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WHAT DOES THAT KNOB/LEVER DO?

Posted: Nov 26, 2013

p>“Why does my regulator free flow at the surface? Is it broken?”

“I just had my regulator serviced, why is it so hard to breathe?”

Today’s blog will answer these questions for you…and maybe create new ones, so let’s get started!

There are two features found on regulator second stages that can answer these questions – the Venturi Lever and the Opening Effort Adjustment Knob.

Venturi and Adjustment

The most common of these two is the Venturi Lever, found on both inexpensive and higher end model second stages. They come in a variety of different locations, but you’ll most often see a switch or lever that says +/- or dive/predive.

Venturi Function: Long Version

Fig 1 Turbulent FlowWhen you take a breath from your reg, there are two ways that air can flow: Chaotically, (fig 1) which is known as turbulent flow, or smoothly, (fig 2) which known as laminar flow. As you can see in turbulent flow, the air is bouncing all around the second stage, which slows the air speed. In laminar flow, this stream of air is able to move a lot faster since there’s nothing in the way of this direct stream of air coming to you, the diver.

Why does this matter?

Bernoulli’s principle states that the higher the speed of a flowing fluid or gas, the lower the pressure. Kind of hard to wrap your mind around, but stick with it here for a bit. This means the harder you inhale, the faster you make that stream of air move, (fighting a current, catching up to your buddy, whale shark just swam by and you don’t have a camera). Calling on good ol’ Bernoulli, this means the pressure is lower in the second stage, which in turn helps pull the diaphragm inwards. Conclusion? This fast moving, smooth moving air reduces the amount of effort needed to keep that air coming to you!

Venturi is enabled in the + or dive setting of your regulator. The – or pre dive setting is Fig 2 Laminar Flowthere because if you’ve ever jumped off a boat or purged your second stage on the surface, you’ll notice that a continuous and massive free flow can occur. This is your Venturi doing its job to the extreme! So, flip it to the other side and we get turbulent flow, slower air, and no massive free flow at the surface. As you begin your dive, flip it back to + or dive to get all that Venturi function goodness coming at you.

Venturi Function: Short Version

The harder you work, the easier it is to breathe.

Opening Effort Adjustment: Long Version

The amount of work it takes to begin the flow of air is called the cracking or opening effort. In regulators without a user adjustable opening effort knob, this effort is set by your friendly local scuba technician during service. Over time, this effort can change which will either make it harder to breathe or make your reg breathe for you!

If you have a regulator with an adjustment knob kind of like the one pictured, you have more control in your breathing effort. While your scuba tech does still set the initial opening effort, you the diver have more control in the overall breathability of your regulator.

What you’re adjusting when you turn that knob is the spring load. In short, the more compressed the spring is the more effort it takes to open the valve to let air flow.

Why does this matter?

If you screw that adjustment knob all the way in, you’ve compressed that spring to the point of “this regulator breathes horribly!” which isn’t pleasant for anyone. The adjustment knob is great for when you’re scooter diving or swimming into current and the pressure of the water is pressing on the purge button – tune it down (compress that spring) and voila! No more free flow. Some regulators are designed to give a slight free flow when that knob is tuned all the out (spring is the least compressed), so think of it like a tank valve – all the way open and a quarter turn back.

Opening Effort Adjustment: Short Version

Righty tighty, lefty loosey. Let your regulator breathe at its best!

More questions? Post them below or feel free to stop by and chat with your friendly neighborhood scuba tech.

Aloha, Kelly

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Teri White
December 1, 2013 @ 9:23 pm

Thank you Kelly for the clarification between the functions of the two knobs! I especially enjoyed seeing the difference in air flow in the two diagrams. I will definitely remember the righty/tighty, lefty/loosey hint:) Teri


Rachel
November 26, 2013 @ 9:25 pm

Great blog, Kelly! Though I've been in the industry for years, I continue to get mixed up about these knobs and levers, but your explanation cleared things up for me in a way that might actually stick this time! One question: what's the deal with regs that don't have either of these knobs?