A change of heart, Scubapro D420

[buddhasummer]

Service / tuning trick:
Make sure there's always a hint of clearance between the back of the diaphragm centre disc and the lever. Perhaps 0.25-0.5mm. Always.

This engineering diagram is a touch fanciful:
View attachment 586972
As drawn, there is zero clearance between diaphragm and lever (bad). There is zero clearance between poppet and knife edge (true). There is zero clearance between the nib at the end of the poppet and the entire lever fulcrum (false). There is zero clearance between the lower hex adjuster and the bottom of the lever fulcrum (too tight). If adjusted this closely, you have a setup for instability.
Why? Because the diaphragm is stretchy silicone. It's "another spring" helping to unseat the poppet.

Any pressure on the lever will lift the poppet. However, only the top hex and spring determine cracking effort. If you add to that unseating force by tightening the lower hex so that there's lever contact with the diaphragm, there's now an unknown additional pressure being applied to the lever by the diaphragm elasticity.
So...you tighten the top hex to try to make the hiss stop. That tries to depress the poppet, but it is very unpredictable. Why? Because with the lever in contact with the diaphragm, as you increase spring pressure, whatever additional seat compression you obtain with the top hex and spring is counterbalanced by the additional force that the poppet is placing on the lever, and the additional stretch that is being applied to the diaphragm. This pushes back, counteracting the additional pressure provided by the top hex. So you tighten the top hex some more...and the diaphragm pushes back...more. Unstable tuning.

Instead, you want cracking effort to be determined solely by the balance between poppet seat and poppet spring pressure. And if you try to "gild the lily" by having a perfect kiss between diaphragm disc and lever, you'll find that as soon as you tighten the top hex to adjust cracking effort, the seat compresses a hair more, the poppet drops a hair in the housing pushing harder against the lever, and now the diaphragm is pushing back, reversing your tune. So now you go and readjust lever height for the third time. This is the technique recommended by the manual, and I disagree with it. It requires too many removals of the diaphragm to finish tuning. Having 0.5mm of "wasted" play between the lever and diaphragm makes for an insignificant decrease in maximum valve opening, for a markedly easier and faster tune.

The likely correct orientation of the lever fulcrum is like this:
View attachment 586971
With this arrangement, the lever is "hanging" on the upper corner of the lever fulcrum. That is why, with the diaphragm off, you can lift the floppy lever ~0.5 cm. Same as with the D400.

And when you lay the reg on its back (mouthpiece down) there should be 1/4mm between the lever and the diaphragm disc here, as set by the bottom hex screw.
View attachment 586974
This clearance is greater with pressure on, than with pressure off. The lever drops a hair when pressurized. Why? Because the IP inside the center balanced compartment is now pushing against the seat. It is trying to lift the poppet and flow, but is counterbalanced by the top spring. That microscopic lift (just short of valve opening and freeflow) pulls the bottom poppet nib away from the lever fulcrum, and the lever drops a hair.

I suppose you could tune it for zero clearance with pressure off, giving you 0.25mm gap between lever and diaphragm with pressure on. You'll have to decide if it's worth the effort. That would be as anal as I would get about it. Once the lever is touching the diaphragm while the reg is under pressure, all stability bets are off, IMO.

With this slight change, tuning the D420 is a piece of cake! And with disassembly/reassembly so fast and easy compared to the usual barrel design, you might just become a convert to the D-series again! Well, except @Luis H , who wouldn't give up his double-hose for any damned single hose toy.

Rob, I'm in awe. Thanks a bunch.

 

[lexvil]

Rob, I'm in awe. Thanks a bunch.

Back yet?

did long dives with mine today, lost my James79 adjuster someplace but didn’t need to change anything, performing perfectly.

 

[buddhasummer]

Back yet?

did long dives with mine today, lost my James79 adjuster someplace but didn’t need to change anything, performing perfectly.

It's been sitting at some location unknown facility in the US since the15th.

With the new knowledge Rob has provided and your liking of it, I'm now beyond excited to get it back.

PS I ordered the C370 kit.

 

[StefinSB]

The little hex that locks the spindle is directly opposite the big slotted spindle screw in V1. The hex in V2 is also in the opposite half of the swivel, but is oriented at right angles to the axis of the spindle screw. Huge improvement in security, keeping the swivel halves at the proper clearance to prevent o-ring extrusion and catastrophic gas loss.

Howany catastrophic gas losses with V1 do we know about?

 

[rsingler]

Howany catastrophic gas losses with V1 do we know about?

Dunno. Why did they change a "proven design"?
All I know is that after servicing my first one for a diver many years ago, and seeing how much depended on 1/6th of a turn and some Loctite, I decided to never buy one. I made do with fixed angle connectors.
Now I'm a convert.

 

[tridacna]

I have about 400 dives on one and 200 on another. Never had an issue with either. I had a failure in Coz some years ago but it was the o-ring on the LP hose that failed and not the Omniswivel. I do service them every two years. I have no reason to upgrade but will do so at next service because the upgrade kits are available. No other reason.

 

[StefinSB]

Dunno. Why did they change a "proven design"?
All I know is that after servicing my first one for a diver many years ago, and seeing how much depended on 1/6th of a turn and some Loctite, I decided to never buy one. I made do with fixed angle connectors.
Now I'm a convert.

Companies change their "proven" designs all the time. I am not arguing the point that the new setup maybe safer but it appears the original one wasnt failing left and right as otherwise we would have heard about it.

 

[tridacna]

Companies change their "proven" designs all the time. I am not arguing the point that the new setup maybe safer but it appears the original one wasnt failing left and right as otherwise we would have heard about it.

You talking about the D420 replacing the D400?

 

[Alessandro Barlettani]

Hi I would like to thank all of you, who with your tests, helped me to set up my D420 perfectly, a special thanks to Rsingler and Buddhasummer. Yesterday I went for a test dive at 40M, after having tuned my D420, 1 "breaking, and impressive, I am used to top regulators, but now I believe that the D420 is he who wins over everyone, as they are I was positively surprised by its performance and I ordered another one, thanks again to all of you.

 

[Alessandro Barlettani]