Venturi: Practical aspects.

[scrane]

I think I am pretty comfortable with the venturi concept and how it accelerates air into the inhalation hose. What I am unsure of is how venturi assisted air influences the breathing characteristics of the double hose regulator. I assume the venturi has nothing to do with cracking pressure. Does the velocity of air have any effect on the effort needed to maintain flow during the intake breath? Does high venturi assist increase the ultimate amount of air the second stage can provide at depth, or will the pressure drop due to increased velocity starve the first stage to negate any gain?
In other words, how does a DH second stage with high venturi assist breathe differently from one with standard configuration?
Thanks.

[luis]

The venturi flow does nothing for cracking effort.

The amount of air delivered by any regulator demand valve is controlled by the inhalation of the diver. You will often hear the erroneous statement that a regulator will give more (or less) air. All regulators better give you just the amount of air the diver demands. Any more air will be classified as a free flow or a leak.

Once air flow is initiated (after the cracking effort is overcome), the venturi flow will assist the diver in sustaining the suction needed to pull down the diaphragm (and continue to sustain the flow needed by the diver).

A properly designed and adjusted venturi flow will only "assist" the diver once flow is initiated and it will stop once the divers stops inhaling. As I mentioned, if it provides more air than demanded it is a free-flow.

The venturi just reduces the effort (by assisting with the needed suction to keep the demand valve open), while the diver is inhaling.

[scrane]

So, will the venturi setting have an effect on the point where free flow might occur or the severity of the freeflow? Or the ability to halt freeflow with the level of the mouthpiece or obstructing the mouthpiece?
Blocking one hole on my Phoenix provides pretty easy breathing. Is it OK to assume that it would have the same effect on my new Kraken?

[luis]

The Kraken and the Phoenix-HPR are very similar. The plastic case (of the Kraken) does have a slightly better horn geometry to match the HPR jet a bit better. There is a performance difference, but it is hard to notice by most divers.

If you want to adjust the venturi up to be aggressive, you need to use a DSV mouthpiece with the flow diverter. If the flow diverter is not in place, it is very easy for the flow to go past the diver and out the exhaust.

I have intentionally adjusted the HPR venturi high enough (by blocking most of the bleed air port) to the point that even with the flow diverter in place, every breath induced excess flow that went out the exhaust. The more I inhaled the stronger the amount of bypass/ wasted air.

With the flow diverter in place, the induced flow tends to eventually stop when I exhale into the mouthpiece. But with the traditional curve mouthpiece (and a strong venturi flow) the flow can easily become an uncontrollable roaring free-flow.

In general (with the DSV diverter mouthpiece) it is very safe to totally block one of the side bleed ports. Most divers will be very happy with the performance with one side fully blocked and the other side open.

I tend to partially block the second side bleed port. But, this adjustment is a bit of a trial and error approach. Half of the second orifice blocked is about the most aggressive venturi without encountering flow bypass issues (out the exhaust). I have also found that my settings can be too aggressive for some divers (too high).

If you decide to push the limits and see how high a venturi assist and performance you can squeeze out of a Phoenix-HPR or a Kraken (with the DSV), I would strongly suggest plenty of pool time testing (in all positions) or test dives where some wasted air (out the exhaust) is not a concern.

Note: Some free-flow can be also caused by malfunctions, like a sticking demand valve lever or icing due to freezing moisture on the mechanism, etc. A strong venturi can also compound those malfunctions, but they can also occur totally independent from each other.