The Isolation Manifold, lessons not learned and a small defence of the IUCRR

[Akimbo]

I think burst disks are a vestige of when scuba tanks had tapered thread valves.

Government requirements for or against burst discs applies to all pressed HP cylinders in the jurisdiction. It basically comes down to which side of the probability compromise the governing committee settles on. Which is more of a risk, escaping toxic gas (the most common industrial gas consideration) or the cylinder exploding?

Unfortunately cylinders still occasionally explode with burst discs and a diver's non-toxic breathing gas can be lost when a blow a disc fails. There are valid arguments on both sides.

I would prefer a prohibition of burst discs on all HP breathing cylinders around the world, but some fool forgot to put me in charge.

 

[Angelo Farina]

I am one who believes the analysis of dive accidents is a great help to the community.

As for manifolds, I have never heard of a closed manifold problem among the people with whom I have dived, but I do have a story that might be telling if I could only figure out what it was telling.

Years ago, when I was finishing my trimix training, I took the final exam. The instructor downloaded the exam in MS Word format from the agency website. IIRC, the exam was about 10 years old. One question had me baffled. I don't remember the exact wording, but it set up a scenario in which a diver is in a dive that seems to be going normally, but when he checks his SPG, he sees that he is going through his gas about twice as fast as expected. I was supposed to suggest a reason.

My instructor explained that the correct answer was that the isolator was closed, so all the gas was coming out of one cylinder. I pointed out that in a normal gear configuration, if the manifold was closed, the diver would not see that I was going through gas too quickly, the diver I would see that he was not going through gas at all. The SPG usually comes off the left post, and the diver usually breathes off the right post. The question would only make sense of the diver had made the highly unusual decision to run the SPG off the right post. My instructor, quite surprised, admitted that I was right and he had never thought of that before. Evidently, neither had any of his students.

So for at least 10 years, people from that agency had been taking that exam. If anyone in that entire agency had noticed this, they either had not notified the agency or the agency had not felt like taking the 5 minutes it would have required to change the MS Word exam and post the new version to the website.

So let's say that in real life a diver with an isolation manifold suddenly has an OOA response during an inhale because of a closed isolator. That means that the diver had not yet checked the SPG at any point in the dive.

Good story. But my setup has the SPG attached to the primary reg. The secondary reg, usually mounted on the left post, has no additional hoses, it is just first and second stage.
On the other hand, my twin tank has no separation manifold. But it has the Technisub spring-loaded reserve on the right tank, the type of reserve which cannot be opened by error when tank is full, and which does not block the filling of the tank if left closed.
You may think that my setuo is a bit vintage (it is, both tanks and regs were manufactured in the seventies). But I consider my setup far safer than "modern" manifolded twins and plastic regs...

 

[Celt]

Government requirements for or against burst discs applies to all pressed HP cylinders in the jurisdiction. It basically comes down to which side of the probability compromise the governing committee settles on. Which is more of a risk, escaping toxic gas (the most common industrial gas consideration) or the cylinder exploding?

Unfortunately cylinders still occasionally explode with burst discs and a diver's non-toxic breathing gas can be lost when a blow a disc fails. There are valid arguments on both sides.

I would prefer a prohibition of burst discs on all HP breathing cylinders around the world, but some fool forgot to put me in charge.

I wouldn't be bringing a burst disc into a wreck or cave, a tank dumping 100cu ft. with no way of shutting it down is going to take a bit of dealing with.

 

[Akimbo]

I wouldn't be bringing a burst disc into a wreck or cave, a tank dumping 100cu ft. with no way of shutting it down is going to take a bit of dealing with.

It is not unusual for some people diving in critical conditions (hard overheads, etc) to replace disks with much higher ratings, or double them up.

 

[p_kos]

It is not unusual for some people diving in critical conditions (hard overheads, etc) to replace disks with much higher ratings, or double them up.

And if a burst disk were to pop in such a scenario having and isolator valve would be nice indeed. Which also suggests that a non-isolator manifold should not have burst disks as part of the system.

 

[Akimbo]

And if a burst disk were to pop in such a scenario having and isolator valve would be nice indeed.

Not just a burst disc, the valve to cylinder O-ring too.

 

[p_kos]

Not just a burst disc, the valve to cylinder O-ring too.

Yes, but the burst disk failing catastrophically seems a lot more likely although I have no numbers to back that up. I also imagine most cylinder orings fail with a slow leak that wouldn’t pose any immediate danger.

 

[Akimbo]

Yes, but the burst disk failing catastrophically seems a lot more likely although I have no numbers to back that up.

I doubt reliable statistics are available but both are quite rare. We have all seen leaking valve stem O-rings but I have never heard of one blowing out. Same with isolation valve O-rings.

At some point you take your chances or stay on deck. You can always adopt a rig like this:

 

[Kay Dee]

Can you explain how the isolator helped and a manifold without one would not?

Pardon delay in reply but promised the missus I’d stay away from the computer for at least a couple of days over Easter (and I don’t check in here all that often at the best of times). Anyway, I see others have stated basically what I would have said, so I wont repeat. Besides, with all due respect, if you can't answer your own question, then, well...........

As for someone else’s question re how others ‘set’ their isolator valve (fully open, half, etc.). Can’t speak for others, nor do I expect others to agree, but when I dived OC mine was two half turns. That is, it took me two turns / flicks of the wrist to close it. And being right handed, it was much easier to turn that isolator valve clockwise than anti-clockwise, so even if muscle memory had failed me, tactile feel would have quickly alerted me to trying to turning it the ‘wrong’ way (i.e. more open). As for accidentally shutting during a dive, sure possible in some circumstances, but that’s part of what religiously checking an spg is about also. But each to his own, one size doesn't fit all, but anyone who goes diving - or fails to check before leaving the fill shop - that the isolator is / was ‘open’, well that’s their own fault they missed a dive, etc. Personal responsibility I believe it is called.

 

[crofrog]

This is a reason to conduct a flow check ~15 minutes.