you find all kinds of bizarre changes over time to deal with what is a design limitation of balanced piston regs - the balance chamber is open to the water, and the piston MUST SLIDE THROUGH AN O-RING, WITH AMBIENT WATER ON THE OTHER SIDE to seal against the seat.
What happens of course is that when gas expands it cools. So the knife-edge at the seat gets COLD. This is not normally a problem for the gas in the chamber, since its very dry (at least you damn well hope it is!)
HOWEVER, metal conducts heat. Well. So the shaft of the piston gets cold. Directly on the other side of the O-ring, it can be below the freezing point of water. The water on the other side is warmer than the metal, and the heat warms the piston shaft. That works well, so long as the water is of a reasonable temperature, as the thermal mass of the piston is rather small (the knife-edge to the midpoint of the shaft does not mass much.)
The problem is that in cold water the water flow through the reg can be insufficient and the small amount of water right at the O-ring can freeze, binding the piston shaft. If you're not moving in the water (thus lessening the flow through the reg) the problem is more acute, of course. Now the piston cannot move to seal against the seat, and you get ever-increasing pressure in the secondary side of the reg. The result is a freeflow as the poppet on the second stage is overwhelmed by the excessive IP, which of course causes more gas to flow, which makes the piston colder, which makes the problem worse. (!) The only "fix" is to turn off the offending first stage (assuming you have another one to breathe off of!) and allow it to warm up and the ice to melt. The problem is also somewhat worse in fresh water than salt (since salt water freezes at a somewhat lower temperature than fresh)
SP has done various things to try to stop this from happening, including putting nylon-like bushings around the shaft, and even going to a partially-composite piston shaft (less thermal conductivity and mass, and ice supposedly won't "stick" to it), rather than filling the balance chamber with "goop" (silicone grease) and using a trim ring with a VERY SMALL port (so the goop doesn't leak out all over the place in normal use; the mess problem, and trouble cleaning it completely during overhauls, is why BP manufacturers don't like the "fill with goop" method of attacking this problem.)
None of these countermeasures are completely effective. None CAN be completely effective, and claims to the contrary are marketing puff - its the nature of the beast you're fighting here.
First thing to do if you're not going to stuff the reg with silicone grease is lose the cute rubber ring around the balance ports (you'll have to take the reg at least halfway apart to get it off though, unless you don't mind destroying it while removing it) if yours has one (Mk10s typically do, I've seen several MK20s and 25s that do not - SP does learn, if slowly.) Besides restricting the flow of water in the chamber and partially insulating it (you WANT lots of flow and you want MAXIMUM heat conductivity!) it also traps potentially nasty (salt, etc) water under it and against the metal, which is not good either. Its a cosmetic thing only - take it off if its there. While you're at it if you have the other rubber thingy around the turret take that one off too - that one does nothing other than trap water where it doesn't belong.
Second thing is to quit trying to use a tool for purposes that are outside of its reasonable design parameters.
The simple truth is that there is no complete fix for this problem. The best bet is to simply not use these regs with ambient water temps under 40F or so, even with the trim ring removed, as while SP will probably continue to claim they've "fixed" this, the problem is one of an inherent design issue with ALL balanced piston regs, unless you fill the ambient chamber with silicone goop or something similar.
If you want to use these regs for extreme cold-water diving then fill the ambient chamber with goop (to exclude water but provide the incomrpessible fluid for the water to act on for the balance function) and use a trim ring with very small (all you need in that case) holes and deal with the potential mess issues. That is the only true "fix" for this problem.
