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How Pre-Dive Switches Work | #scuba #regulator | @ScubaDiverMagazine


How Pre-Dive Switches Work | #scuba #regulator | @ScubaDiverMagazine
Pre-Dive Switches or Venturi Levers help to prevent a freeflowing regulator by disrupting airflow inside a 2nd stage.
#scuba #scubadiving #scubadiver
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00:00 Introduction
00:36 Venturi Effect
02:01 Pre-Dive Switch
02:55 When to Use
05:26 Preventing a Freeflow

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Bloody Marvelous
Bloody Marvelous
8 months ago

I have the MTX and MTXV, and I opened both up.
The MTX doesn’t have a Venturi switch, and the opening is always covered.
On the MTXV you can open the Venturi for additional gas flow.

I therefor have to conclude that a regulator without a Venturi switch exists not to prevent a freeflow, but to allow for the Venturi effect to force more gas into your lungs, which is useful if you’re working harder underwater, like when swimming against a current.

Now, I haven’t looked at cheaper regulators, but I would assume that regulators without Venturi switches have the opening covered to prevent freeflows, and thus aren’t (as) susceptible to it.

Scuba Diver Magazine
Scuba Diver Magazine
Reply to  Bloody Marvelous
8 months ago

Cheaper regs without a venturi lever often will not have a central Valve Spindle or barrel where the airflow is directed straight towards the mouthpiece. That way the airflow is scattered into the 2nd stage and less likely to freeflow.

They also don’t have the same quality of breathe, they’re often set up a little more stiff and they can just freeflow…

john liang
john liang
9 months ago

Thx for explaining this

Lee Miller
Lee Miller
1 year ago

Glad to see I’m not alone in having trouble remembering+/- and should I push the switch away or towards my mouth while diving. I have the same issue remembering which way to turn the adjustment knob, too. I end up going with what feels right at the time.

Bloody Marvelous
Bloody Marvelous
Reply to  Lee Miller
8 months ago

Unscrew the front face of your regulator, and you will see what position does what. I’d say that in almost all regulators, pushing the lever forward will cover the opening, and pulling it back will open it fully.
In other words: Forward is pre-dive. Back is dive.

Joseph dracula
Joseph dracula
1 year ago

#ask mark Hello Mark always enjoy your shows. My question is about wing bladders. What comes to mind is why a 18 pound lift and a 30 pound lift. If the 18 is more than sufficient why do some divers use a 30 pound. Is it because of gear or steel tanks. I have heard of just the convenience for traveling. I noticed most divers are in warm water conditions with no more than a 3 mil wetsuit. Maybe you can go into more detail. Thanks Joe

Scuba Diver Magazine
Scuba Diver Magazine
Reply to  Joseph dracula
1 year ago

It’s a balancing act with wing size. 30lb wings are a good all-rounder for single cylinders and a safe choice. Smaller wings require proper weighting and give you less of a safety buffer.

Ideally you should always be perfectly weighted that you don’t actually need to inflate your BCD, especially to maximum capacity. You basically want enough lift to counteract your negative buoyancy for all of the gear that you’re diving with, including weights.

If I’m travelling I’ll take my little D18 wing as I tend to stay pretty shallow and don’t carry too much gear when I’m on holiday. Diving at home with twin steel tanks I use a larger 40 wing and my drysuit as backup.

It all depends on what you need for the dive at hand. I have the luxury of owning a load of dive gear where I can select a little travel wing but, for others buying gear a 30lb is a better all-rounder choice.

WOW Yummyyy
WOW Yummyyy
1 year ago

LIKE👍👍👍👍👍👍 🤩🤩🤩🤩🤩 ✳✴❇✴

Yggdrasil42
Yggdrasil42
1 year ago

#askmark I’d love to know how a drysuit shoulder valve works. How does it dump air without letting in water? How does its adjustability work?

Scuba Diver Magazine
Scuba Diver Magazine
Reply to  Yggdrasil42
1 year ago

It may vary between designs but, most auto dumps are one-way valves and when you screw the top in it pushes the valve inwards so gas inside your drysuit has to push harder to get out. The clever part is the 2nd seal which only opens when you push the main body inwards in an emergency.
I’ll pull one apart so you can see the insides

Joseph dracula
Joseph dracula
1 year ago

👍😎🤿

Bill
Bill
1 year ago

Constructive criticism: there was one area of content that was not clear. It was easy to follow the open vs covered in the switch, but didn’t go over the switch meaning of + vs -. And then you used terms of on vs off. It left me a little confused when you said on and off. Does on mean activated, thus restricting the air flow or does on mean full unrestricted air flow? My comment is that these terms should be fully explained/correlated to the general audience at the beginning just like explaining an acronym at first use. I love all your extremely helpful instructional videos and value every one of your posts. I do hope you take this comment as a very friendly one from one of your active followers.

Bill
Bill
Reply to  Bill
1 year ago

@elephantprotector yes, it was more clear in the vid 9 days ago. He said when it is ‘on’, it scatters the air. So in this case, ‘on’ means restricting and probably correlated to the actual switch labeling of “-“. In a nutshell, ‘on’ = restrict = “-“. 🤔

Bill
Bill
Reply to  Bill
1 year ago

@elephantprotector my guess is that the first vid was a Q&A, where the second was topic-specific and easier to search for and find.

MrShadowpanther3
MrShadowpanther3
Reply to  Bill
1 year ago

I have one of these on my primary and the + / – always confused me as well. Is “+” Easier breathing or More Resistance to free flowing?!? At 2:18 you can see the + mode would be “smooth airflow” as the – mode puts the shroud over the air inlet. I would think Dive / Surface (or even D/S) would make more sense. + does not mean “more airflow”, it just means “less turbulence / resistance”.
I would blame the manufacturers on this. Then again, D/S is english and maybe they did not want to have to print the relevant characters from every language they sold the regulator bodies in?

Bill
Bill
Reply to  Bill
1 year ago

@MrShadowpanther3 Thank you! 😊👍

Scuba Diver Magazine
Scuba Diver Magazine
Reply to  Bill
1 year ago

This video is a more in-depth look at how the functions work 🙂

Hugh Idiyit
Hugh Idiyit
1 year ago

As a Mares Abyss diver, I had to smile.

Yggdrasil42
Yggdrasil42
Reply to  Hugh Idiyit
1 year ago

Internally it does a similar thing.

Hugh Idiyit
Hugh Idiyit
Reply to  Hugh Idiyit
1 year ago

@Yggdrasil42 Yes, but you don’t need levers and knobs to adjust anything. It just works.

Cecil Lee
Cecil Lee
1 year ago

Thank you for your very clear and comprehensive explanation. I’m new to scuba diving (I just got certified as a PADI Open Water Diver and Advanced Open Water Diver at a resort on Ko Racha Yai 12NM south of Phuket, but am basically a newbie) and am keen to understand just what I’m supposed to do and why. I’ve been staring at the two levers/dials on my second stage and wondering what they are and why they’re there. This video has answered both questions. Wishing you a blessed Christmas and safe and enjoyable dives in the years ahead.

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