Proof that BOV's don't effect -ve pressure tests

Padowan

There's been a lot of discussion about BOVs and what they mean for negative tests. I thought I'd do a little demonstration that proves that a good BOV will not effect your ability to do a negative test and all this talk about needing to have 2nd stages connected to tanks or fingers over 2nd stage inlets to stop air getting sucked in is all rubbish.

Here we see my unit holding a nice negative test with the second stage totally removed and a big hole into the mouthpiece. I air was gonna get sucked in, it would do so here, but you can see from the nice negative it's holding that this is not the case and the Divematics BOV isolates the OC and CC areas completely and effectively.

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Drmike

Whats 'rubbish' is you not realising all BOV are not the same design (some as has been clearly mentioned use o-rings to seal the barrel-inlet-outlet holes and so shouldnt have any issue.....does the divematics?.....


-gotto go plane boarding, but there is a good reason why some valves dont use o-rings.

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Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

Mdemon

Of all people, Padowan is the last person you would accuse of thinking that BOV's are all the same!

Well, anyway, his works. Mine (a BOB H) works. IIRC PChanning's works.

Three models, all working. Happy to recommend anyone to get a BOV.

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colinicky

Think that PChannings using a divematics as well ? so only 2 models so far
Padowan ,thanks for that bit drastic but it definitely shows it

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Colin

I trust my rebreather completely ,
I just don't trust the user

onwards & downwards

pchanning

OK, i'll bite Why do they not use o-rings? If these other BOV's (KISS? - please name them) do not have o-rings for fear of chaffing/ extrusion, trapping grit, whatever, then OK I can appreciate that. But if they dont seal as a consequence, as seems to be purported, whats good about it?

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Cheers
Paul

The key to enlightenment... is survival.

steevke

There is another disadvantage if you do not use o-rings.
In order to get a good seal, you need tight tolerances. However, when you breath out, the warm air heats the inner plug (which will expand), the cooler water otoh contracts the plug housing. This may result in a BOV that is difficult to switch when used in cold water.

Easy way to solve that: increase clearance. However, then you end up with a leaky BOV.

The main reason manufacturers choose not to use o-rings is obvious: if you want to add them you need a 5-axis mill or a lathe with c-axis and driven tools. Machining hours on those is much more expensive than on a standard mill/lathe.

Steven

diverklondike

My Golem BOV does allow me to do a -ve test.

trob09

Glad to hear you've got that sorted out Mark.

Drmike

The worse thing you can do to an o-ring in service is to slide across it (the action of closing the dsv)

The only o-rings that are not static in the whole Rebreather is the ones in the mouthpiece.


If this o-ring fails you can end up with a loop flood.


If this happens when you 4000ft inside a cave (as has happened to me) its not nice.

A well made close tolerance dsv does not need o-rings to seal.

The lack of o-rings takes away one risk of flooding.

The downside is the need to keep the barrel clean/more regular servicing.

If a well made close tolerance dsv leaks (because it hasnt been serviced/cleaned well) then the amount of water ingress will be very small - not a problem.


So basically you have 2 choices:
1) a o-ring seal design that is easier to seal with and requires less service BETWEEN failure - but when it fails it fails suddenly and could cause flooding

or

2) a non o-ring design that requires more service to maintain a seal - but not possible to have a SUDDEN failure - even if it leaks the amount of water that comes in is small.

__________________
Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

Pierre Farrugia

Can someone please tell me which one does not use O rings and those who not ?

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Pierre Farrugia

My wife told me " If you don't quit diving I'm going to leave you"
My reply " God, I'll miss you"

www.atlam.org www.divemed.com