My new G250Vs and ramblings
- Thread starter buddhasummer
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Control is the operative word.
Sales gimmick or not, I think the adjustable vane is a useful feature. It does exactly as it is advertised.
Sales gimmick or not, I think the adjustable vane is a useful feature. It does exactly as it is advertised.
I think it's worth pointing out for other readers that the Dive/PreDive vane must control freeflow. That is its primary function. When the case is designed to facilitate breathing (i.e., with a Venturi effect that reduces inhalation effort to near-zero under a workload - green curve above), you need a way to control that for beach entries or at significant depth where air density further increases Venturi effects.
If you cannot block freeflow with the vane when you bang on the purge button, there are three things to look at.
1) the reg is tuned too hot (cracks too low),
2) the mouthpiece is wrong, or
3) the first stage IP is too high.
#1 is discussed in the curves above.
Re: #2, it seems counterintuitive, but sometimes you don't want as big an opening as possible.
Here are two "Atomic style" mouthpieces.
But only the left one is OEM.
Atomic designed its mouthpiece in conjunction with its retractable vane to maximize flow while preventing freeflow. The mouthpiece on the right will be much more prone to freeflow with a hot-tuned octo on a giant stride.
Finally, in regs that are right on the edge, a higher input pressure from the first stage might be just enough to provide greater throughput promoting freeflow that can be prevented with a little re-tuning of the first stage.
Can’t wait to hear this one.
OP
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Do you guys actually dive these regs? This discussion is reminiscent of some of the early theoretical math work I did. I assume there is a practical component somewhere?
@buddhasummer Perhaps getting them wet may be revealing?
Today.
Fair enough, @couv. The two functions (adding Venturi and preventing freeflow) go hand in hand. If you pulled the vane, that "airflow focusing" function would disappear, and the reg might or might not freeflow (like a 156 case).
And of course, a vane at a blocking angle induces turbulence and thus helps prevent freeflow.
No dance.
You always read the fine print, you bastard. 
Just trying to say that my primary second vane is always on Dive, and my octo vane is on PreDive, unless I'm using it. And I don't crank in that adjustment thingy at all. But I think @lexvil was right. Case design got so much better that they had to put it in to be able to reliably control freeflow, and oops!, performance got even better! You'd have to ask someone even older than you or René Dupré.
