[Drmike]

Quote: (Originally Posted by colinicky)

Mike
That is my exact point though ,I would have thought there should NOT be any way for air to migrate from the CC side to the OC side or vice versa .

Quote from Geordiediver about the V4tec :-
" On fitting the BOV, it was impossible to get my Inspiration to hold a negative pressure test (or indeed a +ve test) - the air appeared to be getting in through the mouthpiece - I am not sure how, but when I streched some clingfilm over the mouthpiece, it was definately being sucked in."

This is one of the comments that started me thinking .If you think about it in relation to a standard inspo DSV it would be like saying " oh I closed the mouthpiece but it leaked water in through the little slot at the bottom which filled the loop !"
I know its a little more technical than that but basically (very) take an inspo DSV & slap a reg underneath & you have a BOV !
Or is this were manufacturers have gone wrong ? In trying to get away from the "Twist mouthpiece" to close the loop they have moved away from the most effective way of sealing ?
Further ponderings to come

When you close the BOV (if its one with no seals on the barrel that seal off the Rebreather inlet/outet) then there is a small gas path possibly to the OC 2nd stage. If the 2nd stage is not connected to a gas supply when you suck a vacuum you could suck in gas through the 2nd stage hose/2nd stage valve through the 2nd stage around the gap and into the loop loosing some vacuum. In practice when you dive the 2nd stage is not 'open' but connected to a gas supply. A shut off valve before the 2nd stage will enable you to hold a negative. If it doesnt then either the diaphram in the 2nd stage has a hole in it and needs replacing, teh diaphram is miss seated, or the exhaust valve is not sealing.


The path around the barrel should be very narrow (close fit of barrel to housing) so although you might be able to loose a small vacum over time through it when its closed water or any significant amount of OC gas shouldnt be able to get through it anyway in use.

In otherwords, if you have an issue just fit a shut off valve (or connect the 2nd stage to a turned off tank) to perform a neg test.
In operation it shouldnt be an issue.

When bailing out the fit of the barrel should be tight enough so water cannot get in. Just like Boris where there are no o-rings in the barrel.

O rings are fine and give a great seal when they are ok, but when they are damaged with frequent open/closing the large gap that they can leave can lead to nasty full floods (has happened to me) this is why a mouthpiece that doesnt use o-rings and relies instead on a tight tolerace to keep water out (yet in fact may let small amout of gas in - failing negative test) is preffered by many. This is why Boris doesnt use o-rings in the mouthpiece and instead relies on fit.

I have a V4TECH and it doesnt let water in and holds a vaccum long enough for me to know the loop is good. If it really bothers you you can always drop your mouthpiece in a bucket of water when doing a neg test. The denser water shouldnt get sucked in (like some small amount of air might)

[colinicky]

That would be a fair test Mike .As you say dump it in water .I suppose it all comes down to how long you need it to hold neg test for you to be happy ? As Simon & Paul have said they have no problems getting a neg to hold on the divematics .I have no problem with the standard inspo DSV holding for 3-4 days .But I know someone who is happy if the hoses pop very slowly back to normal over 2-3 minutes & has dived it like that for years .
Does the divematics use O rings or close tolerance ?

[Padowan]

Quote: (Originally Posted by colinicky)

That would be a fair test Mike .As you say dump it in water .I suppose it all comes down to how long you need it to hold neg test for you to be happy ? As Simon & Paul have said they have no problems getting a neg to hold on the divematics .I have no problem with the standard inspo DSV holding for 3-4 days .But I know someone who is happy if the hoses pop very slowly back to normal over 2-3 minutes & has dived it like that for years .
Does the divematics use O rings or close tolerance ?

O-rings. The rotating barrel actually has 4 rings - 2 that go around the frond and back end of the barrel, and 2 that go round the ports that switch between the loop and the 2nd stage. Not sure if that makes sence if you're not familiar with the thing!

I'll try to draw a picture....

[Padowan]

Quote: (Originally Posted by Padowan)

O-rings. The rotating barrel actually has 4 rings - 2 that go around the frond and back end of the barrel, and 2 that go round the ports that switch between the loop and the 2nd stage. Not sure if that makes sence if you're not familiar with the thing!

I'll try to draw a picture....

Really crap pic, but hopefully you get the idea...