Quote: (Originally Posted by
deedee)
Which version of dump valve ingests water as it burps excess air ? What orientation does the valve need to be for this particular event to happen ?
I always thought positive preasure and correct orientation would prevent above scenario.
Is there a dump-valve that prevents this ?
Darek
Good question.
I would have thought that the only way water can enter a venting CL is if the pressure in the CL is equal to or below the ambient water pressure Wp.
As Wp acts on both sides of the OPV (CL isnt rigid) the effects of Wp across the OPV must cancel out. So we only need to concern ourselves with CL pressure falling to equal or below Wp - in other words we need to have a vacumm in the CL
or CLp = Wp and a damaged leaking OPV is facing upwards.
I would have thought with a normally functioning OPV, on a venting system such as being described, water ingress can never happen as the CL internal pressure is governed by the opening forces of the expanding gas against the OPV (CLf) and the closing force of the spring. (Sf) The spring closing force limits the min CL pressure (in a continuous venting system) to something way above ambient. (how much above depends on spring force, but by definition it will always be above. Think how much it takes you to almost open an opv by blowing into your loop - thats how much +ve pressure you will have as a min in a venting CL)
Min pressure in CL (when OPV closes) is when CLf=Sf
So regardless of orientation I cant see how a properly functioning opv on a venting loop can ever let water in, and I cant see how a venting loop can ever flood (as by definition it is +ve pressure)
One way I
can see it happening is ;
-OPV is damaged/leaking
-Venting uses up all O2 supply
-OPV slowly leaks out gas until CL pressure is same as water pressure
-The orientation of the Rebreather was such that leaking opv was facing upwards and allowed gas to bubble out and exchange with water.
Perhaps someone has some other ideas?
Either way I think its good practice to shut off feed and sit Rebreather down with opv at lowest point of loop.