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 didnt 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 dont 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.
