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

colinicky

How many people have had a loop flood due to the O rings failling?
How often would you strip/clean your non O ring Bov Mike ?

Do Bovs have the same little "drain" hole as the standard inspiration DSV ? If not surely if the O rings fail the only way it could leak is if your OC side leaked into the CC side ?

Am torn between a V4tec or a Divematics at present & after getting mouthfulls of seawater from my inspo DSV & subsiquently messing it up in a big enough way to do 7 hours chamber treatment I have no wish to repeat it !

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Colin

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

onwards & downwards

colinicky

Divematics ,Bob Howell do use O rings

V4tec ,Golem Do not use O rings

not certain on Kiss ,Nemotech or any others

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Colin

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

onwards & downwards

divetheworld

The action of opening or closing an 'o' ringed BOV is the same as opening or closing a standard AP DSV in the fact that the 'O' ring slides over an open orifice. The possibility of the 'o' ring getting trapped or dismounted to cause a failure depends on the size and profile of the groove. Because one person encountered a failure does not always mean a flaw in a design, but highlights a potential failure.
'O' rings tend to settle and remain in place after a short while. Users who have replaced the standard 'o' ring on an AP DSV will be familiar with this concept. Note to be careful.
Alternate designs can limit or even remove this potential failure by inherently safe design. A very good example of this is the Bob BOV which uses an X ring seals, basically a rounded square section. These stay in place by virtue of the nature of the captive profile. In addition the close tolerance of the rotating valve reduces a sudden or unexpected rate of flooding while simultaneously eliminating thermal transitions in the fit which have plagued some past BOV's.
The Nemotech DSV had many users detailing the difficulty in rotating the valve at depth. John Routley found it impossible on his without modifying it.
The upshot of not using an 'o' ring is easier operation with one hand but care must be taken with consistency of production as tolerances and material consistency is critical.

Neither has any distinct advantage over the other if correctly implemented in its design. A lesson to be learned is to wait for the long term verdicts before committing yourself to any particular design.

Brent.

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Mdemon

In the case of the BOB H BOV, I don't buy that. If the o-rings round the holes in the barrel went, no water would get in because of the o-rings at the ends and on the locking plate - unless it came through the second stage. If any of these were to go, I still don't see how enough water could enter to fill the loop. CC is at ambient and so there is unlikely to be any big in-rush of water, especially if face down and ascending. I guess OC there is a tiny chance that loop gas might mix with OC gas... What is your logic please? Happy to be proved wrong. (Happier not to be, obviously... )





I did hear of a total DSV collapse which meant the guy had to bail to offboard. I am much more worried about that... Will try to find out what make it was.

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www.southwestmafia.com

"small minds talk about people, Average Minds Talk About Events, GREAT MINDS TALK ABOUT IDEAS!"

The WRONG Attitude will get you killed.

"Once the agenda-monkeys and perfect-worlders have moved on, perhaps we can do some diving?"

pchanning

Mike,

The Divematics BOV is double-sealed. 1 set of o-rings at the ends of the rotating barrel and 1 set around the holes within the barrel sealing it to the hoses (If any o-ring BOV does not have 2 sets of o-rings - its crap, and I'd say bin it).

For leakage to be an issue both sets of o-rings have to be compromised. Thats one level of redundancy - which is certainly more than the standard Inspo mouthpiece (which has slidy o-rings) - and I have never heard anyone complain about it in this way.

Regardless, even if the 2 sets of o-rings fail, the diver will go OC. Everything is at ambient and it will be akin to breathing off a reg with a hole in the mouthpiece. Annoying, but not the end of the world. Chances are one would be face down so hardly a good flood. Besides there is another reg on the stage.

So in terms of a fault path scenario this means that firstly the breather would have to go wrong or misbehave before switching the BOV, which then suffers critical failures on not 1 but 2 different sets of o-rings, and then should the breathe be wet enough for one to bin-it and go for the stage reg which would not work (because someone is clearly trying to tell me something) and obvioulsy neither would my buddies stage regs.

Thats 3 catastrophic failures before needing to hit my stage reg.

