Balanced Second Stage and Venturi Effect

[youngin9494]

Hey everyone. I'm working on a research paper about regulators and was wondering if anyone knew about balanced second stages and the venturi effect. If a second stage is balanced does that also mean that the regulator will utilize the venturi effect? Or are there unbalanced second stages that use the venturi effect to ease breathing? I'm just having trouble finding out if the venturi effect is due to the "balance" of the second stage or if it just depends on the jet, or valve I'm not sure of the terminology, that the air flows out of. Any info is greatly appreciated, and any resources to point me in the right direction are even better.

 

[buddhasummer]

Have a look at these books:

Scuba Tools - Regulator Savvy Book

and

SCUBA REGULATOR MAINTENANCE AND REPAIR by Vance Harlow

 

[youngin9494]

Thanks for the recommendations, I think I'm going to pick up Regulator Savvy. Hopefully it'll get here before too long.

 

[halocline]

Reg Savvy will have a lot of theoretical information about how regulators work. To briefly answer your question, there is no direct relationship between balanced 2nd stages and venturi assist. "Balanced" means that the pneumatic forces on the valve in the 2nd stage don't change much with changes in supply pressure to the stage. I'm not sure how much you know, but the first stage sends air under pressure to the 2nd stage; that pressure is called intermediate pressure or IP. In almost all 2nd stages, the IP is pushing the 2nd stage valve open, which means you need something pushing back to keep the valve closed until you take a breath. Unbalanced 2nd stages use a mechanical spring to counter the force of the IP air. Balanced 2nds route some of that air into a chamber, which then allows it to push back against the air pushing from the 1st stage. This is helpful if the IP changes as the tank empties, for example. It's a little more complicated than this, but not much, really.

Venturi assist is present in essentially every 2nd stage to some degree or another, and it's adjustable in some 2nd stages. Basically it's caused by air moving quickly through the 2nd stage in a linear fashion; this creates a slight drop in pressure in the surrounding area, which then helps to keep the valve open and air flowing. It's the reason that many 2nd stages will keep flowing if you press the purge. Since all 2nd stages move air through a small valve relatively quickly, they all create some sort of venturi effect.

You might find the first chapter in the Vance Harlow book helpful too; it's a good explanation of the basic air forces at work in the common types of 1st and 2nd stages. You would also benefit from looking at some of the classic regulators with a mentor and take them apart to see how they work. It's amazing how simple some of the great scuba regulators are. If you can get your hands on a MK5 with a 109 2nd stage and go through the rebuilding process with an experienced tech, that would be very helpful, and enjoyable.

 

[youngin9494]

Halocline thanks for the summary. I'm familiar the design of balanced vs unbalanced second stages just couldn't figure out if there was a correlation between second stages that were balanced and those that had the option to adjust venturi effect. I'll see if I can find a copy of the Vance Harlow book to glance over. I was planning on heading down to my LDS later this week to see if they were doing any maintenance.

Thanks again for the responses.

 

[jwelburn]

The mechanism that balances a second stage and the design that takes advantage of the venturi effect are both independent of one another. It would be entirely possible to design a balanced second stage that did not take advantage of the venturi effect. (I am not aware of a balanced second stage that does not use it, but it is certainly possible) There are lots of unbalanced second stages that take advantage of the venturi effect. On many of them you will find a dive/pre-dive switch. However the lack of this switch does not necessarily indicate that the venturi effect is not used to improve inhalation effort, it only means that the flow path of the gas leaving the reg can't be interrupted to "kill" the venturi effect as would be desirable for preventing free-flows as the diver jumps off a boat or during a surface swim.

Bottom line: They are entirely independent features in a second stage.


Did that answer your more specific question?

 

[ams511]

If you want to expand your paper into double-house regulators, I believe the Aqua-Lung Mistral (original version) and its predecessors used the venturi effect. Besides the references given, Vintage Double Hose does have a library of information on older regulators.

 

[youngin9494]

Yea jwelburn that is exactly what I was looking for. What you mentioned about all the balanced second stages that take advantage of the venturi effect is what was tripping me up, but I now understand that they are independent of one another.

If you want to expand your paper into double-house regulators, I believe the Aqua-Lung Mistral (original version) and its predecessors used the venturi effect. Besides the references given, Vintage Double Hose does have a library of information on older regulators.

I haven't looked much into double-hose regs yet. The only pros of them that I currently know of is no bubbles in field of vision and can be better in cold water. Although from my understanding second stage freeze ups are fairly uncommon. And I thought most of double hose regs have been phased out. But they'll definitely be something that I look more into thanks for the tip. That website looks promising, especially the old manuals.

 

[herman]

I haven't looked much into double-hose regs yet. The only pros of them that I currently know of is no bubbles in field of vision and can be better in cold water. Although from my understanding second stage freeze ups are fairly uncommon. And I thought most of double hose regs have been phased out. But they'll definitely be something that I look more into thanks for the tip. That website looks promising, especially the old manuals.

While DH regs were dropped by most manufactures in the late 60s, production cost and regulator trends (fashion if you will) were a large part of their demise. Besides the benefits you state, many of us who dive a DH prefer the way they feel, it's a more natural feeling with a slower more relaxed breathing cycle. They do dive differently and require the diver to be aware of his equipment, something some divers do not like. In the last few years there has been some growing interest in the regs. Newly manufactured and much better replacement parts (hoses, mouthpieces and internal rubber parts) have came on the market and recently, upgraded intenal parts have become available that vastly improved their performance. The "new" AL Mistral introduced a few years ago is an excellent example of why venturies are improtant in a second stage...it did not include one and is one of the major reasons the reg was not liked by the DH community. It is interesting to note, Gagnan spent a lot time developing the ventrui in the Mistral only to have AL leave it out of the later design.