Well, three and a half years out, it's been an interesting evolution. I just took a trip to Cayman Brac, and brought along a few of my "big brand" seconds.
Unlike Regulator Geeks III, this wasn't a deep water stress test, but a week of leisurely bimbles along the reefs and walls of the Brac, switching from reg to another for both immediate comparison and lengthy breathing evaluation at depths from 30 to 110 feet. No current, and I was diving nitrox for dives as long as I could, so I didn't do any hyperventilating tests and waste gas. This was just "plain ol' diving".
I brought my D400 (of course!) tuned as light as I could get it (0.6") with the Venturi collar wide open, a C370 tuned to 0.9", a G260 and an Atomic T3 tuned to 1.0", and the D420 tuned to 1.2", with
@James79 's adjuster on top. In other words, everything was tuned light, but adjustable on the fly.
As you might guess with the regs tuned so light, I was repeatedly embarrassed by freeflows on giant stride until I started remembering to crank them down or change the Venturi switch before jumping off the boat. Little problem.
Short answer? They all breathed great! As with Reg Geeks III, any properly tuned modern reg will do just fine at any recreational depth.
That said, there
were differences.
D400: Inhalation was a sheer delight. Between zero inhalation effort and a wide Venturi gate, the D400 gave me anything I wanted. Sipping was easy, and there was no stutter, even at the lightest of efforts. The round shaft of the lever made for smooth action. However, it quickly became obvious that there was a clear exhalation difference with
every other regulator. It wasn't uncomfortable, but quickly became obvious as I switched back and forth. I came away with a subtle feeling that it could be an issue at extreme depth in current, which reinforces the findings in Reg Geeks III. But it was subtle. Something I don't often pay attention to - the reg was a bit noisy on exhalation, presumably due to the bubble pattern.
D420: By comparison, the exhalation difference of the other center balanced design was HUGE! No effort at all, which is a tribute to Scubapro's specific design inclusions: the guiding internal fins for airflow, and the sculpted exhaust tee (shared with the C370 and S620Ti). As with all of the regs, this baby gave me everything I wanted on inhalation and exhalation in the standard diving position, which was as designed. And since standard position was 95% of the trip, it was a pleasant reg to dive. However, when compared with the other regs, if I had to put words to the feel, it would be "slightly coarse." Both D's had only "average" bubble dispersion, which meant some interference if I looked directly ahead with my head vertical in the water.
G260: This is my "go-to" bulletproof reg, so I had high hopes. Breathing was easy and flows were huge. But to be honest, it didn't breathe as smoothly as the 370 or T3. Bubble dispersion was great. Although it was neutral in the water (large internal volume offsetting big case), with head movement it was more obvious I had something I had to hang onto with my teeth in the mouthpiece. To my surprise (offsetting a past prejudice against small-case designs), I ended up preferring the smaller regs.
Atomic T3: This and the C370 ended up being my all-around favorites for the week. Breathing was light and smooth. The reg was very quiet. Interestingly, at the very beginning of the lightest of inhalations, there was occasionally a tiny "step" in flow; a "tic" in the pattern as I would accelerate from a light sip to a full inhalation. I attribute it to a mechanical phenomenon as the lever leg slid against the poppet foot, though I could be wrong. There was no significance to it during diving; it was just something noted in passing. Breathing remained light and smooth at all depths, attributable to Atomic's superb design and execution, and the AFC venturi vane.
C370: This ended up beating out my D400 for overall delight during diving. When tuned below spec, it breathed sooo smoothly! In fact, when I looked straight down, the valve would open and a bubbling freeflow would gently start. As a result, I dove this reg with a half turn on the knob. It wasn't enough to stiffen up inhalation to any meaningful degree, and eliminated the case-geometry-induced slight freeflow caused by tuning below specification. Best of all, on the deepest dives, the Venturi assist of this reg was smooth and effortless. Bubble dispersion was excellent and the reg was quiet.
In summary (this being a D420 thread), that center balanced valve design did just what it was designed for: provide huge flow at low effort with easy exhalation. That said, it lived up to its "Mustang instead of a Ferrari" manufacturer description in the slight coarseness of airflow. It is indeed a muscle car, and well suited for deep and heavy dives. But like every reg on the list except the D400, it stiffens up when you roll onto your back to look under a ledge.
But perhaps the biggest news is my LDS' response when I described my findings:
my dive shop owner said that it's likely that the D420
is going to be discontinued. We chatted about possible reasons, and the ones that seem most likely are a combination of underwhelming sales (perhaps since it's so different from its brethren), and technician unfamiliarity with proper tuning and service. It's SO easy to service compared with both the D400 and barrel-design regs like the G250/S600/T3. But it is different, and requires a familiarity with center balancing that has virtually disappeared from the technician community over the last 20 years.
If true, I'm disappointed, but only a little. I got SO excited when this reg came out in 2020! The possibility of a new D400 was enticing. But the loss of the coaxial diaphragm/exhaust valve made for one real compromise in performance, and there wasn't enough of an improvement to generate a following among service technicians. I can only imagine what it was like at the HQ technical department during the last two years.
That makes Atomic's foray with the TFX very interesting, indeed! I'm sure inhalation will be delightful. I'm pleased at the return to a coaxial diahragm/exhaust, for it will make the TFX an "any position" marvel. Will it be quieter? Will a larger exhaust valve and redesigned tee improve exhalation WOB?? That is the question. If it does, the TFX has the potential to resurrect the best second stage valve ever. But we are still left with the task of training service technicians on the differences between the TFX and barrel designs. Given what has happened to the D420 over the past 3 years, that may be a tall order.