BTW: The o-rings on my BOV were on there since Divematics started (it was a prototype) and have never been changed. They are lubed maybe 2 times a year. Maybe I have been lucky. Maybe it's a well designed and crafted bit of kit.

FWIW I think Steve has hit the nail on the head. Cost and difficulty. Also, the idea that a manufacturer would design such a highly strung and exacting product for safety reasons(!) is IMHO a nonsense. These things need to be robust, allow for expansion co-efficients (if that is an issue) and fit within a sensible maintenance schedule, like every other bit of dive kit.

Regards
Paul

EDIT: I may have reiterated some of Johns points above - beat me to it (I must not try to cook breakfast and ponce about on t'interweb).

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Cheers
Paul

The key to enlightenment... is survival.

Drmike

You guys are like a dog with a bone - why so aggressive and pedantic?

This is the kind of tone that makes me want to stop posting at times.

Anyway, for what its worth;

When mine flooded the problem was a single torn o-ring on the barrel that seals the exhale side outlet. The o-ring must have torn just before the dive (will still pass neg test if this o-ring is damaged) as I didnt operate the BOV during the dive.

This allowed water in through the BOV 2nd stage and into the loop (ie a single failure point). I didnt realise before I was 4000ft into a cave. This was my experience - this happened to me - and that experience will not change no matter how much attitude you display, what your own experience is with your Ming Dynasty prototype or what colour underwear your mother is wearing.

Now can we drop the attitude?

you know what - just forget it

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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

schford

This thread reads like every one got out of bed on the wrong side today come on its all about sharing information nicely - it shoud be what Rebreather World is about!

Big group hug every one!

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The only bad question is one you did not ask and later wish you had.

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schford

though am sure you woudl all prefer to be in the middle of this one!


Well our male members any way

__________________
Bailout and Kit Management account for Murphy's Law

The only bad question is one you did not ask and later wish you had.

Use of Rebreather World is subject to the Rebreather World Terms & Conditions of Use

divetheworld

Carefully stepping round the toys on the floor.

Paul,
The fact that you havnt changed the 'o' rings on your Divematics might be a good reason why it is so reliable. Perhaps some standard AP DSV users can step in here. When you change the 'o' ring, it does not like to maintain the curve of the barrel and consequently steps out and gets trapped between the barrel and the housing. Ergo, you cant close off the loop without fiddling. How deadly could that be, especially in a situation where your going off loop involuntarily. This settles down quickly with use, but might give you cause to pause. Just a throw away point to keep you safe.

A constructive but critical evaluation of each of the BOV's would be of use. The BOB does have some nice features but they are heavy out of the water and notoriously difficult to maintain without an engineering background and free access to some weird parts. The swivel is a nice idea but in hindsight, a bit of a novelty that could you could be without if supplied with a quality hose swivel. My opinion.

Remember, Rolls Royce engines didnt used to have a head gasket! I remember being shocked by how easy the Submatix DSV was to operate the first time i tried it when I was invited to the UK launch. No 'o' rings, but not built as well as I would like.
Experience of a number of BOV's in use and ones under NDA at the moment I re-illiterate my previous statement that they have no advantage over each other. It is the way they are implemented that makes them succesful.


Brent.

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colinicky

This thread really carried on from my thread here :- http://www.rebreatherworld.com/techn...?highlight=bov

& having looked at Padowans drawing & the stripped down Bob there isn't a great deal of difference in the O rings & their positions .
We have all had experiences in diving or otherwise that people question because they haven't ,thats fair but it does not mean that they are wrong and we are right .

So lets look at what we have learnt :-
General concencus is that non O ringed BOV's are hard or impossible to obtain a neg test .unless you fit a flow stop

Although it may not be possible to obtain a - test (or a poor one ) on a non O ring Bov it does not appear to affect the performance of it .

From reports it appears that the non O ring Bov needs a higher degree of maintanance

There has been a report of an O ring failling at depth which is a serious issue

Personally I am still uncertain between the Divematics & the V4tec .It comes down to how long a -/+ test I need the unit to hold .
Any other sensible comments ?

__________________
Colin

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

onwards & downwards