I recently converted another DA to PRAM configuration, this one in DIN, and rather than using the Duck Bill Eliminator I tried a silicone duck bill on it, in part due to the reports of lower exhalation effort and in part due to the round label can where the DBE can cause some interference with the diaphragm.
It worked great and the exhalation resistance was less than with a duck bill eliminator which in turn is a little better than the older rubber duck bill.
However, this weekend I took my new VDH back plate for a spin, to see how it compared to my Freedom plate, and after a short swim I decided I needed to re-adjust the harness as the can was riding up a bit too high and I needed to shorten the crotch strap and lengthen the shoulder straps. In the process I turned off the tank valve to prevent losing gas while adjusting the harness in the water, and inserted a mouthpiece plug. Then I apparently stretched the exhaust hose.
The end result was that the low pressure in the hose caused by the stretched hose with the mouthpiece blocked and no gas in the regulator, sucked the silicone duckbill back inside the hose. Of course, as soon as I tried to exhale, the positive pressure in the hose forced the duck bill closed, as it was now inside the hose, not outside it, and I could not exhale at all.
Fixing it required separating the cans and then exhaling through it to force 1/4" or so of the duckbill out so I could pull it the rest of the way out. It's wasn't exactly an in-water fix.
I played with it a bit to replicate the failure and I found that with the mouthpiece and inhalation side both blocked a moderate hose exhaust hose stretch would suck it back in the exhaust hose every time.
Now...a disclaimer here is that I am also diving EDPM hoses with 1" ends to accommodate Hope-Paige mouthpieces (I'm really looking forward to the new DSV). One thing I love about these hoses is that they are much more compact - very much like the hoses on my KISS Sidekick. They are much more streamlined in the water, and much quieter than the standard silicone rubber USD pattern hoses. But they have a much greater accordion effect with much more potential volume change that probably exacerbates the duckbill issue.
I mention this as with the new DSVs showing up in the near future, it is possible that folks might start having similar issues, if the hoses get stretched while the gas is off and the mouthpiece is blocked. If that's the case, a duckbill eliminator mod is a good idea to go along with the new DSV.
For that matter the same thing is possible with a mouthpiece plug - and it may be a known issue that I have just never encountered or hear dog before.
Or it could just be a perfect storm of a very flexible silicone duck bill, very stretchy EPDM hoses and an efficient mouthpiece plug.
Anyone have a similar experience?
Larry