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

[Angelo Farina]

Bernoulli and Venturi theories assume no loss and a perfect gas.
Instead a regulator is an high loss device, where the valve is laminating the fluid, so pressure is decreased wasting the associated potential energy.
If the gas was perfect, the expansion would be iso-enthalpic, hence isothermal. Not iso-enthropic (reversible)..
As air is not entirely a perfect gas, the irreversible expansion causes some temperature reduction (Joule-Thomson effect).
So what happens in the reg is very far from what Bernoulli or Venturi theories describe.
What usually is described as "venturi effect" is just the fact that increasing the flow causes some suction on the diaphragm, reducing the depression required for keeping the lever depressed. This reduces also the area of the respiratory cycle, and hence the associated work.
But calculating what really happens is truly complex, do not think that a super simplistic model such as those provided by the Venturi or Bernoulli theories can provide realistic description of what's happening inside a regulator. Proper modelling can only be done with high-end CFD computer programs, such as Fluent, Ansys or Comsol.

 

[Jcp2]

It doesn’t have to be perfect, and the modeling can be done by trial and error with physical models. That’s how these second stages were originally developed before computers were widely available. Theory, physical modeling until good enough, with most subsequent changes just evolutionary either for minor performance or major manufacturing advantage.

 

[Lorenzoid]

Quote from wiki:

But I don't understand that which part of the second stage is mapping to the "constricted section"? The orifice? The mouth piece part? Because the way I see it is that the air is coming from a constricted second (LP hose) to a larger space (second stage), and that seems to be the opposite to the concept.

As I believe others have already alluded, the Venturi effect as it is classically explained with that diagram showing the "constricted section" does not closely map to what's going on in a second stage--or at least those that I'm familiar with. The Venturi lever in my reg simply makes the airflow more turbulent when it is in one position and less turbulent when it is flipped in the other position by moving a baffle in and out of the airflow path. Less turbulent would mean faster airflow toward the diaphragm, but as far as I can determine, the baffle doesn't change the diameter of the airflow path. That is, it doesn't "constrict" the path; if anything, I would think that when the baffle is placed in the path, thereby de-tuning the reg, the path is more constricted, not less constricted. So it really does not appear to me--at least in my regs--that what is occurring can be attributed to the Venturi effect in the classical sense of that diagram with the "constricted section." As far as the term "Venturi-assist," the baffle doesn't "assist" the airflow; on the contrary, it impedes or makes turbulent the airflow. It has occurred to me that calling it a "Venturi-assist" lever may either be mostly a marketing thing, or perhaps in some earlier reg design there really was more of a classic Venturi effect going on.

 

[Bob DBF]

Less turbulent would mean faster airflow toward the diaphragm, but as far as I can determine, the baffle doesn't change the diameter of the airflow path. That is, it doesn't "constrict" the path; if anything, I would think that when the baffle is placed in the path, thereby de-tuning the reg, the path is more constricted, not less constricted. So it really does not appear to me--at least in my regs--that what is occurring can be attributed to the Venturi effect in the classical sense of that diagram with the "constricted section." As far as the term "Venturi-assist," the baffle doesn't "assist" the airflow; on the contrary, it impedes or makes turbulent the airflow. It has occurred to me that calling it a "Venturi-assist" lever may either be mostly a marketing thing, or perhaps in some earlier reg design there really was more of a classic Venturi effect going on.

The Venturi nozzle is the second stage regulator valve and its exit into the regulator body. The air out of the valve is at IP over ambient and flow is determined by how far the valve is open to deliver the air demanded.

The baffle directs airflow. Directing air flow towards the diaphragm will increase pressure under the diaphragm and make it harder to open the regulator. Directing airflow towards the mouthpiece will create a lower pressure under the diaphragm making it easier to open the regulator.

I would venture to say that the baffle on predive is out of the way so airflow flows into the case normally, in dive it directs air flow away from the diaphragm creating a lower pressure under the diaphragm to assist breathing, the exact path determined by the amount of assist required.

 

[Scuba Lawyer]

Ok, this looks like a good place to jump in and ask the following question.

First, I’ve read pretty much every regulator book, reference and manual out there so I’m fairly confident in my abilities to rebuild a number of both piston and diaphragm regs. I’ve rebuilt many for my own use, but never had any formal training course. Anyway, for the first time I have encountered the following FREEFLOW problem in my rebuild of a USD Calypso IV (non J-Valve) reg. In reading this thread I’m thinking I have some sort of venturi issue - but who knows. I’ll ask anyway.

