Report from Fatality at Devil’s Ear

ScubaDadMiami

I have had to replace more than one OPV from silty cave diving. It seems that I just can't clean them out once the fine particulate has worked its way into the valve far enough. After that, the valve works but allows water in when I exhaust gas through it.

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Howard Packer
IANTD CCR Instructor
Miami Beach, Florida
CCRDiveTraining.com

DeepBlueInnovation

I would be interested to hear if other people have experienced the same problem with OPVs. Is this issue native to Optima valves or have other cave divers experienced this with other brands of OPV? Another forum member commented recently that he found disliked the placement of the OPV on his unit (my apologies as I can't remember who or which thread) as when he worked through restrictions he found it was becoming blocked/damaged with debris from the floor.

I don't wish to ignore the other issues raised in this thread, but perhaps some discussion as to OPV placement and operation could be useful. Should standard procedure be to vent gas around the mouth piece in silty environments to prevent the OPV mechanism becoming clogged up?

ScubaDadMiami

Actually, I did not provide complete information in my answer. So, I may have created a misimpression.

The issue with the OPV was actually with my drysuit OPV, not my Optima OPV. I was just extrapolating this to answer the question about how something like this could happen to an OPV at all.

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Howard Packer
IANTD CCR Instructor
Miami Beach, Florida
CCRDiveTraining.com

Drmike

Hmm, why would a staged rebreather thats over inflating with gas and venting from the opv be any more prone to flooding than the breather on your back during ascents? We dont see flooding on ascents, Ive never flooded any of my bobs either (that Im not breathing from) on ascents. You will always have +ve pressure in the loop so long as there is a spring closing force acting on the opv. That force will limit the min loop pressure to above ambient. So I dont buy it. I suspect the opv was leaking and combined with the orientation of the opv, this is what led to the flood.

How many of us do a +ve pressure test....I have to admit I rarely do a +ve


I can imagine a scenario where wet cells led to solenoid firing and loop venting. Over an hour the O2 supply could become exhausted. If the OPV isnt seating properly that only becomes a problem if its facing upwards. Once the O2 runs out CL, pressure starts to drop as gas is leaking out of opv. Once enough gas has leaked out and CL pressure has dropped to ambient pressure then water could begin to drip into the lung through the opv and exchange with gas in the CL (gas bubbling out as water drips in)

If the unit is staged with the OPV facing down and lowest part of loop this risk should be negated, as once cl pressure has dropped to ambient water wont go 'uphill' into the CL through a leaking opv.

So bottom line is if your going to stage your unit it would be wise to make sure your opv is not leaking (+ve or -ve test before hand) and whenever possible stage your unit with opv facing down. As long as the OPV is facing down no matter how crappy it is water shouldnt be able to enter the loop via it.

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Nailer99

Yes- absolutely. This part of the scenario doesn't add up for me. The OPV on my Meg is exactly like the one on my drysuit, and even when the one on my drysuit failed, I was only getting a small amount of water through it- I just can't see how the entire unit could flood catastrophically like that- I'm much more inclined to think that the DSV wasn't fully closed during the hour the deceased was away from his unit.....

luxrok

The unit may have been staged down, however if the CL filled with gas they would have become buoyant and flipped the unit over. In the right conditions moving the OPV to the highest point of the rig. Now when the unit settles to the floor again, the tanks are down and the lungs are up with the OPV in the highest position.

I don't have any information on how the unit was staged, but it is plausible for it to change position substantially with that much volume change.

Hans

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Drmike

Good point. (and all the more reason to shut down tanks to prevent change in status). Would still need a leaky opv (or dsv) to flood though

I dont like the idea of leaving the unit in a condition where it can change itself. ADV leaks, O2 injections etc. Personally I preffer to shut the unit down (where unit allows eg MK15.5) ensure lungs have high ppo2 in them so solenoid doesnt fire (where unit doesnt allow, EG inspiration), but either way Id still isolate o2/dil by turning off tanks.

