Well, 80 cu ft later, I have some impressions of the new D420!
If you'll forgive the shameless plug for my LDS, I spent 90 minutes hovering, on my head, on my back, doing slow loops, purging and hyperventilating from four Scubapro regs. The A700, the G260, the D400 and the new D420 were all attached to the Mk25 EVO. Each reg was tuned to 0.8" cracking effort initially, with subsequent changes as described below. Converting the D420 to left side hose input
literally took 90 seconds with a 6mm hex key and a brass flat to lift off the plastic cover.
Hovering vertical, looking straight ahead, breathing quietly, each reg was virtually indistinguishable on both inhalation and exhalation, with all regs flowing bubbles in front of the mask. A tiny perceived advantage to the D420 on exhalation was assumed to be my bias.
Rolling onto my back, the old D400 quickly showed the one undeniable benefit of a coaxial diaphragm/exhaust valve: there was minimal increase in cracking effort, while each of the other three stiffened up noticeably. The A700, despite its smaller case, seemed stiffest, with the G260 and D420 tied ahead of the A700.
A slow forward loop from horizontal to head down to looking up on my back caused all four regs to breathe slightly wet, with the D400 again being dryest, followed by the G260, A700 and finally the D420 wettest.
In the worst "case geometry" position (hovering about 30 degrees head down, looking at the bottom, the D400 was unchanged, while all three others wanted to deliver air, with the greatest (still mild) chipmunk cheeks coming from the D420. Tuned to 0.8", it was clear that all three regs had exceeded their cracking effort in this position, and slight bubbling from the D420 was audible.
In the position above, where the exhaust valve was highest, it became clear that there was indeed an exhalation advantage to the D420, though the difference in effort was very subtle.
I surfaced and retuned the D420 to 1.1", added a half turn to the knobs of the G260 and A700, and repeated all tests.
Once again, on my back, all three regs other than the D400 stiffened up. The increased cracking effort added to the D420 was not noticeable, and the A700 remained stiffest.
Interestingly, the slow loop after increasing D420 cracking effort to 1.1" significantly decreased wet breathing from that reg. I presume that since the exhaust valve was fluttering less from slight freeflow during the loop, that the case remained dryer. However, as before, all four regs were just slightly wet. Purging while on my back dried all but the D400 (whose exhaust valve was highest in this position). At no time was water accumulation more than a minor inconvenience at the slightly higher cracking effort.
In the standard diving position, the D420 now demonstrated a clear advantage over all three other regs. The D400 was worst with a (very light) 0.6" effort, while the other three were nearly effortless. As discussed in video #2, I believe this is due to case geometry "fault", where the decrease in internal case pressure due to the position of the top of the exhaust valve
preloaded three of the valves, reducing their perceived cracking effort from 1.1" to something closer to zero. The effort required to inhale from the D420 appeared to be the least.
Vigorous high-flow inhalation again appeared to slightly favor the D420, perhaps due to the shorter flow path, though there may be observer bias. The D400 shuddered slightly on the most vigorous inhalation, suggesting that the Venturi collar needed one more click closed. All four regs breathed extremely well, and any differences on inhalation were subtle.
Vigorous exhalation and hyperventilation clearly favored the D420. Exhalation was noticeably easier on forced exhalation, though the differences were small, except for the D400. Hyperventilation with this reg in the "looking down 30 degrees" position made the increased work of exhalation noticeable. With the (smaller) exhalation valve lowest in the water, this is perhaps not surprising.
It is possible, though I do not believe it to be true, that my observations were biased in favor of the D420, simply due to my enthusiasm over this reg.
Bubble flow in front of the mask was minor, but noticeable on vigorous exhalation in the standard diving position. With normal respiration in the standard diving hover, all four regs would occasionally deliver one brief bubble or so in front of the mask, but were generally almost completely clear. From prior comparison, this occasional bubble is still slightly worse than the Atomic M1 with a wide exhaust tee, but the difference is minor.
I had no SPG on this setup, and did not know I had emptied my tank until breathing began to come hard during the final hyperventilation tests. lt became clear that I was sucking dynamic IP down below the regs ability to deliver air well, and all four regs intermittently breathed hard during peak inspiration. It is a credit to SP's engineering that I was still able to complete tests of all four regs at a point where my tank pressure was probably less than 150 psi. All four regs had similar drops in flow delivery at peak inspiration.
Summary:
"On your back" breathing favors the coax diaphragm/exhaust valve of the D400.
All standard positions favored classic case designs with an exhaust valve behind the diaphragm, where case geometry preloads the valve, reducing inhalation effort.
Exhalation work of breathing was subtly better for the new D420, and subtly worst with the D400. Tests at high air density might reveal greater differences.
Overall performance differences among the four regs were small.
Ability to retune on the fly clearly favored the D420
if the decal was not present. The G260, whose knob plug could be pried out easily, was next easiest, though all barrel regs require hose disconnection and interposition of an in-line adjuster if orifice position needed adjustment as well as spring tension. The faceplate screws of the A700 complicate any field work, and the D400 remains very difficult to retune in the field due to difficulty in removal of the case as well as looseness of the valve assembly when the top cap is unscrewed.
My prior video comment that the D420 may not be suitable for scootering due to the large purge lever is likely incorrect. In practice, use of the D420 purge lever was easiest of all four regs, with the stiff rubber cover of the D400 worst. Additionally, in standard scooter position, the portion of the purge lever facing forward is directly above the spring, where purge depression is very stiff. In contrast, the curve of the purge lever at its end looks to be almost parallel to water flow, and I no longer expect inadvertent activation of the purge while underway. Using the purge lever required a change in technique, as best action is at the bottom of the case, rather than the center. With a little practice, purging was easy, and provided brisk airflow.
All-position consistency favored the D400, but this was the only parameter where this reg shined.
Subjective airflow was effortless with the D420, though I am hard pressed to say it was better than the G260. The A700 was a minor disappointment in a side-by-side comparison, but the differences were subtle.
Exhalation effort I truly believe favors the new D420.
Coupled with a near zero inhalation effort to trigger the valve in standard diving position, I think this regulator will match or outperform the other two excellent regulators.
Surprisingly, the higher cracking effort of the D420 did not matter, except vertical in the water looking straight ahead. That was the only position where the minor difference in tuning could be perceived. And the higher cracking effort reduced wet breathing in the inverted roll.
In the normal diving position, SP's engineering of case geometry made for near zero efforts for all three newer regs, while the low spring forces of the center balanced poppet appear to have neutralized the added potential stiffness when lying on one's back looking up.
A Scubapro rep is coming to our LDS for a "new product night" next week. We'll see what supposed advantages Scubapro may be touting for their new toy. More to follow!
