Crossbar and isolator valve: do they help?

[helodriver87]

There is at least one near-miss that was described in an earlier post in this thread that would not have been mitigated by what you just posted.

I looked, but couldn't find it. Got it handy?

 

[stuartv]

I looked, but couldn't find it. Got it handy?

My bad. It was in a linked article.

“Ideal” Manifolds… Not So Ideal?

 

[helodriver87]

My bad. It was in a linked article.

“Ideal” Manifolds… Not So Ideal?

The left post roll off? At least that's what I gathered from reading the description. That doesn't have anything to do with an isolator. Check your valves when you bump em.

I actually had an interesting one last year. On the surface, thankfully. We were setting up a downstream Emerald dive. My doubles were assembled and checked, valves open, and upright in my truck bed. We're going over the details in the parking lot and I hear a "POP-WHOOSH" as gas starts dumping out of my doubles. I tried to shut down the right post but couldn't do it. So we isolated and tried to figure out what it was, as gas was coming out of the handwheel. Ended up being an extruded bonnet nut o ring. That was the root cause, at least. For whatever reason, the valve stem got jammed up when that happened and we had to basically let that side drain and break down the valve to close the HP seat with a flathead. Would've been bad news underwater on a straight bar manifold.

 

[Mark IV]

Really interesting thread. Among other things, it's given me a new appreciation for the set-up I most frequently used back when still cave diving: Three hp120's, one backmounted, and two side mounted. I loved the redundancy, and the easy access to the valves, and I always felt more nimble than with a big honkin' set of doubles on my back, which never felt natural or comfortable, to me. Though I used them when I had to, I could never bond with the complexity of an isolation manifold, or it's awkward location.

 

[stuartv]

The left post roll off? At least that's what I gathered from reading the description. That doesn't have anything to do with an isolator. Check your valves when you bump em.

I actually had an interesting one last year. On the surface, thankfully. We were setting up a downstream Emerald dive. My doubles were assembled and checked, valves open, and upright in my truck bed. We're going over the details in the parking lot and I hear a "POP-WHOOSH" as gas starts dumping out of my doubles. I tried to shut down the right post but couldn't do it. So we isolated and tried to figure out what it was, as gas was coming out of the handwheel. Ended up being an extruded bonnet nut o ring. That was the root cause, at least. For whatever reason, the valve stem got jammed up when that happened and we had to basically let that side drain and break down the valve to close the HP seat with a flathead. Would've been bad news underwater on a straight bar manifold.

I was thinking of the doubles that ended up with all O2 on one side and EAN32 on the other. Just remembering to open the isolation valve before jumping in the water was not going to help that guy.

 

[cerich]

I'm glad for this conversation. I'm not too proud to share my own shortcomings.

I was teaching an Intro to Tech student this past weekend and we were going over failure scenarios. He asked me if it would not be just as good or better, when he hears bubbles, to turn off the isolator first, instead of turning off the actual post where he believes the bubbles are coming from.

At the time, I did not have a good answer for why tuning off the post first is the way it is taught. I admitted that and told him that, since I didn't have a solid reason why the post first is better, I would have to fall back to, "because I don't have a solid answer and either way seems equally valid on paper, we're going to stick with doing it the way I was trained and the way I'm teaching you." I.e. turn off the post first.

This discussion has now given me what I feel like is a solid answer for why we turn the post off first. The vast majority (maybe even all) of the times you have a failure to deal with, it's going to be something that is mitigated by turning off the post. If you turn off the isolator first, you still are going to have to turn off the post. Thus, by turning off the isolator first, all you've done is waste time and the gas you lost during that time. Basically, you will lose more gas by turning off the isolator first, rather than turning off the post first.

The only time that would not be true is if you have a tank valve O-ring or burst disk blowout. And, those are so rare in comparison to other possible failures, that when we hear bubbles we start with the presumption that it will be addressed by turning off the post - because that is what the statistics tell us makes sense.

OK.. let's talk this thru.

The most catastrophic gas loss will come from a burst disc. Isolating will save the most gas, by far.

A tank neck oring is very, very, very unlikely to be a catastrophic gas loss or complete failure.

Most first stage failures manifest at the second stage, when it starts freeflowing like mad, knowing which post to shut down shouldn't be hard to determine. Following a preset right, left center or whatever pattern an agency or instructor pushes is just wasting gas.. if a second starts freeflowing like mad, shut down the post the first stage that is attached to the second stage is on.

A first stage failure that leaks gas, very few are catastrophic unless parts fall off (seen a turret do that) but most are moderate at best and for most divers, even in hoods they can tell which side from the noise. Some can't but most can.

I teach if it suddenly sounds like a freight train behind you (which a burst disc will sound like) of escaping gas, isolate. Otherwise isolate what is obvious or they think it is. If that doesn't fix it, move to isolator and isolate, then sort out if the post you didn't shut down before will sort it out if you shut it down.

I also teach that if you have determined that is isn't a reg issue and it's isolated, that breathing from the side with a bad neck oring or burst disc until the gas is gone is also a good idea.. because the gas is leaving and you may as well use some of it.

It should be a thinking divers game, not route at these levels IMHO

 

[helodriver87]

I was thinking of the doubles that ended up with all O2 on one side and EAN32 on the other. Just remembering to open the isolation valve before jumping in the water was not going to help that guy.

The way I was taught was this: before you splash, check all three valves are full open. If the isolator was closed, open it and stay out of the water until you can validate both pressure and contents. You do that before every dive, no isolator issue should ever bite you.

 

[cerich]

The way I was taught was this: before you splash, check all three valves are full open. If the isolator was closed, open it and stay out of the water until you can validate both pressure and contents. You do that before every dive, no isolator issue should ever bite you.

why would a shut valve that once opened make you distrust your spg?

I agree with reanalyze gas (from both posts with isolator shut actually in this case) but I really don't follow the validate pressure part. Sure, if you have the isolator shut each tank can be a different pressure, but following what you said, that has already been sorted by opening the isolator

 

[cerich]

The way I was taught was this: before you splash, check all three valves are full open. If the isolator was closed, open it and stay out of the water until you can validate both pressure and contents. You do that before every dive, no isolator issue should ever bite you.

oh.. I also do not agree that following what you outlined that a diver can be confident that no isolator issue will ever "bite them" .

 

[helodriver87]

why would a shut valve that once opened make you distrust your spg?

I agree with reanalyze gas (from both posts with isolator shut actually in this case) but I really don't follow the validate pressure part. Sure, if you have the isolator shut each tank can be a different pressure, but following what you said, that has already been sorted by opening the isolator

Might change your gas plan if the equalized pressure is lower than what you were reading previously. Just verifying that you have enough and the right kind of gas before proceeding.