I don't visit Scubaboard much any more either. I got really tired of the drama. I mostly log in after someone PM's me for input or when I have a very specific reason for looking for a particular topic in one of the sub forums.
But...having been invited I'll put in a few thoughts on S600 evolution - starting with the G500.
A couple decades or so after God created the Balanced Adjustable and about a decade after he took an air barrel and poppet from the BA to create the G250, Scubapro created the G500, which was the precursor to the S600. The G500 came in two versions - the Italian assembled G500s with the black purge cover and the US assembled version with the gray purge cover. I was advised a long time ago in a tech service class that the Italian version was superior and over the years, on average, I've found that to be the case. Those Italian assembled black purge covered G500s are superb little second stages and if you find one buy it. If you have one, don't 'upgrade' it.
There was a reason why Scubapro moved away from the G250, other than just the quest for smaller size to keep up with competitors who hyped smaller size second stages. On the earlier Balanced Adjustable and G250, as well as the later G250V, their single adjustment design meant that the engineering and manufacturing tolerances had to be spot on. This included the pressure of the spring, which had to be exactly right over a precise working range, to ensure maximum performance (minimum cracking effort), while having the correct lever height (with maximum working range for the valve). That's the case because the only adjustment is in the seating depth of the orifice (which controls both lever height, which in turn impacts working range of the valve) and the initial cracking effort as the depth of the orifice controls the spring pressure acting on the balance chamber. If the spring is too light, the orifice has to be screwed in too far, which lowers the lever and reduces the working range of the valve which reduces the flow rate of the second stage. If the spring is too heavy, the cracking effort ends up being too high, as the orifice can only be backed out so far before the lever contacts the diaphragm and purge cover, initiating a free flow. As a result Scubapro hand picked (and still does) the springs for these second stages, putting the rejects into other dual adjustment models, where being a bit off didn't matter.
The 'benefit' of the G500 an the "improved" G250HP was that these regulators used an integrated balance chamber and adjustment knob that incorporated a micro adjust feature that could increase or decrease pressure on the spring to allow the orifice to be adjusted to produce optimum lever height, while using the micro adjust to precisely adjust the spring pressure on the balance chamber. It worked spectacularly well in the G500. The G250HP on the other hand was a steaming pile of crap with at best mediocre performance.
The only 'problem' with the G500 (and G250HP) was that the balance chamber was open ended once the cap on the end was removed, so air on adjustments required a special tool that was screwed into the end of the balance chamber to make the adjustment. I don't know why that was regarded as being such a big deal, particularly, as once the adjustment was made, provided the same spring was used in the second stage, there was seldom a need to make that adjustment in subsequent annual services.
None the less, Scubapro redesigned the adjustment knob to allow air on adjustment without anything more complicated than a screw driver. Scubapro also decided that 'lighter is better' and developed a plastic air barrel. Thus, with those two "improvements" the S600 was born.
The changes were backward compatible in the event that anyone wanted to update a G500 to an S600. Not that anyone who understood the performance differences would actually want to do that. However I suspect a lot of people did so because it would 'upgrade' their 'old' G500 to a shiny 'new' S600 - and of course Scubapro dealers could make some money on the 'upgrade' if they marked up the required parts.
Now...to be fair any performance decrease with the new balance chamber design was slight, and probably subjective, but it added complexity for the dubious benefit of some very minor ease in servicing the second stage. It was however a predictable change for Scuabpro to make as they have a history of trying to make servicing easier, not so much to be kind to tech, but in recognition that many techs are just a short step above trained monkeys. That might be hyperbole, but it's accurate to say that most regulator techs in general *might* know how to use a torque wrench but only a small percentage really understand the finer points of some of the operating principles in some of the designs.
As an aside, that's a major reason why Scubapro discontinued the excellent D400. (After of course they gutted its performance by making changes to the lever to meet CE free flow standards, and replacing the aspirator with one that used a plastic orifice, since too many monkeys were damaging the non replaceable orifice in the original D300, D350 and D400).
The major downside with the S600, compared to the G500 was the loss of the excellent metal air barrel. I don't think anyone noticed the lighter weight of the plastic air barrel, but nearly all divers noted the cascade effects of the plastic air barrel in terms of increased dry mouth and reduced cold water reliability.
Metal air barrels conduct heat well, so they get cool along their length due to the expanding air drawing heat from the air barrel. That cool surface then causes moisture in the diver's exhaled air to cool and condense on the air barrel, where it is recycled, reducing dry mouth.
At the same time, that metal air barrel, in contact with the metal nut on the inlet fitting and the metal on the hose end itself, all help transfer heat from the water outside the regulator into the air barrel, which helps prevent ice in the air barrel and on the lever, in very cold water conditions (under 40 degrees). This greatly improves cold water reliability.
Over the course of several years technicians also started noting that the plastic air barrels were prone to cracking where they attached to the metal inlet fitting. This was due to dimensional instability in the plastic used, which caused it to shrink slightly over time, which over the dimensionally stable inlet fitting caused cracks to form in the plastic. Most have just one crack, but I've seen a fair number with cracks on all 4 sides.
Scubapro initially blamed a contractor of using the wrong grind (mix of plastics used to make the part). They replaced them for free (and they still do), and after years of dealing with cracked air barrels, they've went back to metal air barrels to 'improve cold water performance'. That's no doubt true, but I suspect Scuabpro is tired of replacing air barrels for free as well. They'll continue to replace plastic air barrels with new plastic ones until current stocks of plastic barrels are exhausted and then they'll replace them with all metal air barrels.
The good news is that I am not aware of any cracked air barrel actually failing, so it's a purely preventative step by Scuabpro, but one that will ultimately do way with the cheap to produce but lesser performing plastic air barrel in the S600.
The other change in the S600's internal evolution was the perceived desire to use a balance chamber that also removed the spring pressure from the balance chamber and poppet when the reg was depressurized. As I recall this was primarily in response to competitors who hyped this capability. This was however one of those ideas that created more complexity and problems than it solved. At a minimum, it leaves the second stage orifice open to dust, insects, moisture, etc, if the reg is poorly stored. Worse, if the reg is rinsed or soaked when de-pressurized it allows water to enter even if the purge is not depressed. Those ills far over shadowed any benefit to seat life that may result from removing the already low spring pressure in this balanced design.
It's an example of why it's usually not a good idea to let the marketing folks tell the engineering department what to do, and it's a 'feature' that won't be missed on the S600.
It's worth noting that all the bits and pieces are interchangeable, and with the option to 'upgrade' a second stage (including the older G500), or with older parts being replaced with newer ones in the repair process over the last 17 years the design has been around, you're going to see a lot of franken S600s. You're also going to see new purge covers on old S600s and maybe even a G500 or two, so you really can't tell exactly what you have until you open it up.
