I have two containers of air, one at 100 kPa and rising, the other at 30 kPa and decreasing. I need a valve that lets air through when the pressure difference is >70 kPa. Where would I find a valve like that? If nowhere, do you have any pointers on how to make one?
Check Valves are available with adjustable cracking pressures, using a preload spring to control the poppet force, for example: http://www.my-ssp-usa.com/products/valves/check-valves/adjustable-cpa
It's important to note, however, that there is a degree of histeresis with these valves, and once open, they will not close if the pressure difference drops a bit - it will have to drop lower than your application may allow. A Back Pressure Regulator may be more suitable.