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I used Mk20 and MK15 with D400 in down to 42F, no issues on my stage bottle. Would it work in 38F-42F ? likely yes would it be my primary choice for back gas in those conditions ? not the unsealed MK20. I would use something like MK17 for those temperatures. One less thing to worry about during cold dives, which might present other potential issues. There are too many variables to take care of you might as well want to take one out of the equation (or at least minimize the risk). With non sealed pistons you need to make sure they are not overcooled before the dive (if diving at subfreezing surface temps) not having your buddy on the same reg if you need to bail them out, and not using HP tanks as they will increase a chance of freezing the reg.
The D400 is an ecellent second stage. I would keep it also for cold water.
I would spend 100 bucks for swapping the MK20 for an environmentally sealed MK17.
And use a G250V or a G260 as safe/secondary.
I’ve dived my MK20 down to 42 degrees salt water, no problems. 38 to 40 is pretty low but I don’t know what the official definition is of “cold water” for unsealed pistons. Does it need to be 32 for fresh and less for salt? Everybody seems to have a different opinion on what cold water is (and that’s all they really are are opinions) but I never see any hard science on what is “officially” considered the hard bottom water temperature for piston regulators to begin to have problems. My brain tells me that the water has to be damn near freezing for that to happen.
I understand that unsealed pistons can be used but you shouldn’t breath off them until they are submerged. Makes sense since the ambient chamber will be filled with water that is not frozen keeping the reg a constant temp, whereas if breathed off in the air the reg could drop well below freezing temp freezing up any air moisture or then freezing the water it is submerged into.
The point about heavy airflow and the increased cooling caused by pressure drop sounds like it has merit especially if the temp is right to the max of forming ice.
It would be very interesting to do a test in a very cold water tank just to see how low they can go.