One-hand or two-hand valve drills (back mount doubles)

[boriss]

One thing I appreciate about these ScubaBoard discussions/debates is we can think through so much in advance. If I am ever in a situation where my isolator is closed in the future (for whatever reason) I'll leave it off and instantly thumb the dive. I honestly can't say if that would be my response prior to this thread. This isn't a failure mode I've thought through.

I feel like I'm diligent prior to the dive, but then again, so are many divers who end up statistics. It's easy to dismiss incidents as people just being dumb (and wrongfully think this can't happen to me), but these aren't dumb people. It's usually a small oversight or an assumption that cascades.

Thanks all for this discussion!

 

[boulderjohn]

You don't know for sure what the mix is. If you analyzed off the right post and then opened the isolator,

I was not disagreeing with you. I just added that the degree to which the gases mix depends upon several factors.

As I indicated elsewhere, a closed isolator during filling is an error that normally results in uneven amounts of the same gas in the cylinders. Different mixes in different cylinders usually involves additional errors and creates different scenarios. The following examples assume the normal configuration of the diver breathing off the right post with the SPG off the left.

• Let's assume you had nearly empty cylinders and got one of them filled with a different mix. If the left tank was nearly empty as a result, you should notice that with the spg check at the beginning of the dive. You should also have done an analysis. If you did the analysis off the left post, you would notice the wrong mix, so let's assume you did the analysis off the right post and found the right mix. As the dive progresses, you look at your spg, see you are surprisingly empty, and realize the isolator was closed. If you open it and hear the cylinders equalize pressure, you will still be breathing the same mix off the right post. Only the left cylinder will have changed.
• If in the above scenario the left cylinder was filled and analyzed and the right cylinder still nearly empty, then your analysis would depend upon which post you used. If you analyzed the left post, everything would look good. You would not realize the problem until you looked at your spg and saw that it hadn't moved yet or ran out of gas. In that case, the two would mix, and you would indeed be breathing a mixture of unknown composition, although you could make a reasonable guess if you remembered what was in it before. It should be reasonably close to what was in the left, especially if you emptied the right before noticing it.
• The other scenarios for getting different mixes in different cylinders mentioned earlier all require unusual circumstances, including adding different gases through different posts and somehow not noticing that the starting level was completely wrong when measuring the addition of the later gases. I went through as many scenarios as I could, and bizarre possibilities exist, including breathing straight helium at the start of the dive. In every case, though, if the cylinder from which you are breathing is more full than the other, then the composition of what you are breathing will not change. Of course, if you did not analyze that cylinder, then you will not know what is in it.

 

[crofrog]

I was not disagreeing with you. I just added that the degree to which the gases mix depends upon several factors.

As I indicated elsewhere, a closed isolator during filling is an error that normally results in uneven amounts of the same gas in the cylinders. Different mixes in different cylinders usually involves additional errors and creates different scenarios. The following examples assume the normal configuration of the diver breathing off the right post with the SPG off the left.

• Let's assume you had nearly empty cylinders and got one of them filled with a different mix. If the left tank was nearly empty as a result, you should notice that with the spg check at the beginning of the dive. You should also have done an analysis. If you did the analysis off the left post, you would notice the wrong mix, so let's assume you did the analysis off the right post and found the right mix. As the dive progresses, you look at your spg, see you are surprisingly empty, and realize the isolator was closed. If you open it and hear the cylinders equalize pressure, you will still be breathing the same mix off the right post. Only the left cylinder will have changed.
• If in the above scenario the left cylinder was filled and analyzed and the right cylinder still nearly empty, then your analysis would depend upon which post you used. If you analyzed the left post, everything would look good. You would not realize the problem until you looked at your spg and saw that it hadn't moved yet or ran out of gas. In that case, the two would mix, and you would indeed be breathing a mixture of unknown composition, although you could make a reasonable guess if you remembered what was in it before. It should be reasonably close to what was in the left, especially if you emptied the right before noticing it.
• The other scenarios for getting different mixes in different cylinders mentioned earlier all require unusual circumstances, including adding different gases through different posts and somehow not noticing that the starting level was completely wrong when measuring the addition of the later gases. I went through as many scenarios as I could, and bizarre possibilities exist, including breathing straight helium at the start of the dive. In every case, though, if the cylinder from which you are breathing is more full than the other, then the composition of what you are breathing will not change. Of course, if you did not analyze that cylinder, then you will not know what is in it.