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ScubaDadMiami

Going back to the drysuit OPV issue (because this addresses some statements made above), the funny thing about it is that it would hold just fine during descent (negative pressure). The only time it leaked water in when was I was venting gas on ascent.

So, this would mean that the valve would have positive pressure exherted and would vent out gas but would also let in some water everytime that it burped out some exhaust. I am guessing that some crud allowed the valve to function almost normally but would keep a proper seal from immediately forming as soon as the gas exited just from spring pressure alone. Then, the spring would overcome this to force a seal right after. The short delay is likely what let the water in. Of course, the valve would be at the highest point when exhausting gas.

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Howard Packer
IANTD CCR Instructor
Miami Beach, Florida
CCRDiveTraining.com

Drmike

I thought about this too, then realised we need to factor in the resistance to seal. (imagine grains of sand on opv sealing face)

If theres some crud on the valve face that could lead to a resistance to seal. In other words you will need a bigger pressure differential across the valve to seal it tight (rubber is compliant to lumps of crud). On a surface neg test your actually putting a much higher differential over the OPV than will ever be seen in useage - that may mask the presence of crud as it may seal with such a large differential but not when differential is lower when in use.

In use the forces balanced across the valve are the closing force provided by the spring, Sf, working against the opening force of the gas in the lung, CLf, AND any additional force needed to overcome the resistance in sealing caused by the crud, Rf,.

so considering the presence of crud;


The opv will close when Sf=CLf

but

opv will only seal when Sf=CLf+Rf

Sf is a constant so what we see is as Rf increases CLf will reduce
(in other words; the more/larger the crud on seal face the lower the gas pressure in CL will be when valve actually seals)


If Rf is big enough CLf would have to become -ve for the opv to seal. This is effectively why a -ve test on surface (where your actualy making CLf very -ve) may not show up crud in opv (that could cause a leak on a dive) and maybe explain why myself and several boris owners experienced floods due to leaking opvs when those very opvs passed pre-dive neg tests. (opv design now changed)



...or in english, crud on the opv face means the opv will seal less effectively (smaller differential). The more crud the less effective the seal. Too much crud and it wont close at all.

passing a neg test on surface may not mean theres no crud - it may just mean with that much differential (-CLf) Rf is overpowered. When in use Rf may become an issue.

But it goes beyond that;

Given that: The opv will close when Sf=CLf+Rf

Then this can be rewritten as CLf=Sf-Rf

From this we can see that as soon as the force requierd to overcome resistance to seal (due to crud) ,Rf, is equal to Sf then CLf would be zero when opv seals.

Or in english; the more crud on an opv face the lower the pressure in the CL would be when the opv finally seals (assuming it can). To much crud and there will be a point where the gas in the CL is at ambient when opv seals.......and im thinking that could explain how a system can be venting gas AND flooding at the same time. Its because at the end of each vent as the opv closes the gas in the CL is now at ambient rather than higher as it would be in a correctly working system


This could also explain why your drysuit opv doesnt leak during descent (as CLf is -ve and so Rf is overpowered, seal is enhanced) but does on ascent when its +ve and it adds to Rf, seal is weakened.....


If the above is true (is it?)
then that would suggest that just doing a normal -ve test on surface to check out the opv may not ensure there isnt crud in it that will make it leak in use. Maybe better to try to test with smaller surface vacumm or visualy inspect opv seal face before every dive. I think in practice ill try the former and only visual inspect if im going to be staging the unit


ok Im done

Attached Images

opv.JPG (38.1 KB, 389 views)

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bletso

That is why I got rid of my stock OPV and replaced it with a very stiff OPV from a BCD. It prevents the CL's from blowing apart at the seams if I should neglect to shut down the O2 bleed. I just vent though my nose.

Since I did that, I have not had any of the previous issues of unexplained water content in the loop.

Dale