas is the same 1 minute, you have to have a flow rate that is three times higher using the five 4 second intervals. But averaged over a minute the flow from both examples is still .8 cfm.

Thanks, got it. Average to peak flow. I reread your first post and it finally clicked.

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.

Is this referred to as the cracking pressure of the second stage? I have seen the term in a couple of threads but I am not sure what it is.

 

[halocline]

IP is intermediate pressure, which is the pressure that the first stage sends to the second, usually about 130-140 PSI above ambient pressure. If you want to see how a 2nd stage breathes without being fed this pressure, try disconnecting a 2nd stage from the hose and breathing off of it.

 

[copter53]

"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. "

Would you say that as a general rule if one experiences a reg malfunction underwater he should try to breath slowly and never suck harder as an attempted solution?

 

[MauiScubaSteve]

I will continue to teach a "half turn" back off from fully open.

When I was taught to teach diving, the concept of not fully open was described as "proper valve operation" for any valve. If someone is "checking valves" a fully open valve may feel like a fully closed valve, so that person increases effort turning the valve "on" harder and harder, even though the valve is already all the way on. If the valve turns for a while and then stops, the "checker" does not run the risk of damaging the valve.

Luckily for me, if you are using my tanks (or my employers tanks) I (or my coworkers) will be involved in turning on the tanks, so most of the time undue stress on the valve will not happen.

For my students later in life; when someone totally screws the pouch and rightie loosies, there will be some air flowing at the start of the dive. If there is the possibility that people will jump in deep water with valve only "half" a turn on, there is also the possibility they might jump in fully closed. I prefer "half" a turn on for my ducklings.

 

[Hatul]

I will continue to teach a "half turn" back off from fully open.

When I was taught to teach diving, the concept of not fully open was described as "proper valve operation" for any valve. If someone is "checking valves" a fully open valve may feel like a fully closed valve, so that person increases effort turning the valve "on" harder and harder, even though the valve is already all the way on. If the valve turns for a while and then stops, the "checker" does not run the risk of damaging the valve.

Luckily for me, if you are using my tanks (or my employers tanks) I (or my coworkers) will be involved in turning on the tanks, so most of the time undue stress on the valve will not happen.

For my students later in life; when someone totally screws the pouch and rightie loosies, there will be some air flowing at the start of the dive. If there is the possibility that people will jump in deep water with valve only "half" a turn on, there is also the possibility they might jump in fully closed. I prefer "half" a turn on for my ducklings.

Both of my tanks use the new indicator valve:

Vindicator Valve Handle

which makes it easy to see if the valve is fully open -- the indicator is all green.
It's a useful safety feature and I highly recommend it.

Adam

 

[Akimbo]

Both of my tanks use the new indicator valve:

Vindicator Valve Handle

which makes it easy to see if the valve is fully open -- the indicator is all green.
It's a useful safety feature and I highly recommend it.

Adam

That looks like the XS-Scuba Safety Hand Wheel. For something that adds very little cost, I find them a great feature; especially when charging a lot of bottles at once. A couple of charter boat dive masters told me they like them as a quick visual before divers go overboard.