I dive in cold water all the time where I am and the Mk25/S600 is a very popular setup without any reports of trouble. I took the time and checked and the US Navy's list for Authorized for Navy Use list which has only one current in production Scubapro 1st and 2nd stage regulator set on it. That regulator pair on the list for diving to 190 feet down to 38 F is the Mk25/S600. There are two other pairs from Scubapro on the list and either the first or second stage is not in production, but the Mk17 is not part of them. ...
...The data on the list is current from March 19, 2009. If I have made a mistake, please let me know, I just want to have good information here as this is a purchase I plan to make in a few days myself.
The MK 10 is one of the other SP regs mentioned in the list - a 1980's design that is also approved to 190 ft down to 38 F - and one that is also not bullet proof in cold water unless the SPEC kit is installed adn properly maintained. The Mk 16 S550 is the other SP reg mentioned and it is also approved for use to 190 ft down to 38F even though it is an unsealed diaphragm design. It is correct that the Mk 17 is just a MK 16 with 1. an extra HP port 2. a sealed ambient chamber and 3. a new seat alignment system. So in effect if the Mk 16 makes the grade, the Mk 17 is a slam dunk and its abscence from the list is not related to its cold water performance which is superior to that of both the Mk 16 and Mk 25.
Anyone that doesn't think that Scubapro's Mk25 1st stage can do cold water is unfamiliar with the reg as it currently sits.
Wrong...and very presumptive.
I am intimately familiar with all of the various changes made to the MK 25 since it was introduced including:
1. larger ambient pressure holes (2 separate increases in size)
2. mirror polished piston stem
3. changes to the rubber piston strem bushing (both material changes and addition of a second outer bushing)
4. change in the piston head bushing
5. finned swivel cap
6. redesigned LP port turret
Most of the changes are incorporated in annual service kits so unless a major metal part is involved, older Mk 25's get upgraded to the current TIS standard at annual service. Yet reports of problems, while reduced, persist.
None of the changes individually or in total have made the reg completely bullet proof in cold freshwater below about 40 degrees as good cold water technique is still needed to keep it from freezing.
...actually spent the weekend diving with a buddy that used to be a designer for uwatec and scubapro. i had the 'freezing' discussion with him and the interesting thing here was that they were seeing freezing problems only in fresh water; in salt they did some really severe things to this reg (well beyond anything that's been discussed here) setup with no issues.
the other interesting point to note was that when there were freezing issues it almost always involved the second stage and not the first. i didn't understand the explanation entirely, but apparently it's got something to do with the valving on the second.
anyhow, his explanation and history of what he's done with this setup made me more than comfy to go dive this in high 30 degree sw.
Plastic cased second stages that also lack a metal airbarrel (like all but the earliest S600's) have very poor heat transfer that mlimits their ability to transfer heat from the surrounding water to warm intertnal parts around the orifice, seat and and valve assembly that are cooled by adibatic cooling as intermediate pressure air expands as it drops to ambient pressure. In cold water that can result in the internal parts becoming cooled well below freezing. That in turn can cause ice to form on those parts if water comes in contact with them causing either a freeflow or a failure to deliver gas.
In the case of a freeflow, it usally starts out as a small dribble of air between inhalations but that small continuous flow can add to the cooling load on the first stage and essentially undo even perfect cold water technique that otherwise reduces the cooling load and avoids any sudden large demands for gas with large spikes in cooling. In other words, a slight freeflow in the second stage can over time cause a large freeze flow from the first stage.
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That also explains much of the variation in success with the Mk 25. Users differ in their work load, SAC rates, cold water technique, and in what may happen from dive to dive.
For those who are solid supporters of the Mk 25 in cold water use try the following test. Start with a high pressure tank (3442 psi). Take it deep (below 100 ft in 35 degree fresh water, do a fairly hard swim for 5 minutes then let the reg fall from your mouth (a plausible scenario on a cold water dive). I will be extremely surprised if the reg does not freeze flow on you before you can recover the second stage. When you take the cooling that occurs with high flow rates over time, then introduce a bit of water into the freeflowing second stage, the combination will be more than enough to push the first stage into a freeze flow.
That is a tough test for any reg, but a Mk 17 G250V has a much better chance of passing it than a Mk 25 S600 as the Mk 17 has advantages in terms of both heat transfer and a fully sealed design and the G250V has a well designed metal air barrel with extensive cooling area on both the adjustment knob and the inlet fitting and nut on the ends of the barrel.
I used to dive the Mk 25 in cold water (35-39 degrees in freshwater at depths down to 150 ft) and I personally never had a freeze flow, but I understood the limitations of the reg and I knew a good deal when I saw it when the Mk 17 came along. I'd rather not worry about a freeze flow at depth in an overhead environment or with a deco obligation so to me the choice is a no-brainer.
If if you regard the Mk 25 as "good enough" the Mk 17 is simply better in extremely cold water.
