DA Aquamaster once bubbled...
There are some differneces outlined on the tech sheets from SP for the Mk 20 and 25 but, with the exception of a mirror polished piston stem, they involve plastic parts and pretty minor changes in preparation that should be done to a Mk 20 by the dealer at the next annnual service anyway without having to pay for a Mk 25 upgrade.
Also, based on my experience diving both a Mk 20 and a Mk 25 side by side in very cold conditions (upper 30's to mid 40's), and side by side with other divers with the same regs, I would submit there is really no difference in cold water performance between the Mk 20 or 25. Both do very well on low pressure tanks with someone with a good sac rate and both are more likely to freeflow with HP tanks even with a good sac rate.
SP is also currently recommending the IP be reduced to 125 psi (down from 140-145 psi on most Mk20/25's) and that the second stage be adjusted for a higher cracking effort to reduce the potential for freeze up in cold water.
Bascially this is pretty bogus as the lower IP reduces flow rate and reduces the heat transfer demands on the reg and eases the burden a bit on the ineffective TIS system which, in conjunction with the harder breathing second stage should reduce the potential for a freeze up slightly. But it all comes at the cost of your high performcance reg breathing like a rock.
A 125 psi IP is also hard to achieve in some regs as you often have problems getting the IP that low even with the shims and (on the Mk 25) the external adjustment unless you replace the mainspring with a lighter one.
My solution was to put the Mk 20 and 25 on the saftey tanks and go back to using my SPEC equipped Mk 10's for the back gas. Sad commentary on the last 15 years of progress at Scubapro.
