Dive/predive Venturi control?

[soamelt]

What's the point of this switch on the second stage? I have a Zeagle 50d regulator and the only thing I noticed about adjusting this lever is that my reg free flows if I hit the purge valve. It also make the purge extra sensitive when it hits the water with the purge side facing down. I've noticed zero impact on breathing such as flow or ease of breathing. So what am I missing here, what's the point of this control?

Greg

 

[LeadTurn_SD]

The venturi switch enables/disables the venturi-assist in the 2nd stage. This allows the 2nd stage to be tuned "closer-to-the-edge" (of freeflowing).

The "pre-dive" setting should provide just enough increased resistance prevent the reg from freeflowing on the surface, or when not in your mouth underwater. But you should still be able to breath from the reg even with the switch in pre-dive position (and if it also has an external adjuster knob, you should still be able to breath from the reg even with the adjustment cranked all the way down.... but it will be harder).

FYI: For a well-tuned reg without the venturi switch, surface freeflows are avoided / stopped by turning the reg mouthpiece down in the water. Once underwater, you should be able to slowy tilt the reg upright and allow water in to the mouthpiece, and it should no longer freeflow.

I dive "classic downstream" 2nds that don't have external adjustments, and since I've tuned them "close to the edge" they will freeflow easily at the surface if I don't pay attention.

Best wishes.

 

[dkktsunami]

What's the point of this switch on the second stage? I have a Zeagle 50d regulator and the only thing I noticed about adjusting this lever is that my reg free flows if I hit the purge valve. It also make the purge extra sensitive when it hits the water with the purge side facing down. I've noticed zero impact on breathing such as flow or ease of breathing. So what am I missing here, what's the point of this control?

Greg

I'm with you - I notice little or no difference in breathing resistance and leave the lever in the "off" position (pre-dive) and I also leave the resistance knob in a fixed position. These adjustments have never done much for me.

 

[Dockmaster]

The venturi switch enables/disables the venturi-assist in the 2nd stage. This allows the 2nd stage to be tuned "closer-to-the-edge" (of freeflowing).

The "pre-dive" setting should provide just enough increased resistance prevent the reg from freeflowing on the surface, or when not in your mouth underwater. But you should still be able to breath from the reg even with the switch in pre-dive position (and if it also has an external adjuster knob, you should still be able to breath from the reg even with the adjustment cranked all the way down.... but it will be harder).

What he said. I just bought a Flathead7/Z Octo setup and found out that if I don't have my octo set to "pre-dive" I will have a freeflow as soon as I hit the water. I haven't tried breathing from it in the pre-dive" position, so I'm not sure how that feels.

My older setup (Sherwood Blizzard/US Divers octo) never, and I truely mean never freeflowed in the ten years that I used it. I'm sure this had to do to how sensitive it was tuned and the fact that there were no external adjustment knobs on the second stages.

 

[Kern]

The venturi switch enables/disables the venturi-assist in the 2nd stage. This allows the 2nd stage to be tuned "closer-to-the-edge" (of freeflowing).

The "pre-dive" setting should provide just enough increased resistance prevent the reg from freeflowing on the surface, or when not in your mouth underwater. But you should still be able to breath from the reg even with the switch in pre-dive position (and if it also has an external adjuster knob, you should still be able to breath from the reg even with the adjustment cranked all the way down.... but it will be harder).

Best wishes.

This sounds more like a description of a cracking pressure control.

Isn't the venturi (pre/dive) control a vane that fits around the valve spindle in the 2nd stage? Doesn't it redirect the air flow so as to alter the pressure within the 2nd stage, making it more difficult to free flow in the pre dive position?

 

[soamelt]

I'm with you - I notice little or no difference in breathing resistance and leave the lever in the "off" position (pre-dive) and I also leave the resistance knob in a fixed position. These adjustments have never done much for me.

I do actually notice a difference with the resistance knob. It's weird, if I have it at the hardest to breath setting at the beginning of a dive it's fine. However, I start my dive with it at the easiest setting, I can't change it to the harder setting. It's too hard to breath off when I go from easier>harder to breath.

 

[therealwwdiver]

I dive an Atomic ST1 and I can tell a big difference in the resistance knob setting regarding air delivery at depth.

 

[soamelt]

I haven't tried breathing from it in the pre-dive" position, so I'm not sure how that feels.

I don't think you will notice a difference at all, but then again I've only been used this reg to about 75 fsw in south Florida. Maybe it does matter at 150' in cold water?

 

[Kern]

I don't think you will notice a difference at all, but then again I've only been used this reg to about 75 fsw in south Florida. Maybe it does matter at 150' in cold water?

No, depth makes no difference to the pre dive control. It's either set to pre dive to prevent free flows when not in your mouth. Or it's in your mouth & set to dive. There's no a little bit on or half way settings, it's either on or it's off.

 

[LeadTurn_SD]

This sounds more like a description of a cracking pressure control.

Isn't the venturi (pre/dive) control a vane that fits around the valve spindle in the 2nd stage? Doesn't it redirect the air flow so as to alter the pressure within the 2nd stage, making it more difficult to free flow in the pre dive position?

I was giving a simple explanation, and sort of combined the two.

Cracking pressure is increased/reduced with the "breathing resistance knob", venturi effect is controled with a lever (labeled Dive/PreDive or "+" "-") on most adjustable 2nd's (that have both features). Note: When I think about it, I believe there are some regs that now combine the two features.

As far as whether the venturi switch can be set part way on or off.... it should be kept either all the way forward or back.

But using the Zeagle Flathead VI as an example, you can set the venturi switch part way open or closed, (but you should not need or want to).

Moving the lever rotates the vane (which in the Zeagle 2nd and Apex I believe is actually a 1/2 cutaway tube) around the air barrel (Zeagle calls it the valve tube).... Testing the Zeagle 2nd for proper tuning during service involves pressurizing the reg, opening both the resistance knob and venturi lever to their easiest settings, and tapping the purge which should cause an immediate strong freeflow... which is stopped by moving the venturi lever towards the "-" position.... the freeflow should stop before the lever has reached 2/3 rotation towards the "-" position. It is not a pure On-Off switch since it be set in any position between "+" and "-" (but again, why would you want to?).

You should be able to notice a slight difference in breathing resistance between the Dive/PreDive positions on most regs. On some regs it is pretty subtle: Just enough to prevent freeflows. It does not really effect the amount of air delivered.

Best wishes.

PS: After all that, I remember why I like "classic downstream" 2nd stages so much.... no knobs and levers to confuse my remaining brain cells