Pouches are a standard means to pneumatically actuate valves and other
devices.  The blowing pressure times the surface area of the pouch
renders a nominal force, but how much of that do you really get out
from the device?

I found a simple approximate way to compute and illustrate that:
a designer's aid.  See my short technical note that Robbie has put
at  http://mmd.foxtail.com/Tech/pouchforce.html

Johan Liljencrants
Stockholm, Sweden