Must 2nd stage (D400) be tuned specifically for 1st stage (MK17 versus MK20)?

[Leicamshooter]

Must the 2nd stage (D400, R190) be tuned specifically for the 1st stage (MK17 versus MK20)? Can one interchange a D400 or R190 with a MK17, MK20 etc, without having to adjust the 2nd stage specifically for the I.P. in the 1st stage? I may be totally off, but I'm assuming that the I.P. for a MK20 is a bit different than the I.P for a MK17.

Again, thanks for your help!
G Medina

 

[DA Aquamaster]

The IP range for both the Mk 20 and Mk 17 is 120-145 psi.

In order to reduce the potential for a cold water freezeflow the IP on the Mk 20 (and Mk 25) tends to be set around 120-125 psi by most techs.

On the other hand, the Mk 17's I have seen came from Scubapro new in the box with IP's running 145 psi - presumably to maximize air flow since freezing up is not a concern with the fuilly sealed design.

So as a matter of practice a Mk 20 and a Mk 17 are likely to have IP's at the extreme ends of the range.

The D400 is pretty tolerant of IP variance due to the nature of the center balanced poppet design and low spring pressures involved anyway.

The unbalanced downstream poppet designed R190 however would need to be tuned to the higher IP'd first stage as an increase in IP could result in a slight freeflow on an R190 tuned for easy inhalation effort. This would result in a slight increase in inhalation effort on the lower IP'd first stage, but nothing too drastic. And, if the R190 is already set to breathe a little hard, it would probably not have problems on a higher Ip'd first stage.

 

[servicetech]

it would be a good idea to have the second stage tuned to the new first stage.
the d-400 can be a bit tempermental. and running a higher ip is going to make the cracking effort on the second stage alot less. 5 psi plus or minus on a first stage can change your second stage cracking effort more then a bit.
good luck
mike

 

[DA Aquamaster]

the d-400 can be a bit tempermental. and running a higher ip is going to make the cracking effort on the second stage alot less. 5 psi plus or minus on a first stage can change your second stage cracking effort more then a bit.
good luck

The D400 is one of the least tempermental second stages in terms of IP variance. The center balanced poppet design is among the least downstream biased balanced SP designs and a 5 psi variation does not make a significant difference in cracking effort.

You need to remember the D400 was introduced on the MK 5 and Mk 10 first stages. Both those first stages were balanced, but both also had constant diameter piston stems. This meant that, unlike the current Mk 20 and Mk 25 piston designs where the diameter of the seat/knife edge end of the piston is slightly larger in diameter than the shaft to acoomodate the area of the knife edge itself, the IP still varied 5 to 6 psi as the tank pressure fell from 3000 to 300 psi due to the fact that the area of the knife edge was not fully balanced. Consequently, a 5 to 6 psi variance in IP was a fact of life for the D400.

The D400 did get more tempermental with the change to the plastic orifice as they were then harder to fine tune for minimal cracking effort (.6 to .7 inches of water), but the current SP specs call for cracking efforts in the 1.0 to 1.4 range anyway, so this is really a non issue if the tech sacrifices a little cracking effort performance for compliance with the specs.

The other issues with the D400 being "tempermental" were due to the more recent crop of techs not working with many of them and being generally non-proficient with the nuances of their unique adjustment procedure and the critical importance of the proper order of lever and spring pad adjustment.

SP really screwed the D400 with the move to a plastic orifice and with their apparent inability to properly train their newer techs, Please don't make it worse by calling the D400 "tempermental" - it isn't.