Examples: shims on the piston spring (not shown) increase X as the shims get thicker
shims under the HP seat (or a thicker seat) decrease X
shims between the HP seat carrier (not shown) and the regulator body increase X.
Do I have this right?
Piston 1st IP adjuments simplified
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Examples: shims on the piston spring (not shown) increase X as the shims get thicker
shims under the HP seat (or a thicker seat) decrease X
shims between the HP seat carrier (not shown) and the regulator body increase X.
Do I have this right?
Yes, mostly.
When you turn on the reg and pressurize, the spring compresses until the knife edge seals against the seat. The spring is "pre-compressed" with shims, and is compressed even further when pressurized. The more the compression, the higher the IP required to seal.
The only time X actually increases is if you were to shorten the piston when you restore a big nick in the knife edge, or if you were to use a shorter seat.
The only time X actually decreases is if you were to put a shim under the SEAT, or use a taller seat.
I've tried both shortening a piston and shimming a seat, messing with old regs. But the distance X is really predetermined by where the piston bottoms against the body, or by the height of the seat (think Scubapro's three different Mk5/Mk10 seat heights). After that, it's all dynamic spring compression during movement, plus preload added to a spring by a shim. Adding a spring shim does NOT lengthen distance X. When the valve is open, X is determined by where the piston head movement is limited by another part of the regulator body. Without a physical stop, the piston might push out completely past the HP shaft o-ring. You can feel that when you screw a piston reg back together. When you've slid the piston back into place, you then screw the halves of the reg body back together. You can feel the resistance as you begin to compress the spring while you screw the cap down, even though the valve is still open (unpressurized). You can therefore see that adding a shim between the piston and spring would not move the piston, and would not change distance X.
But you are thinking in the right direction.
DYNAMICALLY, the greater the X-travel, the more the spring has to compress, and the greater the required IP to seal.
Now, in responding, I'm stuck in the ancient days of fixed seat position. But with the Scubapro Mk25, for example, you are absolutely correct. Screwing in the adjustment pushes the seat closer to the knife edge, decreasing distance X. And in that case, IP decreases as you indicated on your diagram, because the spring does not have to get as compressed for the knife edge to seal.
When you turn on the reg and pressurize, the spring compresses until the knife edge seals against the seat. The spring is "pre-compressed" with shims, and is compressed even further when pressurized. The more the compression, the higher the IP required to seal.
The only time X actually increases is if you were to shorten the piston when you restore a big nick in the knife edge, or if you were to use a shorter seat.
The only time X actually decreases is if you were to put a shim under the SEAT, or use a taller seat.
I've tried both shortening a piston and shimming a seat, messing with old regs. But the distance X is really predetermined by where the piston bottoms against the body, or by the height of the seat (think Scubapro's three different Mk5/Mk10 seat heights). After that, it's all dynamic spring compression during movement, plus preload added to a spring by a shim. Adding a spring shim does NOT lengthen distance X. When the valve is open, X is determined by where the piston head movement is limited by another part of the regulator body. Without a physical stop, the piston might push out completely past the HP shaft o-ring. You can feel that when you screw a piston reg back together. When you've slid the piston back into place, you then screw the halves of the reg body back together. You can feel the resistance as you begin to compress the spring while you screw the cap down, even though the valve is still open (unpressurized). You can therefore see that adding a shim between the piston and spring would not move the piston, and would not change distance X.
But you are thinking in the right direction.
DYNAMICALLY, the greater the X-travel, the more the spring has to compress, and the greater the required IP to seal.
Now, in responding, I'm stuck in the ancient days of fixed seat position. But with the Scubapro Mk25, for example, you are absolutely correct. Screwing in the adjustment pushes the seat closer to the knife edge, decreasing distance X. And in that case, IP decreases as you indicated on your diagram, because the spring does not have to get as compressed for the knife edge to seal.
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OP
Thanks rsingler. I've been working with SP MK5s and now MK10s. The MK10 Plus threw me a curve with the shims on the HP seat carrier.
So what I get is: while my diagram isn't actually accurate as to the "x" dimension, it works as a practical way of remembering what shim does what, where.
Think of 'x' getting longer with thicker piston spring shims in an UNCOMPRESSED state (eg body not screwed together).
So what I get is: while my diagram isn't actually accurate as to the "x" dimension, it works as a practical way of remembering what shim does what, where.
Think of 'x' getting longer with thicker piston spring shims in an UNCOMPRESSED state (eg body not screwed together).
Yes.
But shims on the seat carrier? I'm not aware of any authorized shim for the seat. Yes, I've tried that for fun, but Scubapro doesn't authorize that. I tried it once when I couldn't lower the IP after removing all spring shims and using the tallest seat on an old reg that I had honed to remove a deep nick. That shortened the piston, and the only way I could shorten "X" was to raise the seat with a homemade shim under the seat in the seat cap. I had to use a thinner seat seal oring to allow the seat to move closer to the knife edge. But that's all unauthorized service.
Looking at the exploded diagram, I do see that white spacer similar to that used on the Mk 20 and Mk25. Bit I didn't think of that as a shim, although using a thick one or two would increase distance X. The risk in changing that thickness is that the HP seat sealing o-ring (PN 01-050-142), or washer (PN 01-070-106) wouldn't seal, and you'd have a HP leak.
But shims on the seat carrier? I'm not aware of any authorized shim for the seat. Yes, I've tried that for fun, but Scubapro doesn't authorize that. I tried it once when I couldn't lower the IP after removing all spring shims and using the tallest seat on an old reg that I had honed to remove a deep nick. That shortened the piston, and the only way I could shorten "X" was to raise the seat with a homemade shim under the seat in the seat cap. I had to use a thinner seat seal oring to allow the seat to move closer to the knife edge. But that's all unauthorized service.
Looking at the exploded diagram, I do see that white spacer similar to that used on the Mk 20 and Mk25. Bit I didn't think of that as a shim, although using a thick one or two would increase distance X. The risk in changing that thickness is that the HP seat sealing o-ring (PN 01-050-142), or washer (PN 01-070-106) wouldn't seal, and you'd have a HP leak.
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OP
Is this better?
OP
OP
FWIW, I'm not trying to adjust the IP wildly, just trying to understand the dynamics. When you buy something off eBay there is no guarantee that all the parts are in the right place, or even that there are all the right parts. One that I took apart looked like a small wad of steel wool was inside. Cleaned up nice and locks well.
That sure is the truth! You never know what you're going to get, but it's a great source for junk to try to restore.
Two spacers on that end cap will increase IP by moving the seat back. (EDIT: removed incorrect assumption that followed.)
And, the line pointing to optional shims under the seat are exactly correct, theoretically. They are what I used when I was playing around. But they're strictly unauthorized, although I wouldn't call them unsafe, as long as you adjust the thickness of the seat sealing o-ring to guarantee a HP seal.
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