Why does my water hammer arrestor ask for 5psi less than house pressure and not 10 psi or 20 psi?
The airