Contributions to this topic have been most interesting and, as a
thread, its longevity seems to indicate interest.  I would like to
add a few comments even though the illustrations already presented are
quite good and don't need any improvement.

One of the parameters not mentioned directly, but which is key to pouch
movement, is the volume of the pouch well.  If the pouch is a perfect
piston (which we know it is not, but for discussion purposes let's say
it is) and the valve travel is about .035 inches, then if we know the
diameter of the pouch well we can compute the volume of air which must
move down the tracker tube to activate the valve.  As has been
mentioned earlier, air doesn't have to move the full length of the
tubing.  It only has to move enough to make the pouch move (inflate).

The speed of that air movement in the valve system isn't based on the
speed of sound as we heard in one interesting theory.  Air moving into
the pouch well has mass and as such is subject to the traditional
formula in which a mass is accelerated by force.  The force in this
case is the difference in pressure of the atmosphere and the pressure
inside the valve chest.

What this means is at small vacuums (differential), the air moves more
slowly into the pouch well.  At higher vacuums (differential), the
force is higher, which makes the rate of acceleration higher because
the mass of air in the passage is constant if we consider air to be
a non-compressible fluid in this application.

We're talking about a tiny amount of air that must be moved.  Maybe
just a 1/2 teaspoon of air.  Like the earlier marble illustration, this
1/2-teaspoon goes into the tracker hole and simultaneously, another
1/2-teaspoon goes into the pouch well at the other end which is enough
for the pouch to move .035 inches.

Several things can slow this process.  The length of tube will add
resistance which in turn will result in slower acceleration of the air.
If the tube is really long, the total mass increases which will also
slow the rate of acceleration.  Previous posts suggest that in most
cases, tube length has no discernible affect.  Undersized holes in the
note sheet, and mis-tracking constrict the tracker hole, and thus,
restrict the air movement which slows down the valve action speed.

From the above discussion, one can surmise that a very small pouch well
has the potential for fast response.  The down side of a small pouch is
that it is weak and cannot move a large valve.  The size of the valve
must be scaled to the size of the pneumatic and its desired speed of
movement.

Double valve systems achieve extremely fast response because the
primary valve is tiny, but fires a much larger more powerful secondary
valve.  This bit of trickery is made possible because the opening
between the primary valve to its secondary partner is huge compared to
the effective size of a tracker hole that fires the whole thing.  There
is no bleed in the secondary valve because the primary when closed,
provides stack pressure to the secondary pouch well.

Understanding the fluid dynamics of the valve comes in handy when we
trouble shoot and fix problems.

Who wants to explain the Duo-Art cross valve?

Bob Taylor
Missouri