Regulator Performance With Increasing Depth

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Where I get confused is that the opening of the valve in a balanced 2nd is still opposed by the balancing force of the air passing into the balance chamber; this balancing force is adding to the force of the "lighter" spring, taking away some of that advantage.... but maybe I'm missing part of the picture.

Maybe some hypothetical numbers will help illustrate. Lets assume for the sake of discussion that you have two identical regs, except one is balanced. Lets assume that they have identical IP at 120PSI. We'll also assume that the surface area on the end of the poppet in the 2nd stage balance chamber is 2/3 of that of the area of the seat inside the orifice. We'll also assume that the seat area is 1 square inch. (It's actually far less, but the proportions are the same) Okay, so the seat is being pushed off the orifice with a force of 120 lbs. This means that the spring in the unbalanced reg is pushing back with a bit more than 120 lbs. In the balanced reg, the spring is only pushing back with a little more than 40 lbs, and the balance chamber is providing 80lbs. (2/3 of 120, due to the smaller area).

So, take a breath, and lets say the IP drops to 100PSI. (Again, wrong numbers, but the principle is the same). Now with the unbalanced spring you still have a 120+ lb force with the spring, but in the balanced you have 40+ with the spring and 67 with the balance chamber, for a total of 107 lbs +.

Now, this does not take into consideration the increased force of the springs as they're compressed a little, but we'll assume that this effect is equal in both springs, essentially taking it out of the comparison. In reality, it's probably not, but my guess (and its a guess) is that the heavier spring will experience a larger increase in force as it's compressed.

I agree that in the real world, otherwise identical balanced and unbalanced 2nd stages breathe very similarly, with the balanced poppet being a little smoother. It's easy to test this by buying two SP 109s (or balanced/adjustable), rebuild one with the current unbalanced poppet (G200, duro) and the other with the balanced poppet (G250, S600) and spring and balance chamber. Awap, who has several and is very familiar with them, claims he can't really tell a difference, and that's an illustration of how subtle the effect can be, as long as the first stage provides a stable IP across the supply range, and has a small IP drop during inhalation. My MK5s drop about 5-7 PSI during a normal breath, which is a very small drop.
 
While we are talking about it, I hope it does not take us off course if we also address how the Kirby Morgan 2nd performs so well. I had a Schematic of the K-M but can't find it right now. As I recall, it has a mechanism similar to what Oceanic uses in the newer Delta 2nds, which Oceanic refers to as mechanically balanced. I have never understood exactly how this functions but I suspect it has a chamber of surface pressure gas that lowers cracking pressure as ambient pressure increases with depth.

Anybody know?
 
For both air and mechanically balanced 2nds, the main spring tension is greater than the secondary spring (and greater than balance chamber for air balanced).

Lets say for example an unbalanced 1st with balanced 2nd set up that drops IP from 140psi to 120psi.

At 3000psi tank. Orifice side (140psi) = Main spring (100psi) + secondary spring (40psi)
At 500psi tank. Orifice side (120psi) = Main spring (100psi) + secondary spring (20psi)

The secondary spring acts just like the balanced chamber and its tension/force decreases as the IP drops. Action and reaction are equal but opposite.
 
For both air and mechanically balanced 2nds, the main spring tension is greater than the secondary spring (and greater than balance chamber for air balanced).

Lets say for example an unbalanced 1st with balanced 2nd set up that drops IP from 140psi to 120psi.

At 3000psi tank. Orifice side (140psi) = Main spring (100psi) + secondary spring (40psi)
At 500psi tank. Orifice side (120psi) = Main spring (100psi) + secondary spring (20psi)

The secondary spring acts just like the balanced chamber and its tension/force decreases as the IP drops. Action and reaction are equal but opposite.

Not so with Scubapro's coaxial valve (Air1 and D-series). With this design, the "air spring" is the primary spring with a relatively low force being provided by the mechanical spring, just enough to maintaihn the downstream override.
 
Thanks everyone!

