AFC OR venturi switch?

[rsingler]

Then you'll love the Atomic, @Divectionist !
There's nothing to fiddle with at all. The vane is out enough at the surface that freeflow is common only if you tune your reg very lightly, and bang it at at the surface.

In the first 10 feet, it retracts enough to facilitate Venturi flow, and as noted above, maxes out around 60 feet. Some folks tune their octo slightly stiffer to prevent freeflow. Others don't bother.

The only thing I'd disagree with is that even a low end modern reg can't match the ease if breathing of the top stuff. It's all tuning. Though I'd agree, the cheap stuff can't maintain it very well.

 

[BoltSnap]

so the AFC maxes out at 60ft... not sure is this is good or bad, let’s say if you decide to 170ft or even tech depths

Meaning it is full effect starting at 60ft. All other regulators their venturi is either on or off.

 

[Divectionist]

Then you'll love the Atomic, @Divectionist !
In the first 10 feet, it retracts enough to facilitate Venturi flow, and as noted above, maxes out around 60 feet.

Does the Atomic have a way to stop venturi at depth or will it sit precariously close to free flowing when tuned for a low cracking effort? I know mine wants to shoot air like a cannon if taken out at depth without blocking the venturi, and I do like to have the option to 'disarm' it.

 

[rsingler]

Does the Atomic have a way to stop venturi at depth or will it sit precariously close to free flowing when tuned for a low cracking effort? I know mine wants to shoot air like a cannon if taken out at depth without blocking the venturi, and I do like to have the option to 'disarm' it.

You're not entirely wrong - light cracking effort and strong Venturi will make a freeflow more likely.
But without being criticial, I would suggest that what you describe may be more of a technique thing than a regulator flaw.

No, there is no Atomic switch to return to predive mode.

But let's look at the physics. When most any typical regulator is held mouthpiece up, the distance from the diaphragm to the top of the mouthpiece is perhaps 2.5 inches. Any reasonable cracking effort is less than that. That's why you can do a "sink test" at home to check cracking effort when you don't own a magnehelic gauge. In other words, ANY regulator turned mouthpiece up while filled with air will freeflow, no matter what the Venturi setting, because the water pressure pushing up on the diaphragm of a reg pushed down in the sink, like water pressure pushing up on the hull of a floating boat, is greater than the cracking effort.

So at depth, any regulator removal that is accompanied by a motion that doesn't even turn the mouthpiece up will be at least +3/4 inches of water pressure on the valve (the distance from the center of the diaphragm to the bottom of the mouthpiece, with the reg held like it is in your mouth). If your cracking effort is very light (~1") it won't take much of an escaping bubble from the case to pull the diaphragm in and actuate the valve.

Yes, an open Venturi vane will facilitate that, but it's primarily cracking effort that triggers the freeflow.

Bottom line: I don't think Atomic's missing "Pre-dive" switch is the problem. After all, you wouldn't twist the Venturi knob at depth each time before removing the reg from your mouth. And diving it without Venturi assist at depth defeats the purpose of the vane with thick air.

A subtle technique change to twist the reg mouthpiece down as it leaves your mouth is perhaps the solution.

But you are correct. With thick air and light tuning, regs are skittish on removal.

 

[DiversL]

So just to clarify, once you go past 60ft or maybe 80ft then “no more improving” , since it is maxed out?

 

[BoltSnap]

So just to clarify, once you go past 60ft or maybe 80ft then “no more improving” , since it is maxed out?

Yes, it maxes out and no additional venturi effect at around 60'.

 

[rsingler]

Well, I would comment that Venturi effects continue to increase as air density increases with depth. So to that extent there is an increased outflow of air from Venturi action due to physics. It's just not due to mechanical adjustments. The point is, there's no need for additional mechanical shifts once air gets thick enough. Physics takes over.

 

[rsingler]

I do like to have the option to 'disarm' it.

I realize I didn't fully answer your question.
While I think we've eliminated the lack of a Venturi Pre Dive switch as a problem, the solution to needing to "disarm" a tendency to freeflow is to screw in the adjustment knob. I might do that to my octopus, for surf entry or at depth, if I tune my regs "light". That's a little cumbersome to do to your primary second stage each time you want to remove it. But if you're doing valve drills at depth, yeah. Just "detune" your reg with the knob until it's back in your mouth. A slightly higher cracking effort while you're mucking about is no big deal, and you can unscrew the knob once you're back to a routine dive.

So to summarize, for both Scubapro and Atomic, the freeflow solution is the adjustment knob, not the pre-dive switch.

Having said that, I ran into an interesting issue with my new D420 doing valve drills. There's a Pre-Dive Venturi adjustment switch on that reg, but no detuning knob. And the air flow path on that center-balanced design is so short, that it would freeflow when tuned below spec, even with the mouthpiece turned down, just because it's Venturi action was so efficient. That reg delivers huge amounts of air effortlessly. So in that case, a very slight turn of the PreDive switch was necessary, coupled with better attention to mouthpiece position during the drills. That new reg is a star!

For all regs, tuning light comes with a price.