Rate of freeflow

[fisherdvm]

Is it first stage dependent, second stage dependent, first stage limitted, or second stage limitted.

Does it depend on regulator design (balanced vs unbalanced first stage, or balanced vs unbalanced second stage).

Do "high flow" regulator designs make it flow faster?

Roughly how fast would a free flow first stage empty an 80CF tank vs. how fast a free flow second stage empty an 80CF?

Is pushing the purge button the same or faster than a true free flow from either the first or second. My guess it is faster, as the seat is fully opened.

 

[editmojo]

Scuba Diving Videos Cutting a Divers Hose Scuba Videos

Not an exact answer, but more fun.

 

[ianr33]

This was an informative thread
The Deco Stop

Time to empty a full tank from a freeflowing reg was depth dependant but around 2-3 minutes at normal depths. A burst LP hose was faster. 83 seconds!

Not a lot of time is it?

 

[24940]

This was an informative thread
The Deco Stop

Time to empty a full tank from a freeflowing reg was depth dependant but around 2-3 minutes at normal depths. A burst LP hose was faster. 83 seconds!

Not a lot of time is it?

Interesting. I ran a test using an AL80 a couple years ago and found little difference in free flow rate from the surface to 90'. The restriction of the valve orifice was the determining factor. As I recall, it took about one minute to drain 1000psi at the surface and a few seconds longer at 90'.

I'll have to try it again with tighter controls, but Kurt's report bears different results than did my experience.

 

[ianr33]

Interesting. I ran a test using an AL80 a couple years ago and found little difference in free flow rate from the surface to 90'. The restriction of the valve orifice was the determining factor. As I recall, it took about one minute to drain 1000psi at the surface and a few seconds longer at 90'.

I'll have to try it again with tighter controls, but Kurt's report bears different results than did my experience.

Kurt was probably using tanks with DIN valves. Might have a larger orifice than a yoke valve? Maybe varies with model/manufacturer? I can see there are lots of variables here.

Never thought about the valve orifice being the limiting factor before. It would make having a high performance reg kinda useless if the tank can not supply as much air as the reg can!

 

[24940]

Kurt was probably using tanks with DIN valves. Might have a larger orifice than a yoke valve?

Bingo! I'm sure that's it. Sometimes, the obvious escapes me.

 

[wedivebc]

I usually perform a leakdown test when teaching tech classes to demonstrate the need for fast valve drills. I usually use Apeks regs for this test. Once when demostrating how much gas can be lost from a cut hose in a classroom I borrowed the shops old Scubapro piston reg. It flowed so much air the scintered metal filter disintegrated and bits of metal embedded in the valve seat and body. It seemed to me more air flowed from that reg than if the tank had the reg removed. I can't afford to repeat the experiment but it was an eye opener.

 

[DA Aquamaster]

There is a great deal of variation in valves.

Way back in the day, the dip tube tended to be pinched closed at the end with two small 1/16' to 3/32" diameter holes drilled crosswise. The good news is that it did a great job of keeping crud out and if you blew a burst disc it took a long time to drain the tank, the bad news was that reg performance on higher performance regs was probably limited by the low flow rate of the valve.

Today dip tubes seem to have the same .250" external diamter and about the same .130" ish internal diameter - and that seems to be true with both DIN, K and convertible DIN/K valves.


The orifice on K-valves however usually seems to be quite small at around 1/16" to 3/32" of an inch. In the case of the former that would represent the potential choke point for the valve compared to the dip tube. On the other hand the orifice in a DIN valve is usually larger, but also involves a hard 90 degree turn rather than a couple 45 degree turns.

Still, the valve is the amjor limitation.

TDI published some test results that indicated that a burst disc would empty an AL 80 in 72 seconds at the surface or at depth, while a ruptured LP hose took 83 seconds at depth or at the surface and a freeflowing second stage took 255 seconds at the surface and 91 seconds at 230 ft.

So in effect, with out considering the valve, the second stage is the limiting factor as the other two failures will vent gas faster and it is the only failuire significantly affected by depth.

That also makes perfect sense as the higher total intermediate pressure at depth compared to the surface would act much like turbocharging in a car or aircraft with the higher pressure resulting in a higher mass flow through the second stage.

 

[fisherdvm]

I've tried to empty my tank for bouyancy check by depressing the purge button. It sure seems like about 3 or 4 minutes to go from 1000 psi to 500 psi in a shallow pool. So I don't think the purge button simulate the maximum free flow described by others here.

I still wonder if a frozen first stage would dump air through the second stage alot quicker than pushing the purge button, as you are dealing with 1000 or 2000 psi as the IP than the 120 to 150 of the normal intermediate pressure.

 

[hudson]

[-]You wouldn't be looking at 1000 to 2000 psi pressure though (assuming you have that much air in the tank).[/-]

Nm, yeah I guess you are right, given it's an open system you should see the flow rate about double at 100ft compared to a purge. I'm a little surprised that the first stage can maintain the IP though during a second stage freeflow, but that's the only explanation.

So maybe a good way to check out the effects of a first stage ff is to dive to a about 300ft and try a purge