Doubles with isolation open or closed Lesson Learned

[Ste Wart]

Leave the isolator open at all times. Job done.

 

[TSandM]

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.

 

[flots am]

After opening my isolator and equalizing both tanks I got up with 705 lbs at 100 feet,

You also slammed a completely empty tank from 0 to half-full in a couple of seconds. If something was going to let loose (burst disk, o-ring, etc) that would be when it would happen.

 

[Akimbo]

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...

IMHO, that is only true if you don’t pre-dive your gear. I normally use the Progressive Equalization technique so the isolation valve is shut unless I am equalizing. Just like all doubles divers that don’t use progressive equalization, I open the isolation ensuring that both cylinders are equalized, and check the pressure. The only other step is I close the isolation before donning the rig.

This is not a new thing. Equalizing reserve gas on multi-cylinder rigs goes back to Cousteau’s first triple tank set and continued on every rig I am aware of that was used on the Calypso. It is also how J-valves on doubles worked.

Progressive equalization can provide one or more “way points” that triggers an event. The first equalization might signal “leave bottom” on a deep dive or “start first leg of your return to the boat” on a shallower dive. Progressive equalization does not mean that you can forget about monitoring your gas supplies. It does give you a hard reserve when you get distracted and underestimate.

Like Cousteau’s rigs, I also dive valve-down to make operating the isolation valve easier. No single technique is flawless. One of the upsides of leaving the isolation shut is you will lose less gas on a HP side failure than leaving it open. A disadvantage of progressive equalization is you could have an isolation valve leak or not close it properly and be losing reserve gas that you expected to have. Valid arguments can also be made for independent doubles, especially if you are a side-mount evangelist.

http://www.scubaboard.com/forums/te...uble-tank-manifolds-bad-idea.html#post6891550

 

[TONY CHANEY]

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.

Not in my case. the isolator was open during the filling and turned off for the travel to the quarry. So in my case gas was known.

 

[Akimbo]

You also slammed a completely empty tank from 0 to half-full in a couple of seconds. If something was going to let loose (burst disk, o-ring, etc) that would be when it would happen.

Here are some tests:

Life Ending Seconds, 3000 to Zero in 72 Seconds

 

[TONY CHANEY]

You also slammed a completely empty tank from 0 to half-full in a couple of seconds. If something was going to let loose (burst disk, o-ring, etc) that would be when it would happen.

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).

 

[TSandM]

My comments were not aimed at people who very deliberately set up their gear and their procedures to dive with the isolator closed. They were aimed at someone who was UNAWARE the isolator was closed when he started the dive . . . which means, I would guess, that he didn't analyze his mix from both orifices before diving. If you haven't analyzed both tanks, you simply do not know what is in the other one. It could be pure O2, or pure He -- I believe I have read of a death where a diver opened the isolator when one tank was pure He, and the diver became hypoxic and died.

There were a lot of steps into the incident pit taken in this dive, and the only one needed to make this a fatality was the wrong gas in the left tank.

 

[Akimbo]

My comments were not aimed at people who very deliberately set up their gear and their procedures to dive with the isolator closed...

Fair enough, but I would hope that anyone would check their pressure with the isolation valve open even on air. Dive shops are busy and there are plenty of them that only fill one set of doubles for every 100 singles. Filling tanks is boring manual work and often goes to the low man on the dive shop hierarchy — who make mistakes. Best to make sure the isolation is open when you drop off the tank and when you pick it up from the shop.

 

[Doc Harry]

When I am diving doubles with an isolator, part of my frequent gas checks includes comparing the expected drop in SPG pressure with the actual change.

A failure of the SPG to drop as expected is a good indication that you isolator valve is closed.