No sure about many mentioned here but they have been brought up in cold water threads. It gets down to roughly 35F where we dive a lot after the second thermocline. I got a freeflow from a Scubapro MK25 pretty quick in those conditions (first day using it). Supposedly the MK17 is better or there is a cold water kit for the MK25. Else most shop owners in the MN, WI area will laugh at you if you go for Scubapro (barely anybody sells them). They have the nickname of "Scubaflo" around here. I got rid of my MK25 and bought an Apeks XTX50 and have never looked back.
The sealed diaphragm Mark 17 first stage is not only 'supposedly' better in 35F degree water, it is at least as good as any of the regulators that are mentioned repeatedly. Apex are fine, I'm sure. So are Mk 17s. I would never voluntarily get into water that cold (35F!!) using either because I would never get in to water that cold.
As I wrote, I dived 20+ year old piston as opposed to environmentally sealed diaphragm type 1st stage regulators in very cold (to me) ocean water, slightly above 40F to the mid 50s. This was not balmy (unless the word was used by an earlier poster to mean a state of insanity rather than warmth). I used SP Mk5s and Mk 10s. Naturally, I used silicone in the ports these regs were built with for that purpose, diving in cold water. These are old piston 1st stages, not the best choice for extreme cold water, but they did work flawlessly for me. I still use them. The same ones. I should point out that when I did these dives I did them in old style Parkway Sharkskin two piece off the rack wetsuits that have limitations, to put it mildly. I still have one, but it lives on the wall to remind me of something important, but I've forgotten what that was. All that ice water on the brain. And elsewhere.
The OP need not trouble himself/herself about the superb regulators I used back then because they stopped making these two models around the same time the Soviet Union began to collapse and we all looked forward to the Peace Dividend that would accompany the end of the Cold War. Too much history for you? That's what the swine in out government figured you'd think as they set about sending US troops all over the world to do 'nation building" (changing things to suit us), plunging us into a vortex of terrorism.
Q. "Will the average American notice that almost all of the US directed terrorism and hatred for the US began just at the end of the Cold War?" Ans. "Naw. They're too stupid and ignorant of history and the logic of cause and effect."
But back to regulators. There are two main types of first stages: sealed diaphragm and open piston. I like open piston better. Easier to work on, simpler, deliver more air. Sealed diaphragm first stages do very well in freezing water, but have issues of their own. For one, they tend to be ugly. Don't get one unless you REALLY think you will be spending time in water much colder than 50F.
I don't dive anymore in cold water, so the choice for me is easy. My old SP Mk5s and MK10s. As good as anything out there, better than most. If you don't think you'll be spending much time in water way down at the frosty end of the thermometer, you choices expand exponentially. Stay with strong established manufacturers with good parts availability and solid records. That's why I suggested doing some serious reading with a strong awareness of who may be selling something. Me, I'm a semi-retired university professor with no economic ties to anything other than TIAA CREF and Social Security.
For the record, I do not sell or have any vested interest in any product, manufacturer or shop.
