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

[Pedro Burrito]

I thought this was interesting. Written by Curt Brown, from Advanced Diver Magazine:

http://www.advanceddivermagazine.com/articles/lifeendingseconds/lifeendingseconds.html

A full 80 cubic foot cylinder will empty in 72 seconds if the burst disk fails on the surface and 74 seconds if it fails at 99 or 232 feet. It will empty in 22 minutes if a high pressure hose ruptures, regardless of depth. If a low pressure hose ruptures, you've got 83 seconds on the surface, 81 seconds at 99 feet and 82 seconds at 232 feet. If your regulator free flows, you've got 255 seconds on the surface, 155 seconds at 99 feet and 91 seconds at 232 feet.

So those videos of diver's dangling SPGs rubbing on rough edges of openings on the Spiegel Grove isn't that big of a deal.

 

[MaxBottomtime]

That's if it's full. If you're at 99 feet, it probably isn't. In that case, make a hasty retreat toward another diver or the surface.

 

[ermaclob]

how likely is that really tho. how many documented cases of a burst disk poping underwaterare there? i know they can on the surface ive seen them. tho ive never been told of one going underwater..... prob because they all died...

 

[Scuba_Noob]

I've had a buddy who had a valve fail on her. I've seen an o-ring burst under as well.

I'm sure the burst disk data is just there so we can see a maximum speed that the tank will drain. It's pretty relevant data.

 

[KeithG]

These are interesting numbers that most divers should be aware of. I was not. Until a recent incident. But everyone needs to be aware your mileage may vary. These are extreme numbers. Your situation could be different. Ours was. No where near the worst case.

I would have thought that the high pressure hose would empty the tank faster than the lp. Nope - seems the orifice in the HP is very teensy. Not much flow to be had. Takes a long time to drain the tank.

My dive buddy experienced a split in her lp hose at the regulator. After establishing how fast it was leaking (there seemed to be lots of bubbles) we spent 5 minutes getting back to the boat underwater. She still had lots of air left. In retrospect it was not a worst case failure. I posted a few months ago on near misses.

Knowing the worst case is good, but people need to not panic, think a bit and react appropriately to their situation.

 

[lindenbruce]

That's still NOT a reason to have gear dangling anywhere, let alone on a wreck. Secure that stuff. Bolt snaps and cord/o-rings/zip ties are cheap. I too was surprised by the fact the LP drains much faster than the HP. B

 

[Diver0001]

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..

 

[Storker]

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.

 

[goflyplanes]

how likely is that really tho. how many documented cases of a burst disk poping underwaterare there? i know they can on the surface ive seen them. tho ive never been told of one going underwater..... prob because they all died...

One of the morons I was diving with last week had an O-ring go at about 8m. I like to think it was God's version of natural selection but that sadly wasn't the case. I feel like from 8m though other than the panic factor you're really not in THAT precarious of a situation.

 

[Hatul]

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.