I am at best a "shade tree mechanic", so as we begin to move further into the physics and engineering behind how regs function, my head begins to hurt a bit :rofl3:

But it is interesting stuff and I do think very worth the effort to try to get as solid an understanding as my feeble mind will allow :)

Best wishes.
 
Darn good post for a shade tree mechanic.
Anyways I have nothing to add other than

Overbalanced is complete malarkey, a myth purveyed by manufacturers and kept alive by professor heads that with no thought follow the well it must be true path syndrome and spread the falsehoods as gospel.

The only thing that will make any difference when your'e deep and your chest is in a vice is body attitude in regards to your reg second, or demand valve as twin hose users know very well..

Some experimentation Akimbo with different types of English for the context interpretation challenged may be reqired.
 
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…1. It's a downstream valve, not upstream…

Oops, that was a typo in my part. I stand corrected. I have been working with some downstream demand exhaust valves lately and my typing was on auto-pilot.

Correction:
More correctly, it is a partially pneumatically balanced downstream second stage demand valve.

For clarity of others who may read this, a downstream demand valve is essentially a relief valve that is forced open or overridden by a lever actuated by the diaphragm. A fully pneumatically balanced valve shares the same failure mode as an upstream or pressure-seated valve in a Scuba regulator — IE the 300 PSI rated second stage hose bursts on a first stage regulator failure. Excessive IP (Intermediate Pressure) causes a downstream Scuba second stage to function like a relief valve regardless of diaphragm force.​
…The diaphragm pushes against the lever, which then pulls the poppet off the seat. This is the same whether it's balanced or unbalanced. In a balanced barrel poppet design, the lever is working against the spring and the air pressure in the balance chamber. There's no less force on the lever (and therefore diaphragm) in a balanced reg, except for the drop in IP in the balance chamber which I noted in my post. Again, there are both large and small balanced and unbalanced 2nd stages; balancing has nothing to do with the size of the diaphragm. You can dispute this all you want, but it's a plain fact. Just in scubapro, you have the R190 and G250V, both larger diaphragms, and the R380 and S600, both smaller, unbalanced and balanced respectively…

I concur with this statement:
Scuba Regulator Savvy by Peter Wolfinger, Chapter 8, How Regulators Work, Balanced Downstream Valves, page 72, bottom paragraph:
…This design dramatically reduces the lever depression force and work of breathing. This reduction force is directly due to the fact that the lever is working against the greatly reduced spring force...

Note that the reference to "work of breathing" in is the context of all other factors remaining the same. Reducing inhalation resistance was the motivation to introduce balanced second stages originally. Since then, other refinements have made the performance of unbalanced designs competitive except for compactness.

Sketch a simple vector analysis illustrating forces and I believe you will understand the concept better. It is not like the diving industry invented pneumatically balanced regulators. The design has been around industrially longer than me and is well understood. The primary purpose of this feature is reduction of sensor area — especially critical in high purity deflecting metal diaphragm applications.

Yes, a partially balanced piston does respond to changes in inlet pressure (IP in this case) compared to the relatively linear-force of short-stroke springs. However, that is irrelevant since the diaphragm area and associated linkage are dictated by cracking forces and stoke for flow capacity.

I didn’t want to get too far in the weeds, but many other factors that influence diaphragm size. Leverage, diaphragm travel, stiction and related system hysteresis, acceptable operating ranges, reliability assumptions, material costs, and manufacturability to name a few. I did not write that balanced valves must have a smaller diaphragm, only that they could. All things being equal, a larger diaphragm will decrease sensitivity to maintenance and manufacturing variations by virtue of the brute force available. Like most engineering tasks, regulator design is a game of compromise and operating assumptions.

… 3. The "best performing" 2nd stage is a subjective statement…
I don’t consider in-situ Ansti breathing machine tests at the equivalent depth of 1,600' subjective. I did not intend to imply that a balanced design could not perform as well or better. Only that this unbalanced regulator is the only one that has been tested and information publicly shared that I am aware of. Commercial diving applications tend to prefer the simplest and lowest maintenance solutions available for consistent long-term performance.

