Free flow at depth?

[ZenSquirrel]

At 30m I use my gas 4 times faster than I would at 0m. If I hold down my purge at 30m would it purge 4 times faster as well? My guess is yes but I don't know for sure.

For those that are wondering, I'm just trying to understand my gear a little better.

 

[Lead_carrier]

No it won't purge 4 times as fast. The gas will only flow out of a regulator so fast. The gas WILL expand as it rises to the surface tho.

 

[norcaldiver]

I could be COMPLETELY way off here, but I would think "on paper" it should actually flow slower at depth.
Here's my thinking. You have 3000psi in the tank that wants to equalize with the surrounding preasure. Now if you were to take a tank to a depth where the surrounding preasure was 3000psi and opened it up, the air inside the thank would not try to rush out to equalize. If the surrounding presure was 1500, it would come out half as fast as at 1ata...
Like and empty tank - the lower the presure in the tank the slower the release of gas, like when it's tottaly empty you can barely hear it push gas out.

Then again, maybe it flows the same no matter what, it just takes less time to equalize the closer the presures are to being the same...

Does anyone know?

 

[ZenSquirrel]

So at 30m it takes 4 times the air to fill my lungs. The one time that I have been that deep I knew that I was using my gas 4 times faster than at the surface but I was breathing at the same rate as always. It did not seem to take the reg 4 times as long to give me a breath. That is why I assumed that it would free flow faster.

If I can get 4 times the air in the same amount of time then I assumed the reg was flowing at 4 times its rate. I don't remember having to "suck" on the reg to get air and the air did not feel "thin", it just felt like breathing at any other depth above 30m.

Just a little more on where I got my guess from.

 

[fins wake]

... at 30 metres you will effectively have four times less volume of free gas.

A free-flow at any depth, even at the surface will empty even a large twinset very quickly, in a matter of minutes. (Take it from me, I've tried.)

Disregarding flow rates, if you get a free-flow at depth and you're on a single tank, you really want to reach the surface as fast as you safely can.

 

[HydroAuto]

Using the general guidelines:
* we are subject to 1 atmosphere at sea level
* 2 atmospheres at 10 meters, air is compressed to 1/2 original volume
* 3 atmospheres at 20 meters, air is compressed to 1/3 original volume
* 4 atmospheres at 30 meters, air is compressed to 1/4 original volume

Lets say you swim down to the bottom of a flooded mineshaft with a spare tank. Assuming you (and your equipment) survived the descent, at a depth of 200 meters you (and your tank) would be subjected to ~210 atmospheres (2939psi). Now the guy at the LDS only gave you about 3000psi in your tank, and you've lost a couple of psi because the temperature at 200m is pretty friggin' cold, you now have a pressure differential of 0 psi.

IE: No gas is flowing out of your tank.

Working backwards from this point as you started your ascent back to the surface, as the pressure differential increases more & more gas would come out of the tank.

So just the opposite of your first assumption, you would free-flow slower at deeper depths than you would at a more shallow depth.

However, the amount time it takes until no more gas comes out would decrease as you descend (read: no more bubbles more faster )

 

[DA Aquamaster]

I am assuming you mean 2000 meters not 200. You are correct that the pressure differential will be lower at extreme depths but this really has no impact as long as the available pressure differential is equal to or greater than the regs intermedate pressure. It is the intermediate pressure that determines whether the reg can operate at its peak potential, what ever the depth.

The larger problem with air at extreme depths is related to viscosity. Air under pressure has its molecules pressed closer together and is in effect "thicker". It is essentially the same thing as comparing water flowing through a restiction under pressure to molasses. The same lungful of air at depth will have a lower flow rate than it will nearer the surface simply because you have a lot more molecules of air in that one breath flowing through the regulator than you would at a shallower depth.

Most high performance regs have received a great deal of design effort to increase air passages and reduce flow restrictions in order to eliminate this as an issue at remotely sane depths (less than 500 ft.)

 

[Diver0001]

ZenSquirrel once bubbled...
At 30m I use my gas 4 times faster than I would at 0m. If I hold down my purge at 30m would it purge 4 times faster as well? My guess is yes but I don't know for sure.

For those that are wondering, I'm just trying to understand my gear a little better.

I'll try explaining this differently than the others. The answer to your question is YES. It will empty the tank 4 times faster than it would have done if you were still on the surface.

In other words, a heavy free flow would probably empty a full tank in a little less than 4 minutes on the surface. At 30 meters you'll have less than a minute.

Does that help?

R..

 

[Charlie99]

Diver0001 once bubbled...
In other words, a heavy free flow would probably empty a full tank in a little less than 4 minutes on the surface. At 30 meters you'll have less than a minute.

Have you actually done a test on this ..... such as measured how many bar you lose from depressing the purge for 15 seconds at 30 meters vs 15 seconds of purge at surface? Which reg?

If the orifice of the demand valve were the only limiting factor, and the IP stayed close to rated value even during freeflow, then you statement is probably true. If on the other hand, the main flow restriction is in the first stage, then variation in freeflow rate vs. depth would much less.

Not having done this test, I had just assumed that it would be a bit faster, but nowhere near the 4 times you describe.

Thanks in advance,

Charlie

 

[divemed06]

Does this mean that a regulator is less likely to freeflow at depth than in shallow water (all other factors being the same)????