IP is fine (and I have actually experienced the same problem described below at varying IP’s). 2nd stage adjusted normally. 2nd stage lever is not caught on anything and moves freely, always popping back up into place when manually depressing it. HOWEVER, when diving with it I will inhale and on occasion it will get stuck in lever down position and freeflow. When that happens, nothing I can do will stop the freeflow - not pushing on the purge button, not exhaling forcefully, not taking the 2nd stage out of my mouth and moving it up and down in the water column or even banging it on my arm. Just solid freeflow. I’ve adjusted and re-adjusted but still the same thing - and only on this 2nd stage (I tried a different 2nd stage on the 1st and zero problem). Parts all look good. I’m at a loss. Happy to have someone show my ignorance. Mark

 

[Lorenzoid]

The Venturi nozzle is the second stage regulator valve and its exit into the regulator body. The air out of the valve is at IP over ambient and flow is determined by how far the valve is open to deliver the air demanded. . . .

Let me see if I understand. Do you mean the Venturi effect occurs because the opening created by the separation between the piston and the valve seat when the valve is cracked open has a smaller cross-sectional area than the valve body through which the air is arriving, and therefore the air velocity increases as it flows through that opening?

 

[Jcp2]

Ok, this looks like a good place to jump in and ask the following question.

First, I’ve read pretty much every regulator book, reference and manual out there so I’m fairly confident in my abilities to rebuild a number of both piston and diaphragm regs. I’ve rebuilt many for my own use, but never had any formal training course. Anyway, for the first time I have encountered the following FREEFLOW problem in my rebuild of a USD Calypso IV (non J-Valve) reg. In reading this thread I’m thinking I have some sort of venturi issue - but who knows. I’ll ask anyway.

IP is fine (and I have actually experienced the same problem described below at varying IP’s). 2nd stage adjusted normally. 2nd stage lever is not caught on anything and moves freely, always popping back up into place when manually depressing it. HOWEVER, when diving with it I will inhale and on occasion it will get stuck in lever down position and freeflow. When that happens, nothing I can do will stop the freeflow - not pushing on the purge button, not exhaling forcefully, not taking the 2nd stage out of my mouth and moving it up and down in the water column or even banging it on my arm. Just solid freeflow. I’ve adjusted and re-adjusted but still the same thing - and only on this 2nd stage (I tried a different 2nd stage on the 1st and zero problem). Parts all look good. I’m at a loss. Happy to have someone show my ignorance. Mark

What happens if you keep the reg in your mouth but put your hands on either side of the exhaust to entirely close any way for the air to leave? Does the diaphragm pop back out?

 

[JackD342]

The Venturi nozzle is the second stage regulator valve and its exit into the regulator body. The air out of the valve is at IP over ambient and flow is determined by how far the valve is open to deliver the air demanded.

The baffle directs airflow. Directing air flow towards the diaphragm will increase pressure under the diaphragm and make it harder to open the regulator. Directing airflow towards the mouthpiece will create a lower pressure under the diaphragm making it easier to open the regulator.

I would venture to say that the baffle on predive is out of the way so airflow flows into the case normally, in dive it directs air flow away from the diaphragm creating a lower pressure under the diaphragm to assist breathing, the exact path determined by the amount of assist required.

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.

 

[Scuba Lawyer]

What happens if you keep the reg in your mouth but put your hands on either side of the exhaust to entirely close any way for the air to leave? Does the diaphragm pop back out?

Excellent question, and one I do not have an answer for as I was not bright enough to think of doing that.

 

[JackD342]

Ok, this looks like a good place to jump in and ask the following question.

First, I’ve read pretty much every regulator book, reference and manual out there so I’m fairly confident in my abilities to rebuild a number of both piston and diaphragm regs. I’ve rebuilt many for my own use, but never had any formal training course. Anyway, for the first time I have encountered the following FREEFLOW problem in my rebuild of a USD Calypso IV (non J-Valve) reg. In reading this thread I’m thinking I have some sort of venturi issue - but who knows. I’ll ask anyway.

IP is fine (and I have actually experienced the same problem described below at varying IP’s). 2nd stage adjusted normally. 2nd stage lever is not caught on anything and moves freely, always popping back up into place when manually depressing it. HOWEVER, when diving with it I will inhale and on occasion it will get stuck in lever down position and freeflow. When that happens, nothing I can do will stop the freeflow - not pushing on the purge button, not exhaling forcefully, not taking the 2nd stage out of my mouth and moving it up and down in the water column or even banging it on my arm. Just solid freeflow. I’ve adjusted and re-adjusted but still the same thing - and only on this 2nd stage (I tried a different 2nd stage on the 1st and zero problem). Parts all look good. I’m at a loss. Happy to have someone show my ignorance. Mark

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?