Venturi Effect

Could someone explain how the venturi effect is connected to the function of a regulator? I presume it mainly deals with the first stage, but not sure exactly the theory as it applies to diving instead of fluid dynamics.

It mainly has to do with the second stage. First remember that air is a fluid just like liquid is fluid. Both have no fixed shape and are easily manipulated by external pressures.

If you don't know already, the venturi effect can be better viewed as the siphoning technique some people use to siphon gas out of a gas tank or water from a fish tank.
If you've never done this before you basically suck enough fluid from a main reservoir to fill your entire hose and pour out the intended end, the fluid will then continue to pour out of that hose until the reservoir runs out or until you physically put a kink or plug in the hose.

This is the same effect that happens in second stages. If you've ever had your reg freeflow on entry because your purge button got hit on impact, this is the venturi effect. Your second stage will continue to free flow until you fix it.

How a second stage works is you have a hard seat blocking air from your hose. It's basically a rubber plug seated on a chimney stack, with a spring pushing down on the plug (in baby terms). As you breath the hard seat cracks off it's seating and air rushes through from your hose. This is because you decrease the air pressure on one side of the hard seat and your hose pressure therefore becomes greater to push against the spring holding your hardseat down on the "chimney stack".
That amount of pressure change needed to "lift" the hard seat is known as cracking pressure.

The air will keep rushing through because of the venturi effect and your hard seat will not close until you actually stop the effect by applying pressure in the reverse fashion. This can be turning your regulator so the mouthpiece is down (water pressure therefore applies the force), or by stopping your inhale (dead air space in your second stage applies the force). For the latter you may ask why that dead air and the venturi effect don't just bust out of the exhaust tee, for that I do not know.

Venturi effect happens in the 2nd stage.

When you inhale, the pressure in front of the diaphragm, (inside the box), is lowered. This pulls the diaphragm inwards, thus cracking the valve & allowing the gas to flow in.

If the venturi is on, the flow will be towards the mouthpiece, thus reducing the pressure in front of the diaphragm & gas flow will continue without any further inhalation effort. It will only stop when the pressure inside the 2nd stage box increases enough to push the diaphragm outwards, thus closing the valve. This can be achieved instantly by exhaling if it's in your mouth. You can also close the mouthpiece orifice by putting your palm down on the mouthpiece sealing it, or putting the 2nd stage, mouthpiece down, into the water, etc. Or by turning the venturi to the off position.

When the venturi is in the off position, the gasflow within the 2nd stage box is redirected away from the mouthpiece towards the diaphragm. This causes a higher pressure in front of the diaphragm, pushing the diaphragm outwards, thus closing the valve. If you dive with it in this position you must maintain inhalation effort to maintain gas flow. When you stop inhaling, the pressure increases in front of the diaphragm & shuts the valve.

Thanks guys!

I never saw a regulator with a venturi effect that could be turned on an off. The venturi effect is caused by the shape of the 2nd stage and how the air is directed to the mouthpiece. Some have a vane or tube that directs the air directly into the mouthpiece tube. This causes a partial vacuum in the second stage that acts as an assist to the vacuum provided my your inhalation effort.
I think the on and off you are referring to is the adjustable 2nd stage such as the Scuba pro 109. The adjustment simply makes the spring that closes the second stage poppet valve stronger or weaker making the vacuum necessary to initiate flow higher or lower but does not change the venturi effect itself.

captain:

I never saw a regulator with a venturi effect that could be turned on an off. The venturi effect is caused by the shape of the 2nd stage and how the air is directed to the mouthpiece. Some have a vane or tube that directs the air directly into the mouthpiece tube. This causes a partial vacuum in the second stage that acts as an assist to the vacuum provided my your inhalation effort.
I think the on and off you are referring to is the adjustable 2nd stage such as the Scuba pro 109. The adjustment simply makes the spring that closes the second stage poppet valve stronger or weaker making the vacuum necessary to initiate flow higher or lower but does not change the venturi effect itself.

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No, I'm not talking about the cracking pressure adjustment. I'm referring to the venturi control, that's found on many, but not all 2nd stage regs, (eg. Apeks) When this is turned to the off position, the gas flow is redirected away from the mouthpiece, & towards the diaphragm. This causes the low pressure at the inside face of the diaphragm to be replaced by a higher pressure, killing the venturi effect, pushing the diaphragm out, releasing the pressure on the lever, & closing the valve.

Sealing the mouthpiece achieves the same result. The pressure inside the box increases until it is high enough at the face of the diaphragm to kill the venturi effect & shut the valve.

PS. If the venturi control in my 2nd stages is set to the on position, it does nothing, other than allow the geometry of the 2nd stage box & it's internals to do the job of supplying a venturi effect & so lowering the work of breathing. That is, you don't have to keep sucking, to maintain gas flow.

Adding on to what Kern said, the Venturi Switch is also called a Pre-dive switch. It's not the adjustable knob seen on second stages such as the SP 109, but rather an actual switch with two settings. It doesn't turn the Venturi effect off, but rather inhibits it from taking full effect.

I believe HOG second stages have them. Sherwood Maximus' do as well.

The adjustment knob present on SP 109's as well as the second stages listed above adjusts the distance of the hard seat from it's seating. Meaning it adjusts the cracking pressure or the amount of effort needed to get air flowing to you.

The wikipedia article on dive regulators has some nice diagrams and gif's that show how the Venturi switch works.
Diving regulator - Wikipedia, the free encyclopedia

g1138:

Adding on to what Kern said, the Venturi Switch is also called a Pre-dive switch. It's not the adjustable knob seen on second stages such as the SP 109, but rather an actual switch with two settings. It doesn't turn the Venturi effect off, but rather inhibits it from taking full effect.

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It alters the pressure differentials within the 2nd stage box, replacing a low pressure zone with a high pressure zone. This stops the venturi effect.

g1138:

The adjustment knob present on SP 109's as well as the second stages listed above adjusts the distance of the hard seat from it's seating. Meaning it adjusts the cracking pressure or the amount of effort needed to get air flowing to you.

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As per what the captain said about the SP109, Apeks , Hog, & similar regs, change the cracking pressure by altering the tension on the spring in the shuttle valve. Increase the tension, increase the cracking pressure, reduce the tension, reduce the cracking pressure. The shuttle valve's rubber seat is always in contact with the seat.