I would add only one thing to Robbie's footnote about the force
being in direct proportion to length, and that is it also depends
on where the pneumatic begins to start closing: it is stronger if it
begins from nearly wide open than if it begins part way.

Hanging the poppets, which allows the technician to screw the pneumatic
up or down the poppet, changes its striking force.  This allows the
technician to even out the keyboard so that every note sounds the same
and none drop out at really low vacuum due to unevenness in the action
or the individual pneumatics.

It is truly amazing how much lower the zero can be set on the Duo-Art
when the time is taken to do this.  This is why hanging the poppets has
become so popular among many Duo-Art rebuilders.

Bruce Grimes