My Scubapro MK 20 & G500 Froze Up For The First Time

[cleung]

I've been diving with my Scubapro MK 20 and G500 which I bought as my original set of gear about 20 years ago. My reg has always performed well over the years and the most recent service was late 2019. Pretty well all of my diving has been in either tropical waters or local Ontario (Canada) where water temperatures averaged about 18 degrees C (or 64 F).

This past weekend, I went on a dive where the water temperatures plunged down to 8 degrees C (46 F) and halfway into the dive, my reg froze up for the very first time. I felt air rushing into my mouth as well as air escaping from my first stage. So I had to abort my dive and surface. Fortunately I was in only 30 feet of water. After awhile on the surface, my reg seemed to work better but I already lost a ton of air.

Two days later my dive group went to another place locally where water temperature was 18 degrees C and my reg worked fine over two dives. So we know the freeze up was definitely due to the 8 degrees C temperature the other day.

In a way, it's a good thing I learned about this limitation of my current Scubapro reg since I won't take this set to a future BC dive trip where water temperatures are usually cold. My local dive shop told me afterwards that I could have turned the breathing mode on my G500 to make it harder to breathe which might prevent some freeze up but no guarantees especially since it's the first stage that is freezing up.

So I would like to hear some comments from those who know regs about my current Scubapro MK 20 & G500 which are serviced regularly and seem to still perform well except maybe in waters below 10 degrees C. Should I simply not dive in conditions colder than 10 degrees with this reg (and rent locally if ever diving BC), continue to dive with it in warmer waters or just upgrade everything since it has been 20 years with this set?

 

[fmerkel]

Wife dives a MK 20 + G250HP for 20 years in Pacific NW water. Has been down to 42*F with no problem. Never frozen up.
A club member did have his MK 25 (2nd unknown to me) freeze up at 80 feet on a 'typical' NW dive, temps probably in the mid-40's. Don't remember the time of year. MK 25's are common around here.

 

[lexvil]

Honestly it should handle that temp just fine, it is one of those things that need to be looked at to make sure all the anti freeze bits are there and in good condition. If it wasn’t in full uncontrolled freeflow adjusting the regulator may have helped.

 

[Angelo Farina]

I would be more worried by the loss of air from first stage. Even if the reg freezes, air should not escape out...
My explanation of what happened is that this was not a freezing event, it was an air loss event.
While loosing air the IP was probably much lower than standard, so you did experience problems breathing form the second stage...
Now, what caused a sudden massive air loss, which self-resolved so that you made two further dives without any further air loss?
This is really very strange...
Only explanation is that the air loss was not inside the reg, but in the O-ring sealing the reg to the tank. This was common years ago, when most regs were still yoke. Now almost every reg is DIN, and this reduced drastically the occurrence of these events.
Did you see exactly where the air was escaping?

 

[rhwestfall]

What IP are you running the first stage at? I am guessing, but think you are at too high of an IP, which exacerbated the adiabatic cooling, causing icing in the first stage orifice (ambient chamber). High IP then was sent to the second stage, with icing at the orifice.

FWIW - We here in the Great Lakes region run IP for cold water diving at the bottom of the manufacturer's specified range, or even lower...

YMMV

 

[rsingler]

The are two possible components to this episode.
The more common is second stage freezing with the valve propped open. As noted above, it is a function of adiabatic cooling as second stage air drops quickly from Intermediate Pressure to ambient in a wet housing. This is addressed by careful breathing technique and not purging at the surface or long test breathing at the surface where damp air can crystallize and start the process before warmer water can begin to melt the ice. The ice then continues to grow until the reg valve sticks open. As noted above, you can reduce the degree of adiabatic cooling by lowering your Intermediate Pressure to the lower limit of mfr spec, or even lower. Some divers will run IP's in the 115 range, instead of 125‐145. A balanced second like your G500, tuned for that IP, will perform just fine.

But your comment about first stage bubbling is another matter. In retrospect, it's perhaps impossible to say where it was leaking, and comments about leaking tank seals have some merit when valve temps drop way below freezing.
But what may have happened is that you have a Mk20 with the smaller ambient holes, and thus less circulation of warmer-than-freezing sea water to offset the adiabatic cooling as pressure goes from tank to Intermediate. This is worsened with heavy breathing. An apple core of ice can quickly impede piston motion to the point that the valve is held open.
When that happens, the second stage (as well as the piston head) is exposed to something approaching tank pressure, or at least well above normal IP, because the valve is held open by the chunk of ice glomming up the main spring.
So at higher pressure, of course the second stage freeflows. But also, the piston head oring designed for Intermediate Pressure is exposed to much more than that, and you may see bubbling out the ambient holes.
Of course in the rush of a freeflow emergency, it's hard to diagnose. But that's one possibility.

I think your Mk20 should be fine for those temps, with a little adjustment.
First, make sure it's been upgraded to the composite piston that Scubapro marketed for the Mk25. Second, lower IP to 125 and consider going even lower. Third, try to accept a slightly higher cracking effort from your second stage, which may help to minimize excessive gas flow from easy triggering. Of course, don't make it harder to breathe than you are comfortable with. Don't test breathe out of the water at the surface - just a tiny check of valve function, preferably with your reg just underwater before submerging.

I'll bet your set will continue to perform just fine at 8°C or below!

 

[rhwestfall]

@rsingler - NICE! Wish I was so articulate...

 

[rsingler]

You said exactly what I said. A lot more concisely.

 

[Couv]

snip...as well as air escaping from my first stage.

This part bothers me. Perhaps I'm misunderstanding where this loss of air came from. @cleung Did you or someone else see or feel air escaping from the first stage?

Per the good doctor's (@rsingler ) example, if the IP did in fact push past the piston head seal, it is very likely that seal "rolled" and maybe returned to the correct position-BUT it is also possible it is still compromised.

 

[cleung]

I would be more worried by the loss of air from first stage. Even if the reg freezes, air should not escape out...
My explanation of what happened is that this was not a freezing event, it was an air loss event.
While loosing air the IP was probably much lower than standard, so you did experience problems breathing form the second stage...
Now, what caused a sudden massive air loss, which self-resolved so that you made two further dives without any further air loss?
This is really very strange...
Only explanation is that the air loss was not inside the reg, but in the O-ring sealing the reg to the tank. This was common years ago, when most regs were still yoke. Now almost every reg is DIN, and this reduced drastically the occurrence of these events.
Did you see exactly where the air was escaping?

There was definitely a big flow of air running into my mouth while I also felt/heard bubbles from behind my head which is where my first stage is near. Also one of the other divers in my group saw air coming out from behind me as well. And yes, it was a yoke system but I most definitely felt the constant rush of air rushing into my mouth. All of this seemed to subside a bit after a bit of time on the surface as I made my way to shore.

I really didn't do anything different on my next two dives 2 days later except it was in 18 degrees C water temp, another location and two other rental tanks. So the water temp was a 10 degrees difference.