Nitrox vs regular service kit - Scubapro Mk2+

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As none of the O-Rings in an MK2+ will see a high ppO2, it matters very little what O-Rings you use. The seat does see a high ppO2, so it is of utmost importance to have a material that can withstand it. I do not know exactly what ScubaPro uses for seat materials…

The yoke and DIN versions both have the retainer o-ring which is exposed to HP, and the HP port o-ring. The DIN version also has the o-ring on the outboard end of the retainer. So two for the yoke version (which nobody would ever use for a deco cylinder) and three for the DIN version. The nice thing about the MK2 design is that none of these o-rings are dynamic, so the mechanical properties are not as important as they would be in say, the HP piston o-ring in a MK25.

I used the SP ‘nitrox’ kit for my MK2+ that I use for 100% O2. Since I’ll only use this reg for deco gas, and the last seat in that reg lasted 20 years, and I’m a certified senior citizen, I doubt I will ever change this new green seat. Hope not! I did use the green o-rings in the kit, but only because they came in the kit. If I do rebuild, I’ll buy higher duro fresh viton o-rings for those spots.

Another nice thing about the MK2 for high FO2 is that the seat is tiny; probably 10% of the size of the MK25. Theoretically that’s less fuel. O2 fires are pretty scary but very rare in scuba and as you mentioned, technique when opening is really important. And just getting the 1st stage and tank valve really clean is important too. Since I don’t dive at home, I’m at the mercy of the shop when it comes to the valve on the deco cylinders, but I have good faith in this particular shop.

 

[Tanks A Lot]

The yoke and DIN versions both have the retainer o-ring which is exposed to HP, and the HP port o-ring. The DIN version also has the o-ring on the outboard end of the retainer. So two for the yoke version (which nobody would ever use for a deco cylinder) and three for the DIN version. [...]

You are absolutely spot on with that, hence why I amended my wrong statement earlier.

I just wanted to clarify what I have written above about the O-Rings on the MK2+ was wrong.

On a yoke MK2+ there is of course one O-Ring exposed to high ppO2, the one on the HP outlet. If it is a DIN version, there are three - The HP outlet and the two on the DIN retainer. Incidentally these are the O-Rings which are made up of FKM in the Nitrox kit. The upper small O-Ring on the piston is also made up of FKM, which surprises me. The lower big one on the piston is the standard NBR.
On paper, ScubaPro does not have a Nitrox compatible yoke version, just the DIN version. That is not really relevant though.

Why that upper O-Ring is made up of FKM is a bit puzzling to me.

I should have added the O-ring of the yoke retainer as well!