I am kinda following, but.. why am I not seeing any pictures?
(also extrapolating a bit from the 2nd stage orifice diameter disparity)
(also extrapolating a bit from the 2nd stage orifice diameter disparity)
SP MK10 PLUS 1ST GENERATION.....?
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CG43
Contributor
Thats all right except the last word of the first sentence should be tank pressure .
There is a method of calculating the actual diameter of the sealing edge from the change in IP when the tank pressure changes.
First calculate the uncompensated defect area = Af
Then you can use this area to calculate the diameter of a orifice = D or in a non-compensated configuration, or the effective diameter = Dg of the annular ring area in a flow through piston stage.
Mit Ap = piston area
delta IP = change in IP
Af = not compensated area
delta t = change in tank pressure
D o = diameter O ring
pi = 3,14159
because of : Ap * delta IP = Af * delta t results : Af = Ap * ( delta IP / delta t )
D or = ( 4 * Af/pi ) : Dg = ( D o^2 - 4* Af/pi ) ^1/2
The orifice of a non-compensated 1st stage has a diameter of about 2.7 mm.
With Subapro, the stem of the piston has a diameter of about 6 mm on the outside.
You can quickly calculate how much smaller the sealing edge can be until you have the same drop in IP as with a non-compensated stage.
CG43
Contributor
Continuation # 12
In a non-compensated piston stage, the large and small piston O-ring seal IP against ambient, whereby the friction forces remain almost constant over the entire range of the tank pressure. In this case, the frictional forces do not cause any significant change in the IP because they are constant.
In a 1st stage with air through the piston, the small O-ring seals the variable tank pressure against ambient.
This results in a variable frictional force which, if it increases linearly with the pressure (which is unlikely), can be compensated for with a tank pressurized surface.
Since the frictional force counteracts the closing, it increases the IP .
Compensation then requires a surface pressurized by the tank . However, since we cannot increase the diameter of the sealing edge of the piston without effort, compensation is not easily possible.
At least friction is always good against oscillation .
In a non-compensated piston stage, the large and small piston O-ring seal IP against ambient, whereby the friction forces remain almost constant over the entire range of the tank pressure. In this case, the frictional forces do not cause any significant change in the IP because they are constant.
In a 1st stage with air through the piston, the small O-ring seals the variable tank pressure against ambient.
This results in a variable frictional force which, if it increases linearly with the pressure (which is unlikely), can be compensated for with a tank pressurized surface.
Since the frictional force counteracts the closing, it increases the IP .
Compensation then requires a surface pressurized by the tank . However, since we cannot increase the diameter of the sealing edge of the piston without effort, compensation is not easily possible.
At least friction is always good against oscillation .
halocline
Contributor
I believe you’re correct with the balancing and shaft/tip diameter. I just didn’t realize that the brass tipped piston were in fact flared at the tip. I’ve seen a drawing (maybe in the MK10+ conversion bulletin) that shows the MK10+ piston drawn with an exaggerated amount of extra width at the tip, but I’ve never seen a MK10+ piston that actually looked like that.
The brass tip of the brass tipped piston is not flared, it is just slightly wider than the shaft.
That made it possible to make the piston sealing diameter edge a bit wider, even when it is almost impossible to 'see' exact, where the sealing edge is
The Composite Piston Tip is flared.
The differences are very small and with a normal caliper hardly to measure, let alone to see it.
This is why I was not completely sure if the theory was concerning the brass tip piston, I was more sure concerning the Composite Piston.
That made it possible to make the piston sealing diameter edge a bit wider, even when it is almost impossible to 'see' exact, where the sealing edge is
The Composite Piston Tip is flared.
The differences are very small and with a normal caliper hardly to measure, let alone to see it.
This is why I was not completely sure if the theory was concerning the brass tip piston, I was more sure concerning the Composite Piston.
CG43
Contributor
Hello Angelo Farina !
I can see that the angle of the conical seat pushes the sealing surface a little towards the outside.
But this sealing surface and a component in the axial direction remains , or what am I missing ?
For perfect compensation you then need a touch line without a surface.
This can be only used as a simplified model ( thats something what I really like ).
I have two old disassembled MK5 1st stage where the piston edge has left a clear groove in the seat.
But here the piston edges are no longer good.
If the seat surface has almost no imprint, I buy the line model.
What is the result of a delta IP / delta tank measurement?
