mk25/s600 or mk17/g250v for Puget sound?

[Torisen]

I'm buying my first set of gear and want to get the best out there, to do this, I'll need your help.

I've seen discussions here and on other boards talking about ScubaPro's mk25/s600 and mk17/g250v reg sets and I'm trying to figure out which would be better for me.

I live in Olympia, WA (an hour south of Seattle) so my wife and I will get most of our dives in the south Puget sound. So far on our dives I've seen bottom temps in the high 40s and low 50s. Currently we have semi-dry suits, but we'll probably move to dry suits (and colder water) after we get more comfortable. We also travel and want something suitable for the tropics.

Cheaper is better, but I have no qualms paying what I need to for the most comfortable, safe, and reliable life support system I can get. I like the look and feel of the mk25/s600 kit better, but haven't used either.

Basically, I want the reg set I'll love to dive with from Naniamo B.C. in the dead of winter to the barrier reef off of Belize in high summer. I'm getting a tech-ish OMS backplate the wing BC and will probably move into caves and wreck penetration both here and in the tropics as I get more experience. My wife is also and R.N. and has her eyes on rescue diving, so I'll probably see some of that too.

So, wise ones, help me find my way?
-Josh

 

[Jorgy]

Both are great regs..............and frankly you would do great with either......

We own a MK25/S500 reg, which my wife dives...........

The MK25/S600 is the one of those regs I see all over the world all the time..........

But for the Puget Sound due to cold water temps, I would pick the sealed MK17 first stage.....

Many on this board have used the MK25 in cold water without issue, but I personally have had freeflows in very very cold water...............

As far as second stages, I have never used the G250V........

Maybe a MK17/S600 combination?

Of course factor in the cost and then the MK17/G250V wins..........

Hope this helps.........M

 

[HatchetJoe]

I can speak for the MK25 and it is a great reg, I have been under the ice and had no problems of any kind. I switched to the G250V as I felt the S600 is lacking, just my opinion. I have no experience with the MK17 so no comment.

I also own Atomic Regs. But since that wasn't the question, I won't get on my soapbox and promote the M1.

 

[Wayfarer2]

I have the Mk25 S600 set and have had it in fresh water as cold as 34 degrees on 3 occasions and had no problems, but I do like the sealed aspect of the Mk17

 

[DA Aquamaster]

I used to ice dive and do deep dives with water temps in the 35-39 degree range (fresh water) with the Mk 25 and never had any issues but I did have buddies with the same reg freeze up on some of the same dives. Consequently I switched to the Mk 17 as it is much more reliable in cold water while the Mk 25 is on the edge below about 45 degrees.

You'll find a few divers who have never had a freeze flow with the Mk 25, but in contrast you will at worst only find a few divers who have ever had a freeze flow with the Mk 17 and in those cases it will be the second stage rather than first stage that froze and / or it would have been caused by excessively moist air passing through the reg.

The G250V has a metal air barrel that provides better heat transfer which makes it much more reliable in cold water than the S600.

 

[Torisen]

Since we're right on the bay here, we'll usually be in salt water, wouldn't the lower freeze point make it almost impossible to experience freezing? Would the titanium mk25T/s600T titanium set be better for avoiding freezing, or is just an expensive gimmick?

Also: is there any comfort disadvantage to the mk17 over the mk25? Wouldn't most of the difference be in the second stage? And wouldn't the metal barrel of the G250v's increased heat transfer make it MORE likely to cause freezing? Don't you want to insulate the cold from the gas expansion from the open water?

The only 2 differences I see on the 1st stages (other than being sealed) is 1 less LP port and no swivel. I can't tell from the pictures, but does anyone know if it keeps the top port (or bottom, depending on how you look at it)? The Scubapro site's comparison feature is horrible, it contradicts itself and has info I know is wrong. It has "Full Titanium body" twice the first for both says no, but the second says yes for both. the mk17 says 4/2 for LP/HP ports but the written description right below that says 5/2 and mentions the swiveling turret. Worse than useless.

