How does balancing or lack thereof affect the possible size of the diaphragm?
The advantages of a balanced 2nd stage are:
1. Lighter spring pressure, which results in less seat imprinting while in storage.
2. Greater tolerance of IP shifts without affecting cracking effort.
3. Theoretically lower effort required to keep the valve open (IOW inhalation resistance, after the initial cracking effort) due to the drop in IP in the balance chamber.
The 2nd stage diaphragm size has nothing to do the issue of balancing. There are large and small balanced and unbalanced 2nd stages.
More correctly, it is a pneumatically balanced second stage demand valve. Look at the exploded view of a balanced second stage and you will see that a pneumatic piston aided by a mechanical spring provides the total force to close the upstream valve. Because a significant percentage of the total force is counterbalanced by the piston, the additional force required of the spring is less. The diaphragm needs to generate sufficient force to compress the spring only, therefore the surface area of the diaphragm can be reduced.
In an unbalanced second stage, all of the force required to keep the valve shut must be exerted by the spring. Likewise, all of the force required to force the valve open must be generated by the diaphragm. More force = larger diaphragm area. Notice the difference in diaphragm sizes of balanced and unbalanced second stages on the market. I believe you will find a clear delineation.
Balancing was initially used to decrease inhalation resistance without increasing diaphragm size. Assisted by breathing machines, sensitive electronic pressure transducers, and fluid mechanics computer modeling software engineers were able to tune both designs to perform optimally, but determined that the smaller balanced diaphragm was a feature consumers were willing to pay for.
Study performance graphs of regulators made in the last 20 years from Ansti breathing machines. You will find a negligible difference in second stage performance as long as the IP pressure is constant balanced or unbalanced.
Balanced first stages provide a more constant IP pressure at tank pressure below ~400 PSI, but there is not much difference above that. As noted in my first post, the best performing second stage on the market is unbalanced.
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.
In practice, I have some 20+ year old unbalanced second stages that have never required seat replacement and still pass the bubble leak test. I have stored some regulators for long periods with the diaphragm depressed, but have not noticed much difference because the seat life is so long anyway.
Item 2:
All other engineering parameters being equal, this may be true, but they are not. The engineering team determines the required IP range and designs the valve so it will close the valve at the high end of the IP while delivering low enough inhalation resistance at low IP to meet the design objective.
Item 3:
I do not believe this is correct. The force is the same. If your hypothesis was correct, it would show on the inhalation curves on breathing machine print-outs, which for practical purposed does not. Theoretically, the friction of additional o-rings on the balancing piston would decrease response and increase cracking pressure. True as this may be, it is not high enough to have a meaningful effect unless the regulator is in terrible need of maintenance.