I can see that the angle of the conical seat pushes the sealing surface a little towards the outside.
But this sealing surface and a component in the axial direction remains , or what am I missing ?
For perfect compensation you then need a touch line without a surface.
This can be only used as a simplified model ( thats something what I really like ).
I have two old disassembled MK5 1st stage where the piston edge has left a clear groove in the seat.
But here the piston edges are no longer good.
If the seat surface has almost no imprint, I buy the line model.
What is the result of a delta IP / delta tank measurement?
I think the missing pictures from @Tanks A Lot previous post are even more critical here for me to get the concept
All I can extrapolate here is some similarity to lateral forces on a tire keeping a car in check, but don’t have the mental capacity to analyze it myself
Tanks A Lot
Contributor
@Mobulai is on the right track, perfect compensation, leaving O-ring drag aside in all subsequent pictures, can mechanically be achieved. My earlier drawings were probably not very informative, so let's try a different approach:
A simplified model of a flow-through piston would look as follows:
If we zoom in on an idealized, razor-sharp edge, our picture would look something like this:
Note how all the forces are acting perpendicular to all surfaces.
Pressure, as I have laid out before, only ever acts perpendicularly, never in a sideways fashion. Because the edge is so razor-sharp and sealing is achieved at the exact outside diameter of the piston, there will simply be no force pushing the piston anywhere, as the force vectors show.
Here comes the really important part: The outside diameter, or better and more precisely put, the point where the piston seals on the seat (its razor-sharp edge), has the exact same diameter as the diameter where the piston seals with the O-ring. The two helper lines I have drawn go exactly through those two points, the razor-sharp edge and the inside diameter of the O-ring.
However, such a razor-sharp edge is not possible from an engineering point of view. More importantly, even if nearly achieved, it would not last long at all, as anyone who has sharpened a knife can attest to… (Why are they always blunt when needed?!).
A real edge is slightly rounded and would probably look like this (I'm sure @rsingler has a great real-world example from under a microscope):
But here is exactly where the problem with balancing starts. Just as before, we have our two sealing points. The inner diameter of the O-ring has not changed, and the green line goes through that point again. However, the point where the piston seals with the seat has changed, it has shifted slightly inwards; the yellow line goes through that point. There is a slight gap between those two lines. The vertical distance between the two dotted lines is exactly what makes our design unbalanced, as represented by the orange line. The collective pink vectors exert a net force to the right, which represented as our orange force vector. This is precisely where our high-pressure supply will push the piston away from the seat. The root cause, as I have repeated many times now, is the nature of pressure: It only acts perpendicularly.
But perfect mechanical balancing can be achieved (ignoring O-ring friction):
What happened here is that we flared our piston outwards at the very top. But if we look carefully, we can yet again spot two dotted lines. However, this time, the yellow line has shifted towards the high-pressure side of the system instead of being pushed into the piston.
If we start on the sealing surface between the piston and seat again, we can yet again spot our pink force vectors, which add up to the orange force vector, pushing the piston to the right. This would indicate that the design is not balanced. But if we look a little toward the right-hand side of the flared piston, we can find the very same pink force vectors adding up to another orange force vector, pushing the whole mechanism to the left. The two orange force vectors are exactly opposite and equal, thereby canceling each other out.
We could have spared ourselves the trouble of analyzing the vectors if we had just followed the green dotted line. Where it seals with the seat and where it seals with the O-ring is exactly in line. Therefore, any forces that may act upon the piston in any direction from the high-pressure side are instantly opposed by equal forces pushing in the opposite direction.
This is how real balancing in a flow-through piston is achieved, by ensuring that the sealing diameter is exactly equal to the inner O-ring sealing diameter.
@CG43 has a point in saying that, mathematically speaking, a touch line is necessary to accomplish perfect balancing. But then again, mathematically, we do have to define a region where the sealing takes place, as otherwise, the calculations become utterly ludicrous. If we incorporate this thinking into a drawing, with an over-dramatically drawn sealing surface, it looks as follows:
As we can see, it makes no difference to the balancing, but one might rightfully argue that we still have a touch line without a surface, at least mathematically speaking. That touch line is exactly where the green dotted line crosses the seat and piston edge
I do think this is pushing the limits of practicality on a subject that, I would argue, has virtually no real-world relevance to begin with. In reality, there is already no discernible difference between a flared piston and a straight one, especially when considering that these high-end regulators are almost always paired with a balanced second stage. Not a single second stage I know of will care about such tiny drops in intermediate pressure, even on the non-flared version. This is already true for unbalanced second second, leave alone balanced ones.
