Venturi effect: How does a second stage mapping to a Venturi tube?

[tursiops]

Bob - If I am reading your last paragraph correctly, I think you have it backwards. I suppose your version could be true for some brands/models, but off the top of my head it is not true for the ones I am familiar with and work on on a regular basis. The "dive" position has a more unobstructed air flow, and the "pre-dive" position moves a piece of plastic into the path of the air flow to direct some of the air elsewhere to "bounce around" in the case, anywhere but towards the mouthpiece.
Another variation utilizes a vane in the mouthpiece "tube" that has the edge facing directly into the air flow for the "dive" position, but slightly tilted or turned (like an airplane tail rudder) for the "pre-dive" position.
The one oddity that comes to mind is older Mares 2nd stages that have the DPD switch, which actually rotate a curved piece of plastic into place behind the lever arm to increase the force required to move the lever. That one is a very mechanical solution and not a Venturi based solution. I not sure if they are even breathable in the pre-dive position. I'll have to try it the next time someone brings me one to work on.

Yes, the more readily the air flows from the hose to the mouthpiece, the fewer losses and the faster the flow....the whole chamber has a lower pressure (this is why Venturi is used to describe it) so the diaphragm is more easily moved to pushing the lever that causes the flow. Screw up that flow path -- with a baffle or whatever -- and the flow gets more turbulent, the flow rate drops, the pressures go up and it is harder to press the purge button. So "dive position" is the unobstructed flow path from hose to mouth; "predive" is a baffled, more obstructed flow path.

 

[JackD342]

You description sounds like an issue I had with an old Mares regulator a while back.

The bottom line was that if the customer pushed too far/hard on the purge button, he was able to move the lever arm past the "tipping point" to where the spring tension held it stuck open rather than it returning to the closed position.

I worked on it for quite a while, and consulted with another experienced Mares tech, with no solution. I sent it to Mares who replaced the lever arm (said it was slightly worn when viewed with a microscope) and reversed the tiny washer so the "sharp" edge faced up towards the locknut and the "round" edge faced down towards the lever arm. So basically they removed some of the friction in the parts holding the lever arm on the stem. While that did work, I have to say that I would also point the finger towards a design that allowed that much travel on the purge button in the first place.

Hopefully something above suggests some other items for you to look at on your Calypso. Maybe the diaphragm is no longer stiff enough and could be replaced? Or the same for the spring?

Added thought: I can't say for sure if I have service manuals for your version of the 2nd stage, but many USD/AL from that period include a spacer on the demand lever stem. Any chance there is a missing spacer?
Also, any chance someone has in the past tuned the unit by actually bending the lever arm, a la the proper procedure for some older Sherwood 2nds?

You should probably try it again dry and this time push the purge in as far as you (reasonably) can to see if you duplicate the issue. If nothing, take the cover off and just press the lever arm in all the way to see if it will move past the tipping point I described. If that turns anything up it hasn't solved your issue yet, but helps you narrow it down.

 

[Jcp2]

I would swap out the diaphragm first and the internal spring second, one at a time to see if the problem follows the part.

 

[Scuba Lawyer]

Added thought: I can't say for sure if I have service manuals for your version of the 2nd stage, but many USD/AL from that period include a spacer on the demand lever stem. Any chance there is a missing spacer?
Also, any chance someone has in the past tuned the unit by actually bending the lever arm, a la the proper procedure for some older Sherwood 2nds?

I have the service manual and schematic. Your thoughts are good ones, and I will check them out.

 

[Scuba Lawyer]

I would swap out the diaphragm first and the internal spring second, one at a time to see if the problem follows the part.

Good ideas all, but I put in a new silicone diaphragm and a new spring so I can't imagine that is it. That said, never say never, I am open to all ideas.

 

[JackD342]

Good ideas all, but I put in a new silicone diaphragm and a new spring so I can't imagine that is it. That said, never say never, I am open to all ideas.

Were they actual correct part numbers from AL? Or something that "should do the trick, they're all the same..." from some other source?

 

[Scuba Lawyer]

Were they actual correct part numbers from AL? Or something that "should do the trick, they're all the same..." from some other source?

Actual correct part numbers and everything, I got the service kit, spring and diaphragm from Bryan at VDH for the 1085 2nd Stage. I've installed the exact same parts on other 1085 2nd stages that I have and never had an issue.

 

[nohappy]

Thank you guys to jump in and try to help. I want to rephrase my original question again. As far as I understood, Venturi effect or Bernoulli's principle is describing the change of the pressure (or velocity) on a single path of a fluid. However, if I read all the explanation correctly, most of them are trying to state that high speed air create lower pressure than the "ambient pressure". Like second stage, high speed air create lower pressure than outer "ambient pressure". Or like the eductor, the nozzle air create lower pressure than "ambient pressure" hence the suction effect. I know it works, but it seems to be comparing two different fluid (or air). i.e. comparing high speed air to the ambient air. I'm aware that if the air goes through a restricted area, it has lower pressure than the pressure at its "original path". How do you prove that it is also lower than ambient pressure? And if you can prove that, how is that relative to Venturi or Bernoulli if you're trying to comparing two different fluid?

 

[lowwall]

Let me try this with a simpler idea.
In your eductor drawing ( this is how regulators are designed) the gas coming out of the nozzle is moving at high speed. As the jet of air shoots down tube 2, the high speed air molecules leaving nozzle 3 "grab/shove/push" other static molecules around them and pull them down the tube with the high speed ones. This leaves the area void of air molecules (vacuum) which causes others from the area behind the jet in areas 1 and 5 to move to fill the void left by the air molecules that were moved away.

The term is "entrainment".

Mares second stages use this in a slightly different way. When the valve opens, gas flows through a tube that is aimed at the mouthpiece. This flow entrains some of the gas in the chamber adjacent to the diaphragm, lowering the pressure there and thus assisting in keeping the diaphragm depressed. The result is low effort to keep the air flowing once an inhalation starts. And no need for additional levers, although they have recently added them on some models for marketing reasons.

 

[nohappy]

Let me try this with a simpler idea.
In your eductor drawing ( this is how regulators are designed) the gas coming out of the nozzle is moving at high speed. As the jet of air shoots down tube 2, the high speed air molecules leaving nozzle 3 "grab/shove/push" other static molecules around them and pull them down the tube with the high speed ones. This leaves the area void of air molecules (vacuum) which causes others from the area behind the jet in areas 1 and 5 to move to fill the void left by the air molecules that were moved away. As long as the jet continues, it will pull more molecules away that have to be replaced, nature hates a vacuum, if the area 1/5 above were sealed off, those areas would become a vacuum as many of the molecules from the area had been swept away with none to refill the space.
Might not be a physics book answer but maybe it will make it more clear.

I like your explanation and I've seen this kind of explanation on the internet. That's very intuitive. But that confuse me too. Because what's the physics principle behind this explanation and how is that different from Venturi?