Hello Mike, hello all -

> ... whereas pipe organ valves of the Peterson type may leak when
> exposed to pressure on the "wrong" side.

I also wondered whether this might happen. But the valves I have have
a closing force of about 0.2 Newtons.

With a wind pressure of 0.05 bar (this is more than double the actual
pressure) and a hole diameter of 4 mm, the opening force on the pallet
can be roughly computed as follows: 5000 N/m^2 * 0.004^2 m^2 = 0.08 N.
This is less than half the closing force, so there's enough safety
margin.

In case this is not enough (for even higher wind pressure?) you could
extend or change the closing springs -- the magnets are very strong.
I did not measure the opening force, but when placed conventionally
inside the windchest of an organ and using the largest pallet diameter
of 2", they would have to reliably open the valve against a force of
maybe 1 to 2 Newtons.

For player pianos, the valves work "the right way" anyway, because the
outside air pressure closes them (somewhat).  Still, because of their
size, organ valves are certainly not optimal for small crank organ and
player piano MIDIfications.

> ... and that someone will inspire me to get off
> my duff and finish my own organ MIDIfication.

Do it!  It's worth the fun when you can just try out an arranging
idea, or quickly arrange a children's song or a theme from TV for
the children.  Also it helps me when I want to calibrate the bleeds.
I have a Noteworthy file with scales, fast repetitions, and trills
which I use to find the ciphers and non-playing notes.

This winter we had sharp temperature and humidity changes: we
experienced both the coldest day (-43 deg C) and the warmest day
(+22 deg C) in any winter since 1870, when they started measuring
in Bavaria.  On some days the organ would play only about half of
the notes before calibration.

Regards

Harald M. Mueller
Grafing bei Muenchen - Germany
http://www.haraldmmueller.de/

 [ 4.448 Newtons = 1 pound force
 [ 1000 Millibars = 1 bar = 14.7 pounds force per square inch
 [ = 406.8 inches water column at temperature 4 degrees Celsius
 [
 [ So wind pressure 0.05 bar is 20 inches water gage, enough to
 [ blow the pipe from the chest (almost ;-)!   -- Robbie