There is, of course, a very tangible difference between the razor-sharp versions and the rather blunt-tipped ones. However, ScubaPro changed these not so much for reasons of balancing but rather to improve the reliability of the sealing. We can easily balance both a blunt piston and a sharp one, it's just a matter of how far outward we flare the very end of our pistons.
A simplified model of a flow-through piston would look as follows:
If we zoom in on an idealized, razor-sharp edge, our picture would look something like this:
Note how all the forces are acting perpendicular to all surfaces.
Pressure, as I have laid out before, only ever acts perpendicularly, never in a sideways fashion. Because the edge is so razor-sharp and sealing is achieved at the exact outside diameter of the piston, there will simply be no force pushing the piston anywhere, as the force vectors show.
Here comes the really important part: The outside diameter, or better and more precisely put, the point where the piston seals on the seat (its razor-sharp edge), has the exact same diameter as the diameter where the piston seals with the O-ring. The two helper lines I have drawn go exactly through those two points, the razor-sharp edge and the inside diameter of the O-ring.
However, such a razor-sharp edge is not possible from an engineering point of view. More importantly, even if nearly achieved, it would not last long at all, as anyone who has sharpened a knife can attest to… (Why are they always blunt when needed?!).
A real edge is slightly rounded and would probably look like this (I'm sure @rsingler has a great real-world example from under a microscope):
But here is exactly where the problem with balancing starts. Just as before, we have our two sealing points. The inner diameter of the O-ring has not changed, and the green line goes through that point again. However, the point where the piston seals with the seat has changed, it has shifted slightly inwards; the yellow line goes through that point. There is a slight gap between those two lines. The vertical distance between the two dotted lines is exactly what makes our design unbalanced, as represented by the orange line. The collective pink vectors exert a net force to the right, which represented as our orange force vector. This is precisely where our high-pressure supply will push the piston away from the seat. The root cause, as I have repeated many times now, is the nature of pressure: It only acts perpendicularly.
But perfect mechanical balancing can be achieved (ignoring O-ring friction):
What happened here is that we flared our piston outwards at the very top. But if we look carefully, we can yet again spot two dotted lines. However, this time, the yellow line has shifted towards the high-pressure side of the system instead of being pushed into the piston.
If we start on the sealing surface between the piston and seat again, we can yet again spot our pink force vectors, which add up to the orange force vector, pushing the piston to the right. This would indicate that the design is not balanced. But if we look a little toward the right-hand side of the flared piston, we can find the very same pink force vectors adding up to another orange force vector, pushing the whole mechanism to the left. The two orange force vectors are exactly opposite and equal, thereby canceling each other out.
We could have spared ourselves the trouble of analyzing the vectors if we had just followed the green dotted line. Where it seals with the seat and where it seals with the O-ring is exactly in line. Therefore, any forces that may act upon the piston in any direction from the high-pressure side are instantly opposed by equal forces pushing in the opposite direction.
This is how real balancing in a flow-through piston is achieved, by ensuring that the sealing diameter is exactly equal to the inner O-ring sealing diameter.
@CG43 has a point in saying that, mathematically speaking, a touch line is necessary to accomplish perfect balancing. But then again, mathematically, we do have to define a region where the sealing takes place, as otherwise, the calculations become utterly ludicrous. If we incorporate this thinking into a drawing, with an over-dramatically drawn sealing surface, it looks as follows:
As we can see, it makes no difference to the balancing, but one might rightfully argue that we still have a touch line without a surface, at least mathematically speaking. That touch line is exactly where the green dotted line crosses the seat and piston edge
I do think this is pushing the limits of practicality on a subject that, I would argue, has virtually no real-world relevance to begin with. In reality, there is already no discernible difference between a flared piston and a straight one, especially when considering that these high-end regulators are almost always paired with a balanced second stage. Not a single second stage I know of will care about such tiny drops in intermediate pressure, even on the non-flared version. This is already true for unbalanced second second, leave alone balanced ones.
There is, of course, a very tangible difference between the razor-sharp versions and the rather blunt-tipped ones. However, ScubaPro changed these not so much for reasons of balancing but rather to improve the reliability of the sealing. We can easily balance both a blunt piston and a sharp one, it's just a matter of how far outward we flare the very end of our pistons.
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