Mk 15 freeze up?

[lautmaschine]

Recently I've been diving my Scubapro Mk 15 in water that is in the low 40s. Wondering if I should be concerned about freeze up? No problems so far, but this reg is not environmentally sealed.

I note in other threads that SP used to make a SPEC silicon kit for this reg, but it is no longer available. Is there anything else I could do? Or should do?

Thanks!

 

[DA Aquamaster]

Freeze potential depends on several other factors - LP vs HP tanks, good cold water techniques, etc.

Some older shops still have the Mk 15/Mk 20 SPEC boots in stock. The major problem is that Scubapro moved away from the environmental silicone used to fill them to avoid cross contamination issues when they went to christolube back in the "all nitrox regs have to be O2 clean" days.

You could use Christolube in a SPEC environmental kit, it would just be more expensive. For that matter you can still use silicone, but some SP tech may have a cow either way as it is messy and is no longer standard procedure on SP regs.

 

[lautmaschine]

I use an HP steel most of the time. I'll check with my dealer on the silicone solution. Thanks. I was wondering if anyone has actually had a Mk 15 freeze on them though.

 

[DA Aquamaster]

The Mk 15 is very similar in performance to the Mk 25. In terms of freeze protection though, the Mk 15 is inferior as it was designed for the SPEC system and really has very little going for it compared to the TIS system in the MK 25. Specifically, the Mk 25 has a composite piston head, a bushing designed to inhibit ice formation, a polished stem designed to shed ice and a rubber boot on the stem designed to crack away any ice that may form and it has larger ambient pressure holes to improve water flow and heat transfer.

So a Mk 15 minus the SPEC system will probably freeze up well before a Mk 25 would, and the Mk 25 is alot less than perfect.

On the other hand, with the SPEC boot and either silicone or Christolube in the ambient chamber, the Mk 15 would be far superior in cold water to the TIS equipped Mk 25.

 

[lautmaschine]

DA Aquamaster,
You truly provide amazing information on Scubapro regs!!! Thank you.

I reviewed the Mk 15 Owner's Manual and note the mention of a Thermal Insulating Bushing (T.I.B). Is this different or superior to the Mk 25 bushing?

 

[superstar]

There is a MK15 spec boot on ebay that is listed with a few other parts. This is the auction number.
130187004155

 

[DA Aquamaster]

The first parts of the TIS system started in the Mk 15 but has continually evolved in the Mk 20 and Mk 25. For tyhe most part, later is better with TIS parts. One of the issues with the piston bushing on the Mk 15 and early Mk 20 was that when used without any silicone in the ambient chamber, they tended to trap water under the bushing and cause some rusting of the stainless steel piston head. This is not an issue on the late MK 20 or Mk 25 as they use a composite piston with a thermopastic piston head.

If you get a SPEC boot, get the later wide boot as they hold the silicone much better and allow for some expansion and contraction. The older stiff and narrow (about 3/8") boots are less effective in retaining silicone and require more frequent repacking. Christolube is not very water soluable but it is thinner than environmental silicone so if you use Christolube, you want to be sure to use one of the newer boots.

 

[halocline]

One of the issues with the piston bushing on the Mk 15 and early Mk 20 was that when used without any silicone in the ambient chamber, they tended to trap water under the bushing and cause some rusting of the stainless steel piston head.

This is exactly what happened to mine; when I took it apart recently the ambient chamber was in great shape except for rust under this bushing on the piston head. After cleaning it up, I put it back together with a very thin coating of Christolube between the bushing and piston. Is there anything else I can do to minimize the problem in the future?

 

[DA Aquamaster]

I put enough Christolube under it and around the stem to ensure there is an excess that squeezes out around the edges. This is not one of the "thin coat is best" applications for christolube.

The problem basically is that the MK 15 was designed to have a silicone filled ambient chamber for environmental and freeze protection (small hard to rinse ambient pressure holes is one of the issues) and no real consideration was given to the potential problem when SP changed the rules, banned silicone and removed all the SPEC boots from their earlier regs. Updating the concept to use Christolube (the approach Scubapro escapee engineers at Apollo took) would have made more sense than just leaving the Mk 15 silicone free with no boot.

The older SPEC adapted Mk 5, Mk 10, and MK 3 all had the same problem but the unbushinged chrome plated brass pistons in the Mk 5 and Mk 3 are not really problematic and the Mk 10, since SP had already made a gazillion of them and a huge number were still under warranty, got an updated piston design and a bushing that reduces the problem.

In comparison the Mk 15 was produced for only a short time in comparatively small numbers, was not well loved by techs as it was harder to work on and was pretty much hung out to dry when the design was modified and updated to create the Mk 20.

 

[halocline]

Okay, I took it apart and put some extra Christolube on that side of the piston head and around the base of the stem so that it squeezes out around the bushing. It raised my IP a few PSI, which I didn't expect but made sense once I thought about it. Thanks for the info!

I don't understand why the MK15 would be considered hard to work on; if you have snap ring pliers that reach the c-clip on the HP end, what's hard about it? It's a few more parts than my MK5, but actually easier to service because you don't have that HP o-ring way down in the reg body.