How Long to Drain a 80 Cubic Foot Cylinder - 4 Failures

[miketsp]

Ever since Curt wrote that article something about the results have seemed counter intuitive to me, namely that a blown burst disk or a blown LP hose will vent the SAME volume of air per second regardless of depth while a free flowing regulator vents more air the deeper you go.

Any theory crafting on why that would be the case?

R..

Overcompensated 1st stage?

 

[Hatul]

Ever since Curt wrote that article something about the results have seemed counter intuitive to me, namely that a blown burst disk or a blown LP hose will vent the SAME volume of air per second regardless of depth while a free flowing regulator vents more air the deeper you go.

Any theory crafting on why that would be the case?

R..

The second stage delivers air at the ambient pressure, which is higher the deeper you go. A blown LP hose delivers air at IP which is about 140 psi and is fixed regardless of depth.

 

[Storker]

The second stage delivers air at the ambient pressure, which is higher the deeper you go. A blown LP hose delivers air at IP which is about 140 psi and is fixed regardless of depth.

Hmmm... Haven't I seen that explanation before?

 

[miketsp]

The pressure in the tank or in an LP hose is independent of ambient pressure.

The pressure of the air from a second stage regulator is equal to the ambient pressure.

The amount of air in an airstream is proportional to the pressure of the airstream, ergo will a free-flowing second stage provide more air (measured as mass, or volume at the surface, or volume at tank pressure) the deeper you go while a direct vent from the tank or the LP hose provides the same amount irrespective of ambient pressure.

The second stage delivers air at the ambient pressure, which is higher the deeper you go. A blown LP hose delivers air at IP which is about 140 psi and is fixed regardless of depth.

IP in an LP hose is not fixed regardless of depth.
It's 140psi ABOVE ambient. Otherwise at 90m IP would be equivalent to ambient and you wouldn't get any airflow.
In the case of an overcompensated 1st stage it will actually increase as you go deeper.

 

[Diver0001]

IP in an LP hose is not fixed regardless of depth.
It's 140psi ABOVE ambient. Otherwise at 90m IP would be equivalent to ambient and you wouldn't get any airflow.
In the case of an overcompensated 1st stage it will actually increase as you go deeper.

It's fixed at about 10 ATM.

Apparently it is of little consequence to the flow rate at recreational depths.

I'm still surprised at the result for the 2nd stage.... intuitively I don't see a big difference between a 2nd stage that's "wide open" and an LP hose that is "cut open"

Evidently there is.... but it was a bit of surprise to me.

When Curt first posted that result I didn't have the self-confidence to question it.... I'm glad I did this time and found a logical explanation. Thanks to Storker and Hatul.

R..

 

[Bob DBF]

IP in an LP hose is not fixed regardless of depth.
It's 140psi ABOVE ambient. Otherwise at 90m IP would be equivalent to ambient and you wouldn't get any airflow.
In the case of an overcompensated 1st stage it will actually increase as you go deeper.

Semantics.
The IP is fixed at 140PSI in relation to the pressure to which the first stage is subjected, hence the term regulator.

The second stage is a demand reg and the size of the orafice and how far the seat and valve moves apart would determine the airflow through the second compared to an open hose. In both cases the primary will, most likely, be full open trying to maintain 140PSI. Regardless of which happens, it is time to beat a hasty retreat as you will be empty shortly.

The few cases I have been involved in have not been complete failures but have put enough air in the water so the distinction between total and partial failure was not acertained untill after the fact.



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

 

[Diver0001]

Semantics.
The IP is fixed at 140PSI in relation to the pressure to which the first stage is subjected, hence the term regulator.

Bob, there are a couple of regulators on the market that "over balance", meaning that the IP in the first stage increases slightly as function of depth. Apeks and Aqualung both have them, perhaps others to do. I think that's what Mike was referring to.

I heard, although I cannot confirm it, that it was originally a design flaw that was subsequently marketed as a feature with the argument that over-balancing compensates for the density of the air at deeper depths, making breathing easier. It stands to reason that this will only work with balanced 2nd stages.

R..

 

[Tigerman]

Ive seen a first stage o-ring blow and it was not on an "idiot diver"s tank, but a very experienced one. **** happens and thats why you have backup plans. In case of blowing o-rings, 2nd stage freeflows and ruptured LP hoses that plan need to be readilly available and used with very little hesitation.

Ive also seen a hp hose blow under water and while it sounds bad with the big bang and the hissing of the bubbles, I was pretty surprised to see first hand how little gas actually appeared from the tank.

 

[miketsp]

Bob, there are a couple of regulators on the market that "over balance", meaning that the IP in the first stage increases slightly as function of depth. Apeks and Aqualung both have them, perhaps others to do. I think that's what Mike was referring to.
..snip..
R..

Correct. I don't much like the term over-balanced because its not the opposite of un-balanced. An un-balanced 1st stage only compensates for ambient, keeping the IP at 140psi or 10ATM above ambient but it does not compensate for drop in tank pressure. A balanced 1st stage compensates for both ambient and tank pressure drop. OTOH the overbalanced or over-compensated 1st stage as defined by APEKs for example "..... implies that as the ambient pressure rises, the difference between ambient pressure and intermediate pressure increases slightly. This would in fact compensate for the effect of increased viscosity reducing the flow rate slightly."
Most manufacturers will tell you that piston regulators can't be overbalanced (only diaphragm first stages can be overbalanced - a simple design change to alter the area exposed to ambient) but then confusion arises in that some of the Scubapro literature talks about overbalancing to discuss the compensation used on their piston regulators to compensate for tank pressure drop.
"Scubapro for example "overbalances" their current balanced piston regulators by designing the seating edge of the piston stem to be slightly larger in diameter than the portion of the piston stem that passes through the high pressure o-ring. This ensures that actual area of the seating edge itself does not affect the balancing. The result is that the IP is indeed very stable and does not drop at all as the tank pressure drops. In an otherwise "balanced" piston regulator (like the older Mk 5 or Mk 10 "balanced" piston designs) the identical diameter of the middle and seating ends of the piston stem result in a 3 to 4 psi change in IP between 3300 psi and 300 psi." (this bit copied from a DA Aquamaster post).
So most discussions on regulators do have a lot of confusion just because of terminology.

---------- Post added March 15th, 2013 at 06:26 PM ----------

..snip..
Ive also seen a hp hose blow under water and while it sounds bad with the big bang and the hissing of the bubbles, I was pretty surprised to see first hand how little gas actually appeared from the tank.

Yep, that's why CMAS used to teach that you should use your knife to cut the HP hose to breathe off in certain 2nd stage failure situations (in the days when it was not very common to see a spare 2nd stage) rather than the LP hose. You get a nice controlled air flow.

 

[billt4sf]

This also shows that for recreational depths in open water, a 13 cu ft should be adequate for all those failures, as you can make it up to 60 ft during a failure before you need to change over to the pony tank, and then a 13 is adequate for a safe ascent to surface.

Yeah, maybe, if you don't panic. For a burst disk (is that an O-ring?) it said you have 72 secs from a full 80 cf -- that's not a lot of time to get to the surface from 60 feet. And then no air to inflate your BCD when you get there. You better have a pony or a buddy!

So we're all practicing oral inflation, right?

- Bill