The diffusion rate depends on the partial pressures of the gas. Once the manifold is open after the initial mixing occurs as the gas flows from the high-pressure vessel into the low pressure vessel gas diffusion will occur based on the differences in the partial pressures of the various gasses

So you can't say with any certitude at what rate the gas will diffuse (lighter gasses diffuse at higher rates) so you can't say how long the gas may or may not continue to be safe to breath.

 

[boulderjohn]

Your decision as to continue breathing the existing cylinder or make make some kind of alternate plan will vary by circumstances.

• If I prepared for a dive by checking the pressure off my left post and analyzing the gas from my left post and then realized during the dive (and breathing from the right post) that the isolator had been closed during filling, then that means the gas I intended to breathe is untouched in the left cylinder, and I should probably switch to the left (without opening the isolator, at least at first).
• If in that circumstance I have a long way to go before reaching the surface and might not have enough gas in that cylinder, I might have to think about the consequences. Do I want to be breathing off the gas in the right cylinder as is later on, or would I be better off with a blend of the two? The answer will depend upon my knowledge of what is in that cylinder, but in most cases a blend of the two would be better.
• It is possible that as I think through the circumstances of the filling, I might realize that continuing to breathe off the current cylinder is a really bad idea--it could have way too much oxygen, for example.

 

[crofrog]

Your decision as to continue breathing the existing cylinder or make make some kind of alternate plan will vary by circumstances.

• If I prepared for a dive by checking the pressure off my left post and analyzing the gas from my left post and then realized during the dive (and breathing from the right post) that the isolator had been closed during filling, then that means the gas I intended to breathe is untouched in the left cylinder, and I should probably switch to the left (without opening the isolator, at least at first).

The recommendation is analyze off the right post, aka the one you'll be breathing from

• If in that circumstance I have a long way to go before reaching the surface and might not have enough gas in that cylinder, I might have to think about the consequences. Do I want to be breathing off the gas in the right cylinder as is later on, or would I be better off with a blend of the two? The answer will depend upon my knowledge of what is in that cylinder, but in most cases a blend of the two would be better.

That's what your team is for.

• It is possible that as I think through the circumstances of the filling, I might realize that continuing to breathe off the current cylinder is a really bad idea--it could have way too much oxygen, for example.

There is an good argument to be made for switching to your teams gas immediately.

 

[Beau Holden]

The recommendation is analyze off the right post, aka the one you'll be breathing from

Why not analyze both posts? Was the isolator closed during partial pressure blending? Was one tank off on mix and has not had time to stabilize? I have many times at quarries and cave, picked my tanks and in the water within an hour. I have never analyzed both tanks, honestly, but this has got me thinking how much trust I really want to put in anyone filling my tanks. If it's banked gas, then of course, no problem (should not be).

 

[Dark Wolf]

Why not analyze both posts? Was the isolator closed during partial pressure blending? Was one tank off on mix and has not had time to stabilize? I have many times at quarries and cave, picked my tanks and in the water within an hour. I have never analyzed both tanks, honestly, but this has got me thinking how much trust I really want to put in anyone filling my tanks. If it's banked gas, then of course, no problem (should not be).

For me, a lot of it is WHO is filling the tanks. Granted, everyone is gonna have a bad day once in a while. I am not trimix certified, as a disclaimer. But one of the semi local mines was the only one that I ever saw fill tanks. I do not recall ever hearing about a bunk fill.

But, like you say, it gets you thinking.

DW

ETA-Meant to write the owner of the mine that filled the tanks.

 

[Beau Holden]

I do not recall ever hearing about a bunk fill.

I have never had a bad fill. I am also not trimix certified, yet. But that one time I have never heard about, I don't want that one killing me.

 

[Dark Wolf]

I have never had a bad fill. I am also not trimix certified, yet. But that one time I have never heard about, I don't want that one killing me.

Could not agree more.

DW

 

[iointerrupt]

All this just seems to point to forking over the $17 and putting Vindicator knobs, that visually show open/close state, on your isolator valves.

For all the talk about 'human factors', I don't know why these things aren't standard across the industry. Standard valves are a terrible design for humans, being unable to tell their state visually. No matter how many times they say "be really really sure about your valve states", people get distracted and make mistakes. It's almost guaranteed given their design.

The Vindicator (or XS Scuba version) adds probably dozens of points where 'closed valve' can be caught and corrected. Pre-filling, pre-analyzing, during setup, pre-jumping in, while jumping in, etc. Big red stop sign right on top of your cylinders for everyone to see.

(I do not own stock in Vindicator Inc, but I wish I did given how many I've bought.)