Dive / PreDive Adjustment

[qsnow]

The answer might be here somewhere, but I've yet to find it in searching around... I'm new to a lot of this stuff and I'm curious as to how the Dive / PreDive adjustment works -- ie. What is it doing / adjusting. My initial thought was that it was to prevent free flows entering the water or at/around the surface, but that doesn't seem to be quite acurate.

My reg is a Zeagle VI/XP, although I'd suspect they all work about the same way.

 

[ptyx]

I think it acts by modifying the shape of the air path and enabling/limiting the venturi effect in the regulator.

When in pre-dive mode, the regulator is supposed to always require negative pressure (suction) at the mouthpiece level to deliver. Put it horizontal or out of the water, and it will stop freeflowing.

When in dive mode, the venturi will do its job and the flow will be self sustaining once started. You need to exert positive pressure/block the flow to stop it from delivering air. This is the part 'below 0' in the the regulator breathing resistance curves.

Why and how the venturi works - I have no idea - but it sounds like a lot of fluid dynamics.

 

[LeadTurn_SD]

Breif "non-technical" description: It is essentially enabling / disabling the venturi effect... on your Zeagle Zx 2nd stage this lever is actually called the "Venturi Control Lever".

Disabling the venturi effect in the 2nd stage helps prevent freeflows when the 2nd stage is out of your mouth. Enabling it once you have the reg in your mouth will make it just slightly easier to breath... but most folks find that the switch does not change the effort of breathing all that much. It does help preventing freeflows however.

Hope this helps.

Best wishes.

 

[qsnow]

So when surfaced, say floating in the water - this may not necessarily stop it from free flowing at times - as water may push on the diaphram? This is more to stop any chance of free-flow when completely out of the water?

 

[LeadTurn_SD]

Hi gsnow,

I have 2 Zeagle Flathead VI's, and do my own servicing on them....

No, the venturi lever is designed to reduce freeflows when the reg is out of your mouth while floating in the water at the surface, but they can still happen.... it depends on where you have your breathing resistance knob set.

Even on dry land, if you have the breathing resistance knob cranked all the way to the easiest postion, and the venturi lever set to the "+" position (venturi ON), then sharply tap the purge, it will intiate a STRONG freeflow (if the reg is properly tuned). Now, with the reg freeflowing, as you move the venturi lever back towards the "-" position, the freeflow should stop before the lever has moved thru 2/3's of its travel. This is how sensitive the Zeagle Zx is supposed to be, if tuned "to factory spec".

Most high-performance 2nd stages will be somewhat prone to freeflows at the surface... with non-adjustable 2nd's, you just need to handle them carefully... submerge them mouthpiece-down, then SLOOOOOOWLY turn them mouthpiece-up so they slowly fill with water, then they should be "ok" while you float on the surface.

Many folks think their regs are improperly tuned if they freeflow easily at the surface with the reg out of their mouth.... this is often not true.

With adjustable 2nd's that are well-tuned, you may need to both set the venturi to pre-dive and slightly de-tune the reg with the adjuster knob until you are ready to dive to prevent surface freeflows...

Hope this makes sense.

Best wishes.

 

[ptyx]

I'm adventuring into not that well known territory here, so experts please correct if I'm wrong.

My understanding is that a regulator will start to flow based on the cracking pressure. At this point the venturi doesn't matter. Dive or pre-dive, if you put it head down in the water it will start free flowing. If it catches a wave or you press the purge button accidentally, same.

The difference will be what happens once the regulator is back in 'normal' position: either above the surface or horizontal/pointing down in the water.

If the venturi is on, it will keep free flowing. In pre dive mode, it should stop by itself.

 

[LeadTurn_SD]

ptyx,

I'm far from an expert (on anything, ), but what you said sounds about right to me

Someone with far more understanding can probably chime in and give a better technical explanation than I ever could..... but from a practical standpoint the venturi lever does seem to help make the reg less prone to free flow, and de-tuning the reg a bit with the adjuter knob also helps.

Best wishes.

 

[Ed Palma]

Cracking pressure is the amount of force necessary to start airflow. this can be measured in cmH20 and can be determined by the how deep you can submerge the reg face down in water before air starts to flow (cm water above the level of the diaphragm)

Now the way I understand the venturi is that given proper flow patterns, a rapid flow of air out of the mouthpiece lowers the pressure within the reg, leading to further actuation of the demand lever (via the slightly sucked in diaphragm). Based on what I've seen, the venturi switch works by re-directing the airflow either toward the mouthpiece (dive mode) causing negative pressures within the box, or toward the diaphragm (pre-dive) which reduces the likelihood of a free flow at the surface.

 

[halocline]

There are a few different venturi assist mechanisms, but they all basically do the same thing which has been explained well. The SP D series uses another dive/predive switch, which puts a spring against the lever in predive position, making it much more resistant to free flow.

 

[qsnow]

Fantastic information Helps my learning process greatly. thanks.

I'll check the tune on my VI as explained above when I get more air hooked up.