arduino based?
If you use a small PIC such as Arduino Nano (small and lightweight) teamed with a barometric pressure sensor (http://www.ladyada.net/learn/sensors/bmp085.html) then you can have the PIC monitor then trigger a solenoid/valve at the right time.
Additionally you can use the PIC to provide other services such as guidance, GPS etc in one small unit.
Hope that helps.
_Chris