Finding Optimal loop volume on Dolphin + bp

Yeang

Hi guys,

It seems like as soon as I get one annoying question answered, another pops out of nowhere!

So I'm trying to figure out what the best way is to optimize my loop volume on my backplated-Dolphin. I can adjust how far the backplate stands off from the Drager shell - the backplate adapter slides back and forth. The original guy who sold it to me was a rather large/tall fellow, and he had it set for almost maximal expansion. This gave me a serious case of chipmunk cheeks when I lay on my back in the pool, as the CLs are further away from my lungs, and also probably contributed to buoyancy issues.

Yesterday, I adjusted the backplate adapter to be much closer to the shell, to restrict the volume of the CLs. I then set everything up (on land), without attaching any Nitrox supply (so the auto addition valve isn't working).

Here is my question: How do I know that the CLs aren't TOO restricted? I tried to answer this question by doing the following...

First I tried to find the minimal loop volume as defined by Rich Pyle's FAQ, i.e. I sucked the CL's down as empty as I could, then exhaled all the air in my lungs through the nose.

Keeping the system at min loop volume, I took in a full complete breath through my nose. I then let this breath into the loop through my mouth. Then keeping the loop at constant volume (no more nose breathing), I tried to see if I could activate the exhaust valve on the Dolphin by exhaling as hard as I could.... the answer was no, not unless I took in some more air through the nose.

I -assume- that this indicates that I am at optimal loop volume, i.e. CL's are almost close to flat when I fully inhale. I'm not sure how the constant mass flow properties of the Dolphin system will change this, as I didn't test it with the flow on.

Any ideas, obvious flaws, or better ways to find optimal loop volume on a Dolphin?

Thanks so much and sorry for the long post. Hopefully this might be useful!

Yeang

CCRDVR

I'm not sure how to figure out the loop volume as you asked. What I would do on ascent to prevent chipmunk cheeks was to adjust the overpreasure valve to it's weakest setting. Thus the gas in the lung would vent out at a much lower preasure. This seemed to take care of the problem for me.

demo0034

It's my understanding that the Dolphin operates on a maximum loop volume at all times since gas is slowly being let into the loop. I believe this is true of all the SCR's. There's no way to manipulate the loop volume while diving.

If you're trying to constrain the lungs to make the maximum loop volume match your tidal breathing volume, then it sounds like you're pretty close, though I wasn't aware that this was common practice on this unit??

shell-back

Not sure what it is you are trying to acheive , ( lower the volume of the loop?) Why.

are you not getting the the dump V to pop off ? or it allways seem to bleed ?

Sorry I,m confused as to what it is your objective is. You must have the gas flow on inorder to get the right amount of clicks and this may change as to what your wearing for a suit.

__________________
The greatest obstacle to discovery is not ignorance -- it is the illusion of knowledge. ----Daniel Boorstin
Billy Beal
SDITDI IT #3507
Rebreather World FIGJAM commitee member

CCRDVR

This may seem kind of silly, but maybe you shouldn't lay on your back. The counter lungs being on your back would be in the lowest positon while you are on your back. This would "encourage" the gas to flow up. Thus adding to the "chipmunk" effect. In addition, that position would make it harder for the OPV to vent.

Yeang

Thanks for all the replies!

Yes, like demo said, I'm trying to constrain the lungs to better match my tidal volume. I guess that's a better way to put it than finding "optimal loop volume" . IMHO, two main reasons for this:

1) The more I can constrain the lungs, the closer the backplate is to the Dolphin and conversely the closer the CLs are to my lungs, so less differential pressure.

2) The more I can constrain the lungs/loop volume, the less buoyancy changes with depth.

Anyhow, this is probably a concern only for a minority of Dolphin users. Most Dolphins operate at "maximal loop volume" as a relatively large flow of gas is being vented into the system as one of you pointed out. With a backplate adapter I have some degree of control of the total volume by constraining the bags (by moving the backplate adapter in and out).

Thanks for the input. I'll try it in the water @ Cocos and see how it goes... leaving tomorrow at 6 am woohoo! Will report back if the Dolphin doesn't get me first.

Yeang

Scuba_Vixen

Did you take the "flaps" off when ther BP adapter was added?

The easiest way to set the maximum expansion of the cls is by where you close your flaps. Some leave no velcro showing, some leave 1/2" to 1" or so.

Exhale your full breath into the loop, and close the DSV and flaps. ... then adjust the adapter. You should be pretty close.