oxyhacker:
SP describes both the MK25 and the Mk14/16/18 as overbalanced. I spent several hours at DEMA trying to find someone at SP who could explain how the Mk25 was overbalanced, and a half a dozen or so SP techs tried and failed to explain. Finally their Euro head of technical services, a very sharp fellow, was able to explain it: the end of the piston that contacts the HP seat is flared ever so slightly (so little that it can still fit though the bushings) so tank pressure exerts some downward force on it, allowing the IP to increase slightly as tank pressure falls in order to keep breathing effor flatter as the tank pressure drops.
A decrease in piston stem diameter (compared to the seat end of the piston) of about .004 is used to compensate for the difference in areas. This is enough to totally balance the piston but is not enough to cause problems with inserting the piston through the HP o-ring and bushings.
Each additional reduction of .001 in piston stem diameter compared to seat diameter would result in an increase in IP of approx 1 psi from 300 to 3000 psi with a 1 sq inch piston head.
I find it a little disconcerting that you talked to half a dozen SP techs who could not explain how that works. SP could maybe stand some improvements in their training program.
Now about the Mk14/16. It's a single, non-sealed diaphragm, so there's no way to build a ratio between the diaphragms into it. So how is it overbalanced? Don't know - no one at DEMA could explain it, and a letter I wrote to SP asking for clarification was not answered!
In a balanced diaphragm reg, balancing the seat carrier is accomplished by ensuring the diameter of the balance chamber and its sealing o-ring is the same as the area of the hard seat.
I have no idea what SP has done with the Mk 14 or the Mk 16/18 but I suppose that changing the diameter of the balance chamber/o-ring in the seat carrier could be used to bring about the same resulting changes in IP as in an "overbalanced" piston reg. Although I really don't see the point of doing that in the first place on a diaphragm reg.
Personally I feel that overbalancing is an interesting technicality, for those who are partial to such things, but more a buzzword than an essential feature, and hardly a feature one should buy a regulator on the basis of.
I couldn't agree more. Ironically a very low tech unbalanced diaphragm first stage would accomplish much the same thing. Given that the IP in a unbalanced diaphragm reg increases as tank pressure decreases, this would create an artificial but generally similar effect to increasing the IP with depth as the IP would increase constantly as the dive progressed and tank pressure decreased. Sort of a poor man's over balancing accomplished in an unbalanced reg.
From the perspective of a tech who likes to tune first and second stages for peak perfromance, I still see the idea of increasing IP at depth through overbalancing the effects of ambient pressure as having a trade off in that care would need to be taken to detune the reg slightly to prevent freeflows at depth where a higher IP (relative to ambient pressure) would occur.
Even a "balanced" second stage still has a degree of downstream override in the poppet/balance chamber design and would be sensitive to any significant increase in IP at depth. So my thought is that if you boost IP enough at depth to substantially increase flow rates in order to compensate for increased viscosity, it would come at the cost of higher cracking efforts at shallower depths.
If the first and second stages have the flow capacity to spare in the first place, why bother? You would just be creating more potential problems to resolve elsewhere in the system and would be uneccesarily complicating the design.
