Crossbar and isolator valve: do they help?

[helodriver87]

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" .

Elaborate?

 

[cerich]

if one side or the other has been filled with a different mix than the other with isolator shut, even opening the isolator will not guarantee the gas will blend. If you didn't watch or fill yourself, analyse both posts.. or it can bite you.

I once picked up a set of doubles that was 10/55 one side and 36 nitrox the other, I ONLY checked both sides because the isolator was shut. The pressure was within 50 pis each side and for a second I considered only analysing gas from one post.. would have been bad.

 

[helodriver87]

if one side or the other has been filled with a different mix than the other with isolator shut, even opening the isolator will not guarantee the gas will blend. If you didn't watch or fill yourself, analyse both posts.. or it can bite you.

I once picked up a set of doubles that was 10/55 one side and 36 nitrox the other, I ONLY checked both sides because the isolator was shut. The pressure was within 50 pis each side and for a second I considered only analysing gas from one post.. would have been bad.

It should still blend. Not instantly, but that's why a closed isolator = surface abort until you know what you're breathing. Anytime I fill my tanks, put my stuff together, or get in the water, I give it a quick check. That should be a foundational part of teaching doubles diving.

 

[stuartv]

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

I'm not sure if we're on the same page or not. To clarify - what I was taught was, if you can tell what side the leak is coming from, turn off that post. If you can't tell, then turn off the right post. So, it's not a preset pattern (unless you have no idea which side it's coming from). If you turn off the post and breathe your reg down, and then you can still hear bubbles, shut the isolator.

What I was taught from there was, immediately find your buddy and have them check your stuff out. That is whether bubbles stopped after turning off the post, if they stopped after turning off the isolator, or if they did not stop.

That last part is what I have had my doubts about (and been asked about). If I shut 1 post and my isolator and can still hear bubbles, then it seems like I would turn off the other post and turn the first post back on while I'm breathing down the reg from the 2nd post. But, my training was not to spend time doing that. Rather, find my buddy and have them examine me.

And then, if I have isolated and tried turning off both posts and none of that stopped the bubbles, I get that it would be good to keep breathing the cylinder that is losing gas, until it goes dry. But, until I've tried isolating and shutting off both posts (or gotten my buddy to examine me), I have to allow that I might be *sure* I've turned off the post that is losing gas and, thus, decide to keep it isolated and keep breathing it until it goes dry, only to find that I got it wrong and it was really the other post leaking all along.

I absolutely agree that the whole thing has to be a thinking diver's game.

 

[stuartv]

It should still blend. Not instantly, but that's why a closed isolator = surface abort until you know what you're breathing. Anytime I fill my tanks, put my stuff together, or get in the water, I give it a quick check. That should be a foundational part of teaching doubles diving.

You have to allow for the possibility that the blender had it closed during filling and then opened it just before they gave you back the cylinders. In other words "my isolator was already open when I checked it" is meaningless.

It seems like I have a vague recollection of a post from @tbone1004 a while back that said that a set of doubles filled with different gases on each side and then left sitting, still did not fully mix after a month or something like that.

 

[helodriver87]

You have to allow for the possibility that the blender had it closed during filling and then opened it just before they gave you back the cylinders. In other words "my isolator was already open when I checked it" is meaningless.

It seems like I have a vague recollection of a post from @tbone1004 a while back that said that a set of doubles filled with different gases on each side and then left sitting, still did not fully mix after a month or something like that.

Sounds like an interesting experiment to me. My hypothesis would be that blending would start immediately, even with minimal pressure gradient and that you should see a difference soon enough to recognize you have different gasses mixing together

 

[cerich]

It should still blend. Not instantly, but that's why a closed isolator = surface abort until you know what you're breathing. Anytime I fill my tanks, put my stuff together, or get in the water, I give it a quick check. That should be a foundational part of teaching doubles diving.

nope, not enough.why would enough gas move thru isolator to blend both cylinder? It wont happen, the lower pressure tank will mix, the higher will be near enough to exactly what it was before for content. Once equal it stops moving.

Anyhow, now you have gone from check before dive one to if find one shut while diving abort. If you are doing the second, you didn't do the first. Which I still contend if not enough.

I have been teaching tech for 26 years and doubles. I have seen a variety of approaches and too many mishaps. and near misses where what should not have happened with blending and filling and marking happen.

 

[cerich]

Sounds like an interesting experiment to me. My hypothesis would be that blending would start immediately, even with minimal pressure gradient and that you should see a difference soon enough to recognize you have different gasses mixing together

Have fun.

Throw an hypoxic mix in one side, nitrox in the other. Let one side be 500 psi higher than the other. Open isolator and analyze from each post. Shut and repeat. let it sit a week, do each post again after dumping say 50 psi with isolator ****, then open and repeat.

I look forward to you posting the results. I actually know what they will be, having done that when I got the bad fill almost 20 years ago..LOL

 

[helodriver87]

nope, not enough.why would enough gas move thru isolator to blend both cylinder? It wont happen, the lower pressure tank will mix, the higher will be near enough to exactly what it was before for content. Once equal it stops moving.

Anyhow, now you have gone from check before dive one to if find one shut while diving abort. If you are doing the second, you didn't do the first. Which I still contend if not enough.

I have been teaching tech for 26 years and doubles. I have seen a variety of approaches and too many mishaps. and near misses where what should not have happened with blending and filling and marking happen.

Because random molecule diffusion still occurs with or without a pressure gradient.

I'm still saying the same thing. If you're checking your valves predive and find a closed isolator, you're done until you can confirm the gas. I didn't say anything about during a dive, although obviously you should still call it and breathe off another source if practical.

 

[cerich]

Because random molecule diffusion still occurs with or without a pressure gradient.

.

Sure but not enough to be meaningful in this case. While it equalized, no gas was going against the pressure from higher cylinder. None. Any molecular diffusion would have to occur at a scale large enough go thru the manifold, that is not happening fast enough to blend both cylinders. The cylinders are still two distinct vessels with a small connection and without pressure to blend, it is not happening at a rate that will make your doubles safe to dive.