scubatoys:
No argument with you that the R190 breathes better than the R380 or the current numbered R390. They have identical poppet assemblies and internal arrangements but the R190 benefits from a large diaphragm which means more surface area available to generate the same force required to operate the lever. The R380/390 (and the S500/S550/S600 for that matter) have a lot of marketing appeal based on the selling of divers on the idea that small regs are better, lighter in the water etc.
To be honest though they are all good regs. The problems with both the R190 and R380/390 almost always stem from improper tuning as either can be made to breathe very well through proper orifice and lever height adjustment, flow vave adjustment or reversal and through the removal of the plug installed in one of the holes in the valve body itself (put there to comply with a really detrimental CE freeflow requirement.)
Many non adjustable regs perform far below their capability due to design mods and factory adjustment specs designed to meet european norm feeflow criteria that for the most part have nothing to do with real world diving requirements.
But Maxpower, if you are concerned and want to upgrade, skip the S550. Spend $30 more and get a G250 HP. It is fully diver adjustable, shares the same poppet assembly as the S500/S550/S600, is internally identical to the S600 and yet has the same larger diaphragm as the R190 and consequently offers an advantage over both the S550 and S600.
The MK 16 first stage also works very well in cold water. It is a diaphragm reg and while the ambient chamber is not sealed, it is very open allowing a huge amount of heat transfer, is well separated form the colder parts of the reg and uses a teflon coated spring that resists ice buildup. We have not had problems with freezeflows with either the Mk 2 Plus or the Mk 16 while ice diving in water as cold as 34 degrees.
An adjustable second stage like the G250 is also advantageous for cold water diving as it can be cranked down a bit to prevent any umintentional freeflow on the surface or in the early parts of the dive.
