Doubles with isolation open or closed Lesson Learned

[flots am]

Not if you follow a basic rule to slowly open and close all valves (unless doing an emergency shut down where speed will save gas).

The OP was out of air. I'm pretty sure "slow" wasn't on the menu.

 

[Garth]

i've not heard of many cave divers doing progressive isolation valve closure.

OP, How long have you been cave diving?

 

[mdwypro]

wasn't talking about cave

---------- Post added October 24th, 2013 at 12:48 AM ----------

1- My buddy was close by. 2- I dove my first dive with the same tanks and the isolator was open. 3- I never close the isolator and it was closed accidentally before the second dive. 4- I had enough cubic feet to dive the 15- 20 minutes bottom time planned since the first dive. 5-. I ended up with the same pressure or cubic feet in both tanks I would normally on a similar dive dive. 6- I mentioned Cave training due to the instilment of work it out do not panic. 7- I filled both tanks and isolator was full open. Anything is possible but two different gasses in one set of doubles is a stretch. Again anything is possible but it is so unlikely it should not be part of your check list. 8- Yes I made mistakes, I would not end a dive for a faulty transmitter. because I dive with my SPG. 9- I do not do two cave dives on one set of tanks and the isolator stays open.10-Opening the isolator causing a blown burst disc again is possible, but it is another stretch anything can happen but that causing a burst disc to go is unlikely. 11-Also the tank was not empty, it had 110 psi or 3.52 cubic feet of gas. 12- it was a mistake to open the isolator and not close it. It reduced the amount of breathable gas. not by much, but breathable gas is less than if I left it closed. 13- For an instructor to say wtf, to me or anyone else, he doesn’t deserve the title. Go yell at your children and see if they listen. 14- I posted this to share an experience and gather data. I try to always dive safe. Most of you guys are the best. You come up with knowledge advice opinion and experience, teaching me and other people reading this without being VULGAR or Demeaning. You guys are the best. 15- God Bless

 

[j2s]

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.

Ditto.....And ditto to Ste Wart......Get tanks filled w/manifold open and don't mess w/it unless there's a problem.....It's not 'rocket science'.......

 

[Dr. Lecter]

11-Also the tank was not empty, it had 110 psi or 3.52 cubic feet of gas. 12- it was a mistake to open the isolator and not close it. It reduced the amount of breathable gas. not by much, but breathable gas is less than if I left it closed.

As Agillity correctly pointed out, you and I were both incorrect that equalizing reduced your breatheable gas as #11 disproves #12.

 

[mdwypro]

What is equivalent psi in dissimilar tanks?
120X.032 (baseline for one 85lp) =3.84 cubic feet of usable gas
3.84/.064 (baseline for double 85lp) =60 psi
One tank has 110 psi =3.52 cubic ft. One tank has 2000= 64 cubic feet you must subtract
64.00-3.52=60.48/2=30.24 cubic feet to equalize both tanks. To equalize both tanks and stay above or equal to 110 psi you lose 3.52 cubic feet of gas

 

[Dr. Lecter]

What is equivalent psi in dissimilar tanks?
120X.032 (baseline for one 85lp) =3.84 cubic feet of usable gas
3.84/.064 (baseline for double 85lp) =60 psi
One tank has 110 psi =3.52 cubic ft. One tank has 2000= 64 cubic feet you must subtract
64.00-3.52=60.48/2=30.24 cubic feet to equalize both tanks. To equalize both tanks and stay above or equal to 110 psi you lose 3.52 cubic feet of gas

This isn't right, it isn't even wrong.

You have two tanks of equal volume, and attached to each is a regulator that will not deliver gas under 110psi. In Tank L you have 2000psi, and in Tank R you have 110psi. Thus, you have 1890psi of breathable air in Tank L and 0psi of breathable air in Tank R.

Upon equalizing, each tank will contain 1055psi or 945psi of breathable air, for a remaining total of 1890psi of breathable air distributed between both tanks. While you've cut the breathable pressure in half, nothing has changed because you've doubled the breathable volume. You've doubled the breathable volume because Tank R wasn't empty when you equalized -- just drained to exactly the point at which the regulator stops delivering gas. Although you'd lose access to 110psi/3.52cf if you dumped Tank L into an empty Tank R, here you lose nothing because Tank R is already filled to the point at which the reg stops delivering gas.

If you want to show me that 1890psi in an LP85 is a different volume of gas than 945psi in what is effectively an LP170, by all means do so.

 

[DaleC]

wasn't talking about cave

---------- Post added October 24th, 2013 at 12:48 AM ----------

3- I never close the isolator and it was closed accidentally before the second dive.
6- I mentioned Cave training due to the instilment of work it out do not panic.
7- I filled both tanks and isolator was full open. Anything is possible but two different gasses in one set of doubles is a stretch. Again anything is possible but it is so unlikely it should not be part of your check list.
8- Yes I made mistakes, I would not end a dive for a faulty transmitter. because I dive with my SPG.

I was thinking of the other points in regard to item #6. IMO cave training, like solo training, should emphasize teaching one to avoid creating a problem more so than teaching how to cope with one.

For #3, a simple valve drill at the beginning of the dive would have resolved the downstream problem. Why don't you do this?

If you admit #3 happened, # 7 is not an unlikely event. What is even more likely is to fill one tank and have half the gas you think you do. What do you do to safe guard against this?

If you dive with your SPG, why do you have a transmitter. Either you use it or you don't. Imagine a pilot having two instruments to measure altitude and choosing to ignore the one that showed a descent. Safe practice? In this case your ambiguity regarding it created an issue. If using an instrument you do not trust creates potential problems based on false assumptions. Why do you do this?

I think answering the above three questions will illuminate what the real lessons were that should have been learned from this incident. Opening the isolator doesn't even register. 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...

You may feel people are being harsh but this is the technical sub forum and you are claiming to be cave trained. "Assuming" is something that is taught against in OW when BWRAF is introduced. Because of the high stakes involved with this form of diving I think it can be expected for the kid gloves to come off and the discussion to be much more.. frank.

 

[waterpirate]

I think your first event, was a failure to catch this pre-dive. Everything else you experianced or theorised could have been a non-stater if you had caught it. As others have said, you need to look at your pre-entry procedures and correct bad habits/procedures.
Eric

 

[mdwypro]

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.