Effects of depth on IP and balanced and unbalanced 2nds?

[84CJ7]

I found this page which is mostly irrelevant stuff (to me) about constant mass flow and rebreathers:
Understanding Constant Mass Flow • ADVANCED DIVER MAGAZINE • By Paul Raymaekers
Which also has this very relevant paragraph:
"When we go diving, the water pressure increases with 1 bar every 10meters we descent, so the IP of the normal scuba regulator will also increase by 1 bar for every 10 meters of depth. With an IP set at surface at 10 bars absolute, when diving at 20 meters depth the IP will have increased by 2 bars till 12 bars absolute, or at 50 m till 15 bars absolute… and so on."

With a balanced second the IP is acting on both sides of the seat in the second, so the increases to IP from ambient pressure effect the 1st and 2nd stage evenly.

So I feel like I am missing something, how does an unbalanced 2nd hold back the increasing IP? Does ambient pressure acting on the back side of that tiny seat make any difference? I would assume not if balanced 2nd's use that same space for other things.

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This part is more DIY forum stuff:
I will have follow up questions as I am tuning a MR22/abyss and am curious about the effects of depth on tuning.
One would think a person with a balanced first and unbalanced second could just crank the IP up to 14 bar to see if the spring pressure (orifice adjustment) is sufficient to hold back what the IP will be at 40 meters (other factors aside) but as with all things I am guessing its not that simple.

EDIT: As an experiment I cranked the IP on my MR22 up to 14 bar, set both the abyss and the protons adjustable orifices so that they had enough spring preload to just hold back the pressure (shaking them lightly made them sneeze) and then set the lever height to just click slightly (abyss is a bit tight but that cover isn't coming off again) and then backed the IP down to 10 bar. It will be interesting to water test them.

 

[beanojones]

You are forgetting that it is ambient pressure inside the second stage at depth (and also inside the ambient sensing chamber in the first stage, so the IP raises as the ambient does). IP is actually (IP over ambient). Subtract out the ambient and you still have the IP which should nominally be the same at any depth.

There are first stages where the IP over ambient does rise with depth (usually called Over balanced first stages, often the dry chamber diaphragm first stages), though the MK15 was supposed to be overbalanced. DRIS did some dives with an actual IP gauge and measured how much they IP over ambient rose to at depth. I know my Micras leak noticeably at over 80 feet when they are on Apeks DST first stages.

Those overbalanced first stages do cause leaks with properly adjusted unbalanced second stage at depth. ANd sometimes all sorts of other weird stuff including becoming easier to breathe with low tank pressure.