Sorry, but you're wrong again, on both points. It's really simple; in the case of the unbalanced piston first stage, the supply pressure is pushing on the seat, trying to open it. IP is the force required to keep it closed. IP must overcome both the spring and the supply pressure on the seat. As that supply pressure drops, so does IP. The spring pressure remains the same, so the only variable is the amount of air pressure on the seat, and that's directly related to the pressure in the tank. 3000 PSI, lets say for argument .01 sq" seat size, that's 30lbs of force. With a half tank, 1500 PSI, the force is 15 lbs, and that's actually what the IP drop would be with that size seat. In actuality, the seat size is a little smaller to limit the amount of IP drop, but it does occur linearly, whether or not you think so…
I did not write that there was no change, only that the change is within an acceptable tolerance (operating range).
…Your "drop more or less linearly over the supply range" is technically true of all regulator designs, if you have sensitive enough instruments to find it. However, it is misleading since the drop is within the operating range of the second stage…
I happen have an unbalanced first stage apart on my bench at the moment, which has been on working dives below 1200' for months at a time. The port diameter at the seat is 0.78" in diameter. Therefore, the pneumatic force is 1.434 Lbs at 300 PSI and 14.335 Lbs at 3000 PSI or a delta P of 12.901 PSI. This is well within tolerance of most second stages on the market — typically 135 PSI ± 10 PSI. When this first stage is properly tuned, it delivers 135 PSI ± about 6.5 PSI over that inlet range.
All this might be relevant if the high or low IP within the operating range directly correlated to respiratory work loads, but it doesn't. The change in inhalation work load is numerically negligible over the design IP range and is imperceptible by humans.
The problem is you appear to actually take superficial marketing drivel at face value. The reality is that static IP is a meaningless predictor of dynamic inhalation resistance under a work load. It is just easy to measure static IP with primitive bench instruments, which may be a reason for the misguided fixation.
Dynamic IP is also marginally relevant.
Inhalation resistance through-out the entire respiratory cycle, at depth, and under maximum work load is what counts. Perhaps that is why virtually every major regulator manufacturer in the world has invested a quarter million dollars or more on an
Ansti breathing machine for their engineering department? It is not that marketing hype has no basis in science; it is just simplified to the point, some might say intentionally, that it obfuscates and implies performance benefits that are not justified.
… The bit about the 2nd stage not making a difference in how this IP drop affects breathing is, of course, wrong as well. Balanced 2nds are, by design, less affected by IP drop, (thats the basic reason they're balanced) because the balance chamber contains air at IP. When IP drops, so does the pressure in the balance chamber, which of course lowers the cracking effort...
I did not write that is does not make a theoretical difference. What I wrote is it does not make a practical difference on modern regulators since they can be optimized with many techniques beyond balancing. BTW, the primary practical reason for balancing is peak flow rates, which can indirectly be reflected in IP variations. Secondarily, is performance at the very low range of inlet pressure is far better. However, this seemingly desirable characteristic is potentially dangerous since virtually no warning is provided that cylinder pressure has dropped to the IP.
What matters is how much energy must be expended to breath throughout the operating depth range under maximum work loads, not the mechanics to get there. There is irrefutable evidence that there are dozens of engineering solutions to archive the same goal; as evidenced by years of quantitative data produced by test labs for the EU, the world's Navies, and development engineers — in and outside of the Scuba industry.
The world's navies and commercial diving industry do not choose demand regulator technology without valid reasons. They would be delighted to use a high-production and cheap regulator if it met their requirements. Since you insist on ignoring the most relevant qualitative performance data available, then perhaps you should wonder why they consistently spend 4-8x more than for a top of the line Scuba product — especially when the dominant products are unbalanced.
Regulator design cannot be reduced to simple arithmetic. Decades of computer modeling, fluid dynamics analysis, finite element analysis, prototypes, and testing has produced many different designs that easily exceed the needs of advanced recreational divers. I am not a regulator engineer (thankfully), but I have had the privilege of working on several projects with some very capable ones. My task was to help them appreciate some unique demands of the application, test models, and help analyze the results. I am far from expert, but I don’t entirely lack qualifications or real-world experience.
