Tank blowout - airloss rate

[rjack321]

HP ports have a tiny orifice - for exactly this reason. They don't need to flow basically at all. LP ports have no such restriction. So yes, LP hose will drain a tank faster.

For the incident in question, the rate isn't calculate-able because nobody knows the size of the gap.

 

[Ed Palma]

thanks folks! I read the other thread, and in addition to the other posts here, it clarified that point for me (HP vs LP). As for the neck o-ring, I just got that anecdote from the spare-air site (from the spare-air thread). I wonder how often that happens?

 

[ClayJar]

"Obviously you'll lose more at deeper depths."

Not necessarily. If an LP hose (or its O-ring(s)) blows or you free-flow, then you would lose more at deeper depths, as the pressure (and therefore, the quantity of air) increases parallel to ambient pressure. However, if an HP hose (or its O-ring(s)) or a valve O-ring (i.e. at the yoke) blows, you'll be losing air from the tank pressure to ambient, which is greater at deeper depths, meaning the delta-P across the breech (and by relation, the flow rate) is inversely related to ambient pressure. Although I do not believe this relationship is significant for recreational diving (I would simply consider HP blowouts to be constant rate vs. depth), it does run counter to the obvious rule that applies to LP blowouts.

As far as draining a tank faster if an LP hose bursts vs. the HP hose, the LP hoses are designed to allow as much air through them as possible with minimal restrictions (you want to be able to hoover the tank if you need to, after all). The HP hoses, on the other hand, are normally equipped with a very small (almost pinhole) orifice on the end that screws into the first stage, as there is no need for large flows through the hose. In the event of an HP hose (or SPG) blowout, this very small orifice restricts the flow to a mass flow rate much lower than that of an LP hose blowout.

 

[Santa]

Just a battered old rental yoke. I witnessed 3 blowouts while I was there - which is more ´than I've seen in all the rest of my diving.

I think the problem was that they couldn't get that white paper tape for the tanks so they used a stickier black type. I believe it stuck to the o-rings and pulled them out a bit upon removal - so if you weren't careful you'd squeeze'em.

But I'm definately getting some true redundancy for some of the diving I'm doing.

 

[Santa]

I had a 140 or 160 bars when it went and surfaced with around 10. Including the safety stop that lasted me for 7-8 minutes - all inclusive ;o).

Regarding depth being a factor I'm not thinking so much about pressure differences as just plain breathing expenses. Obviously that's a factor.

About the size of the gap - that's an unknown for sure. I get my bearings from the fact that my buddy who's a very experienced DM said that judging from a purely visual impression it was "as big a blowout as he'd ever seen" ;0). But you're right.

It blew at the neck of the tank by the way - but it's a thought that you'd never be able to tell what had blown while wearing the BCD - so you'd have to assume the worst.

 

[Santa]

How did your buddy and the other divers around you respond? It seems to me that this would make a huge cloud of bubbles and attract attention. You wrote that you went for your buddy (the closest diver). Did he notice it before you swam towards him?

Like your signature ;0)

Can't say for the rest but the closest one who had around 40 dives gave me the double-golf-balls - and my buddy was maybe 7-9 metres above and to the side so I dove beneath him knowing the bubbles would alert him to the problem. It was a conscious decision to go with him, partly so as not to spoil the dive for the other paying divers partly for the added safety. I'd done some 30 dives with that guy while I was there and I'll admit that we we're in the habit of being "loosely associated depending on depth and conditions" at the time but hey ...;0).