Quote: (Originally Posted by
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.
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
