Richesb:
The Mk25 is a super reg. If you can afford it buy it.
It will outperform all the others.
I agree, but the 300 SCFM flow rate of the Mk 25 is about twice as much as you will ever need given that the highest performing seocnd stage available only flows 65-70 SCFM. Two divers breathing 70 SCFM from the first stage (not that it will happen as no diver is ever going to use that much - even on a very rapid inhale) would still leave 37 SCFM for inflating etc on the "lower" but still very high performance MK 17 at 177 SCFM. So why exactly does anyone need 300 SCFM?
Richesb:
But you may never notice this performance since the capacity to deliver more air is something that only comes into play deep diving and working at depth.
As in REALLY deep diving. Assuming you have a high average RMV of .8 CFM and you triple that under a high work load (2.4 cfm at 1 ATM) at 900 feet you'd still be using less than 70 cfm and if you are inhaling half the time while working that is still only a flow rate of 140 cfm and at that point you are outbreathing the second stage anyway.
So unless you plan to go deeper than 900 feet or share gas deeper than 450-500 feet under high workload conditions, the Mk 25 is still way overkill. And that is not even taking into account the fact that trimix and heliox slip through regulators 20-30% better than air. Besides, with those kind of depths and gas flows, you better be surface supplied or you are screwed.
Richesb:
Another point you have missed is that any diaphram regulator actually performs better as you dive deeper due to external pressures.
Well...no. All first stages are balanced to account for changes in ambient pressure so all of them, "balanced" or "unbalanced", "piston" or "diaphragm" maintain the same relative difference between ambient and intermediate pressure.
Apeks claims to have "over balanced" diaphragm first stages where supposedly the difference between ambient and intermediate pressure increases with depth. Theoretically it would help maintain flow rate as the viscosity of the thicker air increases, but practically speaking this effect is really small. More importantly it would create tuning issues that would require the second stage to be detuned at shallow depths to prevent it from freflowing at deeper depths. Alternatively you would have to remove most of the downstream bias of the balanced second stage poppet and if you do so it will no longer vent excess pressure in the event of a high pressure seat leak - a definite safety problem.
Generally speaking "balanced" designs, piston or diaphragm have higher flow rates than "unbalanced" designs as they can accomodate larger orifices with no concern for excessive change in IP as tank pressure falls. With an unbalanced first stage (all of which are currently piston designs) an increasingly larger orifice requires an increasingly larger piston head to keep the change in IP as tank pressure falls from exceeding a maximum of about 20-25 psi.
Richesb:
Piston regulators deliver a more even flow over the entire depth range. But they also have more moving parts and require more service when used regularly.
No. Unbalanced piston designs are very simple with very low parts counts and no dynamic o-rings exposed to more than intermediate pressure. It does not get any simpler or more reliable than that.
Generally speaking balanced piston designs are simpler in most cases and at least no more complex in the other case than most balanced dipahragm designs from a parts count perspective - although there is a fair amount of variability between designs. However nearly all balanced piston first stages have only one moving part so from a dynamic perspective they are a lot simpler.
Diaphragm designs are similar in terms of parts count and variability in parts count. However nearly all of them have more than one moving part - a seat carrier, a pin to transfer pressure from the seat carrier to a pad, the pad that presses against the diaphragm and the diaphragm itself which flexes if not actually moves. There are also usually 3 springs as opposed to the 1 spring normally found in a piston design.
In my experience, salt water intrusion, corrosion, rust or other contamination into a first stage will disable a diaphragm first stage long before it would disable a piston first stage.
Also in my experience a good diaphragm design like the Scubapro Mk 17 will display no more lag or drop in IP on inhalation on a test bench than a good piston design like the Mk 25. Diapragm designs have made significant advances in recent years, while piston designs plateaued about 20 years ago.