… 4. This one is really bizarre, although I guess you're just confusing pressurized with non-pressurized. An unbalanced reg uses a much heavier spring; therefore when the reg is not pressurized, there's more force against the seat, and consequently more wear in storage, all other things equal, than a balanced reg...

I think I see the miscommunication.
…Item 1:
The force to keep the second stage demand valve closed is essentially the same with a balanced or unbalanced second stage. This statement is true when the regulator is not pressurized, which is the vast majority of the time, but not while it is in operation...

By "This statement" I was referring to your original statement under Item 1, not my preceding sentence. I believe we are in agreement here. However the main point I tried to make is still valid. The argument is theoretically correct, but has little practical value.

It was common practice to store mixed gas diving helmets and masks with their Dial-a-Breaths fully backed off based on this logic. Note that a Dial-a-Breath has a far greater adjustment range than a Scuba regulator's spring force adjustment — like 14-18 turns to compensate for an 80-250 PSI over-bottom inlet pressure. After noticing that it was not a universal practice with all companies, we looked at repair records. Nobody can say conclusively without a controlled study, but the repair logs and spares consumption indicated no perceptible difference. This is a testament to seat materials being better than originally assumed.
 
While we are talking about it, I hope it does not take us off course if we also address how the Kirby Morgan 2nd performs so well. I had a Schematic of the K-M but can't find it right now. As I recall, it has a mechanism similar to what Oceanic uses in the newer Delta 2nds, which Oceanic refers to as mechanically balanced. I have never understood exactly how this functions but I suspect it has a chamber of surface pressure gas that lowers cracking pressure as ambient pressure increases with depth.

Anybody know?

The KM second stage is a fully adjustable unballanced regulator with a secondary spring that gets compressed and thus stiffened when the adjustment knob is screwed in. As the diver goes deeper the adjustment knob can be screwed out to release spring pressure off of the valve poppet and thus reduce the cracking pressure.

I believe the KM Superflow regulator also has a larger valve I.D. which allows a higher volume of gas to flow than your typical recreational scuba regulator because it was designed for a working diver who needs the flow as opposed to a diver that is just crusing along enjoying the sights.

Schematic: http://www.kirbymorgan.com/PDF/Blowaparts/scuba_regulators_blowapart.pdf
 
The KM second stage is a fully adjustable unballanced regulator with a secondary spring that gets compressed and thus stiffened when the adjustment knob is screwed in. As the diver goes deeper the adjustment knob can be screwed out to release spring pressure off of the valve poppet and thus reduce the cracking pressure.

I believe the KM Superflow regulator also has a larger valve I.D. which allows a higher volume of gas to flow than your typical recreational scuba regulator because it was designed for a working diver who needs the flow as opposed to a diver that is just crusing along enjoying the sights.

Schematic: http://www.kirbymorgan.com/PDF/Blowaparts/scuba_regulators_blowapart.pdf

Thanks for the schematic.

It is a classic downstream design which appears to use an o-ring as the LP seat. Interesting. And then it has the additional spring with mechanical adjuster similar to the newer Oceanic Deltas. The older Deltas had the 2 springs with one spring adjusting pressure on the downstream end of the poppet. The newer Deltas have a contraption that looks something like the KM adjustment tube but perhaps a bit more complex. I don't see as many o-rings in this KM contraption, but I do see something called a piston that makes me think there may be a chamber inside that adjustment tube. I would like to better understand what is going on inside there. It is clearly more parts than you need to simply apply more spring force to the end of the poppet.

Edit: After looking at the schematic a bit more, that o-ring must be the seal between the inlet nipple (tube) and the case. So, an LP seat must be integral to the inlet valve (poppet).
 
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For both air and mechanically balanced 2nds, the main spring tension is greater than the secondary spring (and greater than balance chamber for air balanced).

There is only one spring in most second stages. What springs are you talking about?
 

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