Doubles with isolation open or closed Lesson Learned

[TSandM]

My husband pointed out the fallacy in this to me today. Assume your regulator stops delivering gas at 150 psi. If you breathe your right tank down to 150 psi, where the reg stops delivering gas, and switch to the other tank, you can breathe that down to 150 psi. Then you have two tanks with 150 psi in them. On the other hand, if you open the isolator, you will be able to breathe until the pressure reaching the first stage is 150 . . . in BOTH tanks, which is exactly the final scenario if you DON'T open the isolator.

 

[RJP]

I am waiting to hear an answer from my Instructor I will not tell you that you don't have the same cubic feet of gas, I am saying you will have a more time leaving the gas in one 85 then equalizing in two in two.

Perhaps you can also tell us whether the transfilled tank got hotter or colder when the isolator was opened - and why? (That's another good one.)

 

[Akimbo]

… If you breathe your right tank down to 150 psi, where the reg stops delivering gas, and switch to the other tank, you can breathe that down to 150 psi. Then you have two tanks with 150 psi in them. On the other hand, if you open the isolator, you will be able to breathe until the pressure reaching the first stage is 150 . . . in BOTH tanks, which is exactly the final scenario if you DON'T open the isolator.

I concur. AOTBE, a doubles set with an isolation valve has the same amount of usable gas as independent doubles (side or back mount). This is true regardless of whether you leave the valve open or closed since you can breathe the isolated cylinder down with the other regulator.

The only case where an isolation valve manifold can deliver more usable gas than IDs is with one or more failures (leaks). The downside is the isolation valve provides more opportunities for operator error and misdiagnosis of problems… as in this case.

BTW, the complete method to determine usable gas is to determine the lowest pressure that delivers acceptable inhalation resistance for your regulator and add the bottom pressure in gauge pressure, not absolute… unless you can suck a significant relative vacuum! It may not make a big difference in 100', but gets pretty important when sizing bailout gas in deep water. And yes, you will be able to access some of it on ascent, if you haven’t blacked out first.

 

[Dr. Lecter]

Your still not considering you need to keep t 3.65 cubic feet of gas in the tank that has the 2000 psi to maintain above 110 psi maybe I am splitting hairs, but it still comes out to less breathable gas, even though you have the same cubic feet of gas.
I am waiting to hear an answer from my Instructor I will not tell you that you don't have the same cubic feet of gas, I am saying you will have a more time leaving the gas in one 85 then equalizing in two in two.

Yes, I am. Re-read my post above about doubling the breathable volume while halving the breathable pressure.

 

[rjack321]

The real issue is being an assumptive diver: Assuming the isolator is open, assuming the contents of both tanks will be the same, assuming a gauge is faulty...

Ding ding. Dale nails it. The whole 150psi left in a tank once the regulator stops delivering gas is just arguing for arguments sake. Its irrelevant.

 

[ajduplessis]

It can be terribly dangerous to open an isolate in mid-dive, because you simply do not know what was done the last time the tank was filled. If you were getting a partial pressure fill, it's possible that the contents of one tank are very different from the contents of the other. If you encounter a closed isolator during a dive, it is safest to abort the dive. Of course, you didn't discover your problem until you had no choice, which surprises me a bit. A flow check is part of the pre-dive sequence for all technical and cave dives, at least in the folks I dive with.

In this case you also did not analyse your gas, if you did the mix would be wrong.

 

[Jerry_]

Your still not considering you need to keep t 3.65 cubic feet of gas in the tank that has the 2000 psi to maintain above 110 psi maybe I am splitting hairs, but it still comes out to less breathable gas, even though you have the same cubic feet of gas.
I am waiting to hear an answer from my Instructor I will not tell you that you don't have the same cubic feet of gas, I am saying you will have a more time leaving the gas in one 85 then equalizing in two in two.

In your post (both initial and the one above) you mention TIME left, rather than usable gas.
How do you estimate time (and time difference left), if not based on the remaining volume of gas and your current/standard air consumption (sac)?

If you base your "time left" on the expected time left indicated by your computer (but you thought your sender was faulty?!), pay attention that the computer estimates your future air consumption, based on the sac of the recent period measured by the decreasing pressure. Therefore any estimate a computer can do will be completely wrong if another factor (f.i. Equalizing) also changes the pressure.

The only point I can see here is, as mentioned by RJP: did the temperature of your gas change when you equalized? (Expanding gas, lost energy, lower volume) if so, what was the change impact in time ( i.e. imediately, after two and after five minutes .., when the gas got back to ambient temperature)

 

[TSandM]

In this case you also did not analyse your gas, if you did the mix would be wrong.

Depends on the post you analyze. For example, if I'm having my 32% tanks refilled, the partially empty side would still show 32%; the other side would have a much higher O2 content.

 

[ajduplessis]

This is for informational purposes only, use it or don't.

My right post valve failed during a dive. You can see the o-ring pushed out. I was able to switch to my backup and shut the right post leaving me with all my gas. This would not have been possible in SM or ID configurations.

 

[ajduplessis]

Depends on the post you analyze. For example, if I'm having my 32% tanks refilled, the partially empty side would still show 32%; the other side would have a much higher O2 content.

I agree. My apology.