A balanced second stage still has to have some downstream bias to act as an overpressure relief valve in the event of a first stage leak. I have noticed that most balanced second stages I have worked with when adjusted for minimum inhalation effort at an IP of 140-145 psi will begin to freeflow when the IP reaches about 175-180 psi - so you have about 30-40 psi to play with in a second stage using a balanced poppet.
At 230 ft and you would have an increase in ambient pressure of 7 atmospheres compared to surface pressure of 1 atmosphere, or about 100 psi greater pressure. Assuming you have an IP of 140 psi at the surface (155 absolute), it would now be 240 psi (255 psi absolute), but still 140 psi over the ambient pressure.
If the reg were overbalanced so that it gained 5 psi of IP for each atmosphere, it would have an internal pressure of 275 psi (290 psi absolute) at 230 ft - about a 13% increase. That would be the most you could push the overbalancing without reducing the down stream bias of the second stage if you wanted a reg that could go to 230-250 ft.
But a 13% increase in pressure is not going to make much difference in the mass flow of the reg as it is basically a linear equation. And if the reg really needs what amounts to "supercharging" to push enough gas through the internal passages, you need a better reg in the first place. Most engine designers would not even bother with a supercharger or turbocharger boost of only 13%.
It also causes more problems than it resolves, as you now have a reg that will demand a degree of adjustment as you descend and ascend as the IP is constantly changing realtive to ambient pressure. That makes no sense and I suspect that no designer would intentionally do that to a scuba reg.
So...I have two thoughts on "overbalancing" and both are a bit cynical:
1. It is just marketing hype to sound cool and sells regs with no real impact on IP; or
2. It is an unwanted/unintended side effect of dry sealing the first stage. If the seal and pad do not perfectly transfer the ambient pressure inside the reg to the seat, you will get less than perfect IP compensation with depth. Rather than saying "our reg's depth compensation sucks" we turn it over to the marketing folks who call it a good thing and claim the reg is "overbalanced".
I suspect number 2 has everything to do with it for Oceanic and Apex.
But it gets confusing. Scubapro defines "overbalancing" differently. They used a straight piston stem on the Mk 5, 10, 15 and early Mk 20. What this meant is that the outside diameter of the piston stem at the seating edge was exactly the same as the outside diameter of the piston stem where it passed through the HP o-ring. But the seating edge itself took up a small but still significant bit of the diameter meaning the area at the end of the piston that was acted on by downstream pressure from the tank was slightly less than the area where the stem passed through the HP o-ring. This meant the "balanced" Mk 5, 10, etc, were not perfectly balanced. On average this meant a drop in IP of about 4 to 5 psi as tank pressure fell from of 3300-300 psi. Still not bad, and well within the tolerance of a balanced poppet, but still not perfect.
So Scubapro decided to expand the diameter of the seating end of the piston slightly to increase its diameter so that the diamter of the inside of the seating edge was equal to the diameter of outside of the stem where it passed through the HP o-ring. The result was an IP that did not vary at all and Scubapro called it "overbalancing". The big difference here, beyond the obvious, is that the effect is intentional rather than an artifact of and engineering deficiency.
All that said, I'd love to hear what Oceanic has to say and what the change in Ip with depth happens to be. In either case, whether intentional or not, I suspect the change is slight - perhaps 5 psi per hundred feet.
you feed us with the best info we can get and know how to help a diver in need.:thumbs_up:
