Doubles with isolation open or closed Lesson Learned

[mdwypro]

Recently I had a discussion about isolation. How in the old days I heard you would close your isolation until you breathed the tank down, then open the valve and then close and breath the other side down. Maybe I am a little confused with the exact scenario.
Soon thereafter having brain freeze I closed my isolator unintentionally. I have an SPG on one side and on my right post I have a sender. I notice my air was about 600 lbs on my sender during the dive, but my SPG read 2100. Did not pay it much mind and figured electronic sender was bad. I also could see that the SPG pressure came down (mainly from wing inflation). I was having a very comfortable dive after unhooking the grapple on the St Albans. The grapple brought my sac rate up a bit, but I relaxed and went on with my dive. About five minutes latter my primary regulator started getting very hard to breath. Doing a recent adjustment, I thought it was the Regulator, as I took a couple more breaths I realized I was out of air, so I went to my necklaces after first deploying my 50/50 at 155. It was a great feeling when the necklaces had plenty of air. I reached back and opened my isolator which now I consider a mistake. It equalized both tanks, but it gave me less time than if I would have left the isolator closed for marinating a higher pressure in one tank would give me more breathing time than sharing the gas in both. Thanks to cave training, I didn’t panic. I could have gotten to 72 feet faster than I should have if needed, and I could have spent more time at 20 feet on 100 percent. These were thought if I was out of air.
When I downloaded the pressure left in my right tank when breathing was no longer possible it had 110 lbs at 145feet. After opening my isolator and equalizing both tanks I got up with 705 lbs at 100 feet, I was already breathing off my necklace. My pressure increased in the almost empty tank as I came up. The 110lbs got up to 125 at 100 feet. One or two more breaths. After deco final assent I ended up with 615 in both tanks. They where two low pressure 85 steel.

 

[Dr. Lecter]

Paragraphs are your friend and ours.

That out of the way, I'm curious how you came to have the iso fully closed: was it a brain fart that you meant to fully open it, or, you closed it all the way and forgot to back it off a 1/4 turn?

I recently moved from 'all three knobs full open before donning the rig' to 'posts full open, iso full closed and back 1/4' and had my very first 'whoops, my SPG isn't moving' dive shortly thereafter. I've considered diving with it closed and equalizing every so often...but I'm not at all sure the extra task loading is worth the added protection of not having to close the iso in an emergency.

Also, why do you think opening the isolator was a mistake? Whether 2000psi in your left tank or 1000psi in both, you're still looking at the roughly the same volume of useable air, and equalizing removed the risks posed by having all your eggs in one tank.

 

[TONY CHANEY]

Damn, been there and done that. I was worried that a tank would roll open during the drive to the quarry (prior to having steel plugs) so I turned the isolator to isolate the tanks. So there we were at about 85 feet into the dive and I notice that my guage has not went down too much. SPG on the left tank as well as the drysuit and backup reg. Rt had primary and LPI. Primary reg. started breathing hard so I switched to my backup reg. So now I remember shuting my isolator. I SHOULD HAVE LEFT IT THAT WAY! So I open the isolator and hear that long equalizing sound and watched the guage drop to about 400 psi. I had a deco tank and was not in too much trouble. Time to say goodbye to fellow divers and went to the surface for a good long surface swim to the exit. I have never shut it off since and still check it prior to every dive.

 

[mdwypro]

Brain fart or I was upside down yes I know which way to turn I usually never close it. Yes I realized it was a mistake to open the isolator when I downloaded my transmitter info. By opening the isolation it brought my useable pressure down because of the space taken up by the gas. I am going to do the math soon. But I was on my last breath and according to what my transmitter read I had 110 psi left. By opening the isolator I have the same cubic feet of gas, but I would reach that 110 psi a lot sooner with the gas spread in both tanks. I also open my isolator all the way and don't turn it back a quarter. Only reason is that is what I was taught. I also found by turning it back a quarter turn may seem open on the surface but it may not flow through isolator at depth at depth.

 

[Michael Guerrero]

Paragraphs are your friend and ours.

Absolutely no value added here, I just wanted to say publicly that I love the quote. Personally, I'm a big fan of line breaks.

---------- Post added October 22nd, 2013 at 10:14 PM ----------

OK, so maybe value added here, but I'm not understanding the problem. Like the good doctor said, what's the difference? By turning the isolator, all you did was equalize the gas between two cylinders instead of one. You didn't diminish your amount of gas. 100 psi of gas across two tanks is not the same as 100 psi in one.

So, what is the problem exactly?

 

[Dr. Lecter]

By opening the isolation it brought my useable pressure down because of the space taken up by the gas. I am going to do the math soon. But I was on my last breath and according to what my transmitter read I had 110 psi left. By opening the isolator I have the same cubic feet of gas, but I would reach that 110 psi a lot sooner with the gas spread in both tanks. I also open my isolator all the way and don't turn it back a quarter. Only reason is that is what I was taught. I also found by turning it back a quarter turn may seem open on the surface but it may not flow through isolator at depth at depth.

I'd be interested to see the math. While I understand that you doubled the unusable volume of gas by spreading your 2000psi over both tanks (hence why I said roughly) above, twice that tiny amount is still a small amount and as you said you had sufficient pressure to return with over 600psi spread across both tanks plus your deco bottles... bad gas planning, but not cutting it life or death close. So, I'm not seeing the mistake in prioritizing redundancy over useable gas supply. Had you not equalized and then suffered a serious problem with the left tank, like a blown burst disc, something tells me you'd be calling out a different mistake.

I haven't noticed that having it a 1/4 open, rather than full open, fails to provide enough flow to equalize. Perhaps I will as time goes on, but so far I'm liking the faster isolation ability without the task loading of manually equalizing a shut isolator during the dive.

 

[DaleC]

Not to be a downer but the main mistake I see is diving with two different pressures and assuming the transmitter was faulty. If you don't trust it, don't dive it but if you dive it, respond to what it's saying.

 

[Mikko Ilari Laakkonen]

Seriously wtf?? You're cave diver and you still count gas volume by pressure instead of amount of gas?? You continue a ceiling dive with equioment failure, run OOA and still want to forget redundancy? Where was your buddy?

Please seek further basic technical diving training before continuing ceiling dives!

- Mikko Laakkonen -

I love diving and teaching others to dive.

 

[Agility]

By opening the isolator I have the same cubic feet of gas, but I would reach that 110 psi a lot sooner with the gas spread in both tanks.

I disagree.
You still had 110 psi in your right tank, it was not empty (maybe unuseable but not not empty).
By distributing the volume from the left tank the pressure would fall in the left tank and rise in the right one.
You would still be using the same volume of gas at each breath, so the pressure would only drop half as much as if you were breathing from the isolated left tank.
You would reach 110 psi again at exactly the same time.

 

[mdwypro]

its about 2 cubic feet less of gas. I will post more math latter. I had it all typed and spell check killed it. 1.76 cubic feet less base was.032 one tank. the simple figure is I could not breath with 110 psi x.032=3.52 divide by 2= 1.76 less cubic feet of gas by oppening isolator