Piston 1st IP adjuments simplified

[Couv]

Tigger, not stupid at all. In fact you can use them as they are with a pneumatically balanced second stage like a G250, Balanced Adjustable (156), a 109 that's been converted to balanced, or any assortment of pneumatically balanced second stage. I would prefer one with an adjustment knob-not necessary, but it's an additional measure.

However, I've read a lot of your post and I know this creep is still going to bug you. @halocline and @rsingler have successfully dealt with issue; so search their post for the secret. Me, I'm for downgrading any MK10 Plus to a regular trouble free sharp piston first stage. You'll need the old style piston (I know someone who has them ), a standard seat retainer, and a standard seat. In addition to having a piston design that worked very will from the second generation MK1 (precursor to the MK5) MK5/6/7/8/9/10 there are aftermarket seats available for a reasonable price.

Seat retainers can be sourced from junk MK5s & MK10s

Edit: Ooops, something just caught my eye. IP of 120 is a bit low. First shim the spring so that the IP is within specs. See the bulletin inserted earlier. If still no joy, it might be time for a new spring.

 

[rsingler]

To make that decision, I'd do a long test, e.g., 12 hours or more. If it indeed locks up at 155, even though it's over spec, your regs should tolerate it fine.
Your reg in use won't be an issue, and your second stage should be tuned to your instantaneous IP for best performance.
But the SM set that's idle is more at risk. I suppose if it starts to bubble as IP creeps up, a quick small purge of the second stage will give you another 10-20 min until you're ready to breathe it.
This is the frustrating thing about the Mk10+. It acquired a reputation for creep that made it rather unpopular at the time it was introduced. Can't figure out what the problem is, because SP subsequently adopted the same rounded edge piston for its Mark 20 & 25, that persist until today. Since the 10+ uses the same Mk25 seat, the only things I can think of are the radius of the piston knife edge, and the stability of the shaft as it travels to its sealing point. The 10+ has only the HP o-ring to steady it, while the Mk25 had those plastic spacers above and below the o-ring. Perhaps there's some microscopic variability in landing point?

Personally, I wouldn't accept that amount of creep in my regs. On the other hand, if those are new seats and polished knife edges, perhaps some time will let the seat mold to the knife edge and it will settle in.

 

[rsingler]

I have quite a few sharp edge (Mk10) sitting here. I'll trade you for your Mk10+ any time, just for the challenge of seeing if I can make it work.

Rob

 

[Couv]

I say trade ONE and see who comes up with the solution first.

 

[rsingler]

Lol!
Tigger, you don't have to trade at all. If you get frustrated, send me one and I'll share what I find. We can work the issue together on SB, and I'll send it back to you.
Rob

 

[raftingtigger]

I have quite a few sharp edge (Mk10) sitting here. I'll trade you for your Mk10+ any time, just for the challenge of seeing if I can make it work.

Rob

PM sent

I have to see what I have, but I really want a consistent fleet.

 

[halocline]

The reason the MK10+ tends to creep more than the MK20 is probably due to a few factors. 1) The piston head is smaller, therefore the total seating force pushing on the seat is lower for a given IP. Make sense? 2) The piston shaft alignment is probably not as accurate in the MK10+. Somebody earlier in the thread brought that up. There's a 'spring centering' bushing for the MK10+, and you'll want to make sure that's not missing. 3) I suspect that the piston edge for the MK10+ is not quite as well designed as the MK25 piston. The MK20 went through three piston designs and the last one really locks up better than the first two. The MK10+ piston was much earlier than any of those pistons.

I have several MK10s, one conversion kit to MK10+ that I haven't used. So I don't have a lot of experience with the MK10+. I've worked on a few but never owned one. I have over a dozen seats for the MK5 and 10, but only a couple of MK20 seats. So I'll keep diving with the MK10s. The only advantage of the MK10+ is higher air flow; I don't think I could tell any difference in use so that doesn't matter to me.

20 PSI creep over several minutes would be totally unacceptable to me. If you were diving a single tank, and as such cycling the first stage every few seconds, it doesn't really matter, but with doubles you are allowing one of the first stages to sit closed for several minutes, so the slow creep could really cause problems.

 

[rsingler]

I have to think about that #1, @halo. I think a lot depends upon the counterspring. A larger piston head should require a heavier spring to ensure that constant IP being exerted by a larger head on a constant size knife edge yields the same pressure on the seat. There may be one too many variables to say what's happening at the Mk10+ seat. It could go either way. SP's original Mk10 spring may have been too heavy when the contact area for that seat changed (increased?) from a knife edge to a radius (and that could be the Mk10 Plus's problem), but piston head size is only part of the issue.

Concur with #2 & #3.
Re: #3, perhaps that's why hand polishing works for me - it may subtly change the radius of the rounded knife edge.

 

[halocline]

I have to think about that #1, @halo. I think a lot depends upon the counterspring. A larger piston head should require a heavier spring to ensure that constant IP being exerted by a larger head on a constant size knife edge yields the same pressure on the seat.

Yes, you're right about the spring. The net seating force should be the same, sorry about that.

But I still think there's something about the smaller piston head that makes lock up less reliable. The only difference between the MK5 and MK10 is the piston head size, and all of my MK5s lock up more reliably for longer than my MK10s. Same seat, same piston knife edge.

 

[lexvil]

Yes, you're right about the spring. The net seating force should be the same, sorry about that.

But I still think there's something about the smaller piston head that makes lock up less reliable. The only difference between the MK5 and MK10 is the piston head size, and all of my MK5s lock up more reliably for longer than my MK10s. Same seat, same piston knife edge.

Not that it has anything to do with the MK 10 proclivities but didn’t they move the HP shaft seal to the bottom of the bore, closer to the piston head?
I no longer have a MK 10 or plus (I have some parts if anyone is in need) I’ve had good luck getting them to perform reliably with less creep using tribolube 66, the thicker stuff seems to stick better.