what i meant was less water pressure in the system. 70 psi is 70 psi. If you read 70 psi on a pressure gauge, you can bet that there is 70 lbs of pressure in that system.As far as the diaphragm, you are right, it can have more expansion at 50 psi with 28 psi of air. It can expand even more at 70 psi if your settings aren't correct. And that can cause a diaphragm to be damaged.
Oh yeah, i did graduate High School.