Is the Scubapro MK10 a balanced piston 1st stage?

[Leicamshooter]

Is the Scubapro MK10 a balanced piston 1st stage? I know the MK20 and MK25 are, but when did scubapro move away from the downstream piston to the balanced piston. I suppose the current MK2 is a simple downstream type. Again, thanks for your valuable info and help!
G Medina

 

[DA Aquamaster]

The Mk 2, Mk3, Mk 200 and Mk 2 Plus are all unbalanced downstream "flow by" piston designs.

The Mk 5, Mk 10, Mk 15, Mk 20 and Mk 25 are balanced piston designs.

However it should be noted that the Mk 5 and Mk 10 use a piston where the piston stem is the same diameter as the seating end of the piston so the actual area of the knife edge seating surface on the end of the piston is not, technically speaking, balanced. This means that as tank pressure drops from 3000 psi to 300 psi you can still get a decrease in intermediate pressure of about 4 psi.

So the Mk 5 and Mk 10 are balanaced relative to the Mk 2, Mk 3 etc, where the drop in pressure is 20 to 25 psi across this range, but are not balanced to the same standard as the Mk 20 and Mk 25. In the latter models, the seating end of the piston is oversize relative to the stem so that the area of the seating surface itself is accounted for and the piston is truly balanced with no IP change across the 3000-300 psi range.

 

[guruboy]

However it should be noted that the Mk 5 and Mk 10 use a piston where the piston stem is the same diameter as the seating end of the piston so the actual area of the knife edge seating surface on the end of the piston is not, technically speaking, balanced. This means that as tank pressure drops from 3000 psi to 300 psi you can still get a decrease in intermediate pressure of about 4 psi.

So the Mk 5 and Mk 10 are balanaced relative to the Mk 2, Mk 3 etc, where the drop in pressure is 20 to 25 psi across this range, but are not balanced to the same standard as the Mk 20 and Mk 25. In the latter models, the seating end of the piston is oversize relative to the stem so that the area of the seating surface itself is accounted for and the piston is truly balanced with no IP change across the 3000-300 psi range.

Can you explain how this "seating surface area" affects the balancing? Why isn't it balanced if the outer diameter is the same on both ends?

 

[nohappy]

@DA Aquamaster I'd like to know why exactly, please.

 

[DA Aquamaster]

Can you explain how this "seating surface area" affects the balancing? Why isn't it balanced if the outer diameter is the same on both ends?

@DA Aquamaster I'd like to know why exactly, please.

No matter how sharp, the seating edge on the piston still has area, and that area is not addressed by the diameter of the piston where it passes through the high pressure piston stem o-ring.

On the late MK 20 and on the Mk 25 they evolved to a rounded seating edge with significantly more area, and this was offset by increasing the diameter of the seating surface end of the piston, so that the cross sectional area being acted on by the high pressure gas from the tank is the same area as the cross sectional area of the piston stem.

That also requires enough give in the piston stem o-ring bushings to allow the larger piston end to pass through the o-ring and bushings.

 

[nohappy]

@DA Aquamaster what do you mean "that area is not addressed by the diameter of the piston where it passes through the high pressure piston stem o-ring"?
Why is that making it unbalanced?

 

[DA Aquamaster]

The pressure acting on the piston stem acts on the full diameter of the stem. The pressure acting on the area of the inside of the seat is acting on the diameter of the piston inside the knife edge of the seat where it contacts the seat. That's a finite but still measurable difference in area.

But don't take my word for it:

1) Get an IP gauge for $20;
2) connect a Mk 10 to a full 3000 or 3442 psi tank and check the IP; and then
3) check the IP on a tank with 300 to 500 psi left in it.

You'll see a reduction in IP of around 4 to 6 psi on the second tank.

 

[halocline]

The pressure acting on the piston stem acts on the full diameter of the stem. The pressure acting on the area of the inside of the seat is acting on the diameter of the piston inside the knife edge of the seat where it contacts the seat. That's a finite but still measurable difference in area.

But don't take my word for it:

1) Get an IP gauge for $20;
2) connect a Mk 10 to a full 3000 or 3442 psi tank and check the IP; and then
3) check the IP on a tank with 300 to 500 psi left in it.

You'll see a reduction in IP of around 4 to 6 psi on the second tank.

I have always had more drop in IP from 3000 to 300 PSI on MK10s. I attribute this to increased friction at the HP piston o-ring at very high tank pressures. There might be 5PSI drop in IP from 3000 to about 2000 PSI and then another 5 PSI drop to 300PSI, so it's not linear. That's what makes me think it's increased friction from slight extrusion of the o-ring when subjected to full tank pressure.

I have also found that my MK5s drop a little less than the MK10s, and the only reason I can guess for that is that the MK5 has a higher absolute pressure acting on the piston head (larger surface area) and as such, the increased friction at the HP o-ring is a proportionally smaller force.

I also suspect, but don't really know, that the bushing system in the MK15-20-25 helps with the friction at the HP o-ring, as my MK15s drop even less than the MK5s, and I believe they both have straight (not flared) piston shafts.

 

[DA Aquamaster]

Exactly, the larger piston head reduces the change in IP. It's a major reason Scubapro went back to the larger piston diameter on the Mk 15, and the subsequent Mk 20 and 25.

 

[rsingler]

@DA Aquamaster what do you mean "that area is not addressed by the diameter of the piston where it passes through the high pressure piston stem o-ring"?
Why is that making it unbalanced?

Although the sizes of the areas in question are exaggerated for visual purposes, and the dimensions are arbitrary, perhaps this picture will make sense of the cause for slight unbalancing due to the slightly decreased diameter of the seating edge, compared with the stem diameter.

As these pics show, a "knife edge" isn't a knife edge:

As Pete Wolfinger's Regulator Savvy discusses, that few thousandths difference in OD vs knife edge contact circle is several psi of imbalance between 3000 and 300 tank pressure.
Think of it as little tiny tank gremlins hooking their fingers under that tiny curve and lifting up on the piston. If the side of the piston stem were perfectly straight, with an absolutely perfect knife edge contacting the seat, the gremlins couldn't get their fingers underneath, and the reg would be balanced.

In the case of the Mark 25, the very end of the piston stem has an increased diameter. Think of the gremlins as standing on the shoulder of that increased diameter and jumping down to increase the force on the seat. At the same time there is a second set of gremlins lifting up on the curve at the bottom edge, and they cancel each other out.
Because the circle where the enlarged knife edge contacts the seat is exactly the same size as the piston stem, it's as though the piston stem were perfectly straight with a perfect knife edge, and the reg is closer to perfectly balanced.