Question about Scubapro V.I.V.A. system

[SteveMKentucky]

I recently purchased a Scubapro MK25 G260. I was surprised when I found that it had two external adjustments. The first is a typical breathing resistance adjustment (which I'm familiar with) but the second is something called a V.I.V.A. control knob. I reviewed the owner's manual which gives a brief description of the system but it really made little sense to me.

From the manual:

V.I.V.A. (patented)

V.I.V.A. is the acronym for “Venturi Initiated Vacuum Assist.” High speed air flow passing over the vane creates a low pressure area inside the second stage housing. This depression pulls the diaphragm inside the housing, maintaining pressure on the valve lever and keeping the valve open without requiring an additional effort on the part of the diver. On some SCUBAPRO second stages the Venturi effect (V.I.V.A.) can be adjusted during the dive by changing the position of the flow vane via the knob positioned on the outside of the second stage.

also from the manual:

If the second stage is equipped with a Venturi effect adjustment system, this device must be turned to the minimum (marked “MIN”) or the “PREDIVE” setting while at the surface (Img. 6). At the beginning of the dive the adjustment knob must be turned to the “MAX” or “DIVE” position.

Can anyone explain to me just what this thing does? Also, if I forget to turn it to dive what will the result be? The fact that the manual uses the word "must" implies that it's important.

 

[rsingler]

The VIVA vane changes turbulent flow inside the 2nd stage into smoother, straighter flow. As you know, occasionally on surf entry when not breathing from your reg or when you drop your second stage out of your mouth and it lands mouthpiece up, the reg begins to freeflow violently. This is due to the Venturi effect: the flow of air out of the mouthpiece from the opening of the second stage valve creates a low pressure inside the reg case which keeps the diaphragm sucked in and keeps the valve open. Again as you know, you stop the freeflow merely by putting your thumb over the mouthpiece hole. When the pressure rises inside the reg case, the diaphragm returns to its normal position and the valve closes.
When the VIVA is turned to the Dive position, there is no obstruction to airflow, and the reg will freeflow if the valve is opened briskly. This is a good thing, because as you dive deeper, the increasing density of the air is offset by the venturi effect and helps keep the valve open so that breathing effort doesn't rise.

In the PreDive position, the VIVA vane turns sideways a little. If you open the valve briskly, the rush of air out of the valve is turbulent, and no relative vacuum is created inside the case. Therefore the reg won't freeflow. Many divers enter through the surf with the vane left in PreDive, in case the reg is knocked from their mouth, so it won't suddenly dump a lot of air. They then turn the vane to the Dive position before descent.

Note: the VIVA does NOT change the point at which the valve opens. In other words, you don't have to suck harder with the vane left in PreDive. In fact, during quiet breathing, you may not be able to tell any difference at all. This is to be compared with screwing in the adjustment knob, which WILL create a noticeable increase in required breathing effort.

The VIVA's value really only becomes apparent at depth, or during heavy effort. It is not adjustable in some older and lower end regs, but instead is set by the technician at just a minimal degree of tilt that will self-terminate a freeflow in one or two seconds.

"MUST" be set to the Dive position? Not really. If you forget, you MIGHT notice a minor increase in breathing effort at 60' swimming against a current. But it's still a very nice technological tweak that makes regulator performance just a bit better, especially for the machines that measure this kind of thing for regulatory approval agencies and advertising.

 

[SteveMKentucky]

The VIVA vane changes turbulent flow inside the 2nd stage into smoother, straighter flow. As you know, occasionally on surf entry when not breathing from your reg or when you drop your second stage out of your mouth and it lands mouthpiece up, the reg begins to freeflow violently. This is due to the Venturi effect: the flow of air out of the mouthpiece from the opening of the second stage valve creates a low pressure inside the reg case which keeps the diaphragm sucked in and keeps the valve open. Again as you know, you stop the freeflow merely by putting your thumb over the mouthpiece hole. When the pressure rises inside the reg case, the diaphragm returns to its normal position and the valve closes.
When the VIVA is turned to the Dive position, there is no obstruction to airflow, and the reg will freeflow if the valve is opened briskly. This is a good thing, because as you dive deeper, the increasing density of the air is offset by the venturi effect and helps keep the valve open so that breathing effort doesn't rise.

In the PreDive position, the VIVA vane turns sideways a little. If you open the valve briskly, the rush of air out of the valve is turbulent, and no relative vacuum is created inside the case. Therefore the reg won't freeflow. Many divers enter through the surf with the vane left in PreDive, in case the reg is knocked from their mouth, so it won't suddenly dump a lot of air. They then turn the vane to the Dive position before descent.

Note: the VIVA does NOT change the point at which the valve opens. In other words, you don't have to suck harder with the vane left in PreDive. In fact, during quiet breathing, you may not be able to tell any difference at all. This is to be compared with screwing in the adjustment knob, which WILL create a noticeable increase in required breathing effort.

The VIVA's value really only becomes apparent at depth, or during heavy effort. It is not adjustable in some older and lower end regs, but instead is set by the technician at just a minimal degree of tilt that will self-terminate a freeflow in one or two seconds.

Perfect answer. Thank you. So I should probably enter the water with the VIVA control in pre-dive then switch as I descend?

 

[BoltSnap]

Perfect answer. Thank you. So I should probably enter the water with the VIVA control in pre-dive then switch as I descend?

I turn it into "Dive" position when the mouthpiece is in my mouth, in "pre-dive" position when it is out of my mouth. You do NOT want the "VIVA" (venturi) control switch in the "Dive" position if the mouthpiece is NOT in your mouth!!!

I always have the cracking pressure adjustment knob set in the easiest breathing setting, out all the way, and the Viva in the "Dive" position when the mouthpiece is in my mouth and I am underwater.

 

[rsingler]

Yeah, maybe switch to Dive just before descent.
As long as you don't forget, or as long as it's not too much trouble wearing thick gloves.
To be honest, I just leave my primary on Dive, and keep my reg in my mouth or in my hand on entry, so freeflow isn't an issue. One less thing to forget.
On the other hand, my octo is left on PreDive. The opening effort of the valve is unchanged, and it won't freeflow if it comes out of its keeper.

I might actually start in Pre Dive if the entry were truly rough, but usually I don't move it from the Dive position.

 

[buddhasummer]

As always Dr. Singler is right on the money. Great explanation Rob.

 

[rsingler]

You're kind, Simon. We all jes' divers.

Oh, btw...150 of those o-rings are shipping out from Precision tomorrow. PM me with how many you want.

 

[ams511]

What o-rings?

 

[ams511]

@SteveMKentucky there is a YouTube video on this: