beginning with doubles

[shoredivr]

One perceived challenge of doubles is that many start with them at the same time they begin tec training, and we therefore tend to associate doubles with tec diving, which probably isn't appropriate. Tec IS a task loaded environment, but the doubles are only one small piece of that. So, if you can carry them and afford them and start with them in a prudent manner (your pool sounds like a good idea), why not?

Yes...

 

[Walter]

I could be wrong, but doesn't the isolated manifold only introduce a failure of the isolator itself compared to a non-iso manifold ?

You are correct, but OTOH, adding the isolator introduces a failure point with no real benefit.

So there is then no advantage to the non-iso version as you always lose all the gas in both tanks with a failure that an isolator could help with ?

The only time an isolator would help is it you have a failure of the tank neck o-ring or the burst disk. While a failure of the isolator valve is extremely rare, it is much, much more likely than blowing a tank neck o-ring or losing a burst disk during a dive.

 

[Colliam7]

You are correct, but OTOH, adding the isolator introduces a failure point with no real benefit. ... The only time an isolator would help is it you have a failure of the tank neck o-ring or the burst disk. While a failure of the isolator valve is extremely rare, it is much, much more likely than blowing a tank neck o-ring or losing a burst disk during a dive.

Interesting point. I have seen several occurences of a blown yoke O-ring. I have never seen a case of a blown DIN O-ring (not saying that it doesn't happen). I have also never seen a case of a blown tank neck O-ring. In your experience, is a tank neck O-ring failure, or a burst disk failure during a dive for that matter, a) unheard of, b) extraordinarily rare, or c) just uncommon? Maybe a hard question to answer. But, you make a good point - introduction of the isolator valve adds several possible failure points, including the connections on both sides, as well as the valve. I have had small leaks there but not a catastrophic failure, so I wonder about relative frequency.

 

[Walter]

B for burst disks, tank neck o-rings and isolator valves. Burst disks and tank neck o-rings are closer to A, but not there.

 

[TSandM]

I wonder if the concern about burst discs came out of cave diving? Most people who have had them fail say that they failed on land, after the tanks were filled (that was certainly the case with the one that blew on us), but I know some manifolds are designed so that the burst discs stick out. Coming in contact with the roof of the cave might shear one, I suppose. (I would have thought that vanishingly unlikely, but on our recent trip to Florida, we picked up TWO manifold valve knobs that sheared -- scooters and cave roofs are not a good combination, I think.)

The one that always boggles my mind is the tank neck o-ring. It's being replaced once a year, if the tanks are undergoing visual inspection as they should, and once it's there, it's not moved or stressed again except by continuous tank pressure. It seems wild that one of them would fail, but I guess it has happened.

 

[ClayJar]

The one that always boggles my mind is the tank neck o-ring. It's being replaced once a year, if the tanks are undergoing visual inspection as they should, and once it's there, it's not moved or stressed again except by continuous tank pressure. It seems wild that one of them would fail, but I guess it has happened.

If the valve is not installed properly, the neck O-ring can extrude. I've only seen that once, and although the neck O-ring was partially extruded, it had not yet failed (defining failure as loss of containment). Generally speaking, for a neck O-ring failure there would almost have to be damage to the neck or valve, grossly improper torque on the valve, or lube on the O-ring.

In other words, don't let Joe-Bob the muffler guy mess with your tank valves. (It's not like anything in the viz class is rocket surgery. :biggrin

 

[limeyx]

The only time an isolator would help is it you have a failure of the tank neck o-ring or the burst disk. While a failure of the isolator valve is extremely rare, it is much, much more likely than blowing a tank neck o-ring or losing a burst disk during a dive.

I'd surely love to know where those numbers are coming from.

 

[Walter]

I'd surely love to know where those numbers are coming from.

What numbers?

 

[limeyx]

What numbers?

The analysis of in-water failures of isolators compare to tank neck/valve failures that finally convinced you the isolator cures fewer failures than it solves.

 

[MBH]

I had a Tank neck o-ring extrude on a hp119 last September. It was a super bad leak, loud as hell, and it still took over an hour to bleed off the gas in that tank. Knowing what I know now, I'd isolate, and then breathe down the tank with the blown valve to neck o-ring failure. I could get a lot of mileage on that leaking tank before I needed to go back to my intact tank.