I have seen a few folks with nice dual tank tech setups up here with the mk25s on each and that top port and swivel made for some really clean hose routing. Other regs seem to require a hose to go straight towards the tank and make a pretty harsh bend.

Also x2: Is the swivel on the mk25 a potential point of failure to worry about? As my dad taught me about cars, every moving part you add doubles your potential failures.

Thanks for all your help!

 

[nilsdiver]

The cooling and freezing of a regulator comes from the temperature drop in going from the high pressure to the intermediate pressures and then again in going to the ambient pressure you breath. The pressure change the regulators performs causes the cooling. The regulators rely on the water temperature to warm them and prevent the freezing. So if the water is not warm enough to counter the cooling of the gas expansion you get a frozen regulator.

There fore you do not want the regulator insulated, and the g250v metal barrel becomes a feature.

 

[Torisen]

OK, so boyle's law, P(ressure) = T(emp)/V(olume)

I get that, so is the problem water freezing against the regs because the regs are cold from gas expansion, or is the problem with moist air coming out of your tanks and that moisture freezing inside your regs?

And if you're relying on the water to warm the regs enough to counteract the cooling from the expanding gas, wouldn't you always freeze if the water was 32F or close enough to not warm your reg?

And isn't your air supposed to be dehumidified to prevent corrosion in a steel tank? If that was done properly, wouldn't internal freezing not be an issue? Could that be the reason so many people with mk25/s600 setups have never had problems? Their air is dry = no moisture to freeze internally.

Or am I not understanding the problem?

I love diving, but I just like to understand as much as possible before I trust my life to my gear.

thanks again for all of your input!

 

[beester]

DA Aquamaster will be able to explain everything much better then most here... but I'll give a shot.

The freezing risk (free flow )comes from water particles freezing, forming crystals and blocking or opening valves. This freezing is caused by adiabatic cooling. Gas expansion causes a cooling effect just like gas compression causes a warming effect (just touch your tank when it's filled).

Your 1st stage will decrease pressure from 3500psi (230 Bar) to on average 150 psi (10 bar) + surrounding pressure (absolute pressure). This means that in your 1st stage you get a very very strong cooling effect (we are talking -40 F and more). Even tho the water is very very cold (36-40 F) it's still alot warmer then the air in your 1st stage. The water surrounding your 1st stage (and also 2nd stage) will in effect warm your regulator again.

In this situation more metal on both 1st and 2nd stage means more chance for the water to warm your regulator. This also goes for your hoses. If you 'protect' your hoses with hose protectors.. (plastic or rubber) they will isolate them from the warming effect of the water.

Next to that there are a few points:

- The more flow you demand of your regulator, or the more flow it gives, the more adiabatic cooling will happen. So if you are putting your BCD inflator, dry suit inflator, 1 stage on 1 1st stage and are at the same time breathing and putting air in your bcd or drysuit you are increasing the flow and the cooling effect. Not good. Most people will put different demands on separate 1st stages (single tank with Y valve or doubles)
- Piston regulators (MK25) are in their nature more suspect, since they are open to the water and they are very high performing (flow). Both these things are sources for freezing. Membrane regulators (especially sealed membranes) are much less suspect (MK17). With 2nd stages, the more metal the better. With cold water diving a G250v is better than an S600.
- Don't talk only about equipment. Your procedure for cold water diving is also important: Make sure the air you fill is dry, make sure not to test your regulator out of water in winter (no warming effect from water), make sure never to get water in your regulator 1st stage, separate demand by using 2 1st stages, this will decrease flow per 1st stage.

Since you are talking about Scubapro, if diving cold water take a MK17/G250V combo. The flow it can give is less than the MK25 but more than sufficient for all uses (both recreational and tech) and it is a much better cold water regulator.

 

[Mitten Diver]

I dive an MK25/S600 here in the Great Lakes aka cold water without any free flows. I dive a single tank with a drysuit so all air sources off one MK25. I would recommend two things, first don't let the LDS tune your reg on the edge of free flowing. You will get more than enough air at any depth, I love this reg. Second remove all of the rubber strain relief crap at the regulator it allows the water to assist in warming the air. Beester is correct that this will help.