SP MK3 1st Stage?
- Thread starter HenrikBP
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I agree it is one of the older pre-SPEC capped Mk 3's. (light yoke, 2 LP ports, rounde cap).
OP
HenrikBP
Contributor
Thanks for the replies guys.
It was a freebie from a seller due to a slight mix-up. I'm exited to have it.
I'm guessing since it's based on the MK2, that it's an unbalanced 1st stage?
Henrik
It was a freebie from a seller due to a slight mix-up. I'm exited to have it.
I'm guessing since it's based on the MK2, that it's an unbalanced 1st stage?
Henrik
Yes, the basic layout and design is identical. The Mk 3 used a piston with a smaller diameter head. The smaller head diameter creates a bit more drop in IP as the tank pressure falls from 3000 to 300 psi. but it is not a major difference.
R
redacted
Guest
I thought the IP drop over the tank pressure range in an unbalanced piston was caused only by the change in pressure on the HP seating surface. How does the piston head diameter effect IP drop?
The area of the orifices in both is the same. So the change in downstream pressure exerted on the seats setting on the orifices when the tank pressure drops is also the same.
That downstream pressure is countered by air pressure in the compression chamber acting on the top of the piston head. If the area of the piston is smaller, the pressure must be greater to provide the same amount of force.
To give a simple example, If the downstream force pushing on the seat is 10 lbs and the area of the small piston head is 1 square inch, you would need 10 psi of pressure acting on the piston head to balance the downstream pressure. However if the area of the large piston head is 2 square inches, you only need 5 psi to create the same 10 lbs of pressure.
So for the same change in downstream pressure, you need less change in pressure on top of the piston head to keep it balanced with the large piston head compared to a small piston head, so the larger piston means less change in IP due to changes in tank pressure.
Technically if you used a really large piston head, the IP drop would be very minimal even with an unbalanced regulator. The practical limits are imposed by the size and weight of the regulator body and compression cap needed to accomodate the larger piston. The larger piston also has more mass and more inertia and the response time gets slower as the piston gets larger and heavier.
I suspect lighter weight and faster response time is why Sp tried a smaller diameter piston in the Mk 3 and Mk 10 before returning to the larger diameter pistons in the Mk 15 and Mk 200. With the composite piston first used in the Mk 20, it became a moot issue as they then had a piston that was both large in diameter and light in weight.
That downstream pressure is countered by air pressure in the compression chamber acting on the top of the piston head. If the area of the piston is smaller, the pressure must be greater to provide the same amount of force.
To give a simple example, If the downstream force pushing on the seat is 10 lbs and the area of the small piston head is 1 square inch, you would need 10 psi of pressure acting on the piston head to balance the downstream pressure. However if the area of the large piston head is 2 square inches, you only need 5 psi to create the same 10 lbs of pressure.
So for the same change in downstream pressure, you need less change in pressure on top of the piston head to keep it balanced with the large piston head compared to a small piston head, so the larger piston means less change in IP due to changes in tank pressure.
Technically if you used a really large piston head, the IP drop would be very minimal even with an unbalanced regulator. The practical limits are imposed by the size and weight of the regulator body and compression cap needed to accomodate the larger piston. The larger piston also has more mass and more inertia and the response time gets slower as the piston gets larger and heavier.
I suspect lighter weight and faster response time is why Sp tried a smaller diameter piston in the Mk 3 and Mk 10 before returning to the larger diameter pistons in the Mk 15 and Mk 200. With the composite piston first used in the Mk 20, it became a moot issue as they then had a piston that was both large in diameter and light in weight.
DA explanation is correct, but I would like to expand on that.
The pneumatic pressure forces are the product of the pressure times the area. The pressure times the area becomes a force. In our US system of units: psi x inches square = pounds
In an unbalanced piston regulator, the IP change as a function of tank pressure is driven by the ratio of the piston head area to the volcano orifice area, the larger the area ratio the less pressure fluctuation.
One way to increase the area ratio is to increase the piston head area as DA mention. The other is to decrease the volcano orifice area. This is the reason why in an unbalanced regulator the volcano orifice opening area tends to be small (and consequentially they will not flow as much air in a fully open position).
When the piston geometry was changed so the input tank pressure did not affect the piston valve opening/closing motion (as we know it in a balanced piston). The area ratios did not mater anymore and the orifice opening can then become very large. In many cases much larger than needed for the max flow rated actually needed (but it makes for good advertisement).
In a balanced piston regulator the primary forces that determine piston opening motion are the pressure force on the head of the piston against the spring force. There are other secondary forces, like friction and small unbalanced areas, but most of the time they are kept small by design.
In an unbalanced piston regulator we have three primary forces:
The tank pressure times the orifice area
The IP times the piston head area
And the spring force.
For more information I recommend the Scuba Tools book, "Regulator Savvy" from Pete Wolfinger. It has very good explanations with excellent diagrams.
Scuba Tools - Regulator Savvy Book
The pneumatic pressure forces are the product of the pressure times the area. The pressure times the area becomes a force. In our US system of units: psi x inches square = pounds
In an unbalanced piston regulator, the IP change as a function of tank pressure is driven by the ratio of the piston head area to the volcano orifice area, the larger the area ratio the less pressure fluctuation.
One way to increase the area ratio is to increase the piston head area as DA mention. The other is to decrease the volcano orifice area. This is the reason why in an unbalanced regulator the volcano orifice opening area tends to be small (and consequentially they will not flow as much air in a fully open position).
When the piston geometry was changed so the input tank pressure did not affect the piston valve opening/closing motion (as we know it in a balanced piston). The area ratios did not mater anymore and the orifice opening can then become very large. In many cases much larger than needed for the max flow rated actually needed (but it makes for good advertisement).
In a balanced piston regulator the primary forces that determine piston opening motion are the pressure force on the head of the piston against the spring force. There are other secondary forces, like friction and small unbalanced areas, but most of the time they are kept small by design.
In an unbalanced piston regulator we have three primary forces:
The tank pressure times the orifice area
The IP times the piston head area
And the spring force.
For more information I recommend the Scuba Tools book, "Regulator Savvy" from Pete Wolfinger. It has very good explanations with excellent diagrams.
Scuba Tools - Regulator Savvy Book
As far as I can tell, the basic reason for Scubapro to go back to a larger piston on the Mk-15 (from the Mk-10) is to increase the magnitude of the primary forces (the basic primary forces being the pressure force on the piston head against the spring force that pushes the piston open).
By increasing the primary forces, the secondary forces become less significant. Secondary forces include friction, inertia forces (mass times acceleration), and the sealing force of the soft seat against the piston end (I can't think of any others at this moment, but there may be).
I almost forgot one important primary force is the hydrostatic force of the ambient pressure against any the pistons or diaphragm first stages (balanced or unbalanced).
By increasing the primary forces, the secondary forces become less significant. Secondary forces include friction, inertia forces (mass times acceleration), and the sealing force of the soft seat against the piston end (I can't think of any others at this moment, but there may be).
I almost forgot one important primary force is the hydrostatic force of the ambient pressure against any the pistons or diaphragm first stages (balanced or unbalanced).
R
redacted
Guest
That might explain why my Mk10s demonstrate greater IP inflation problems at higher input pressures than my Mk5s as a result of friction at the HP o-ring. I am assuming it is friction caused by extrusion as changing to a polished piston shaft and a Urethane 90 o-ring seems to reduce or eliminate the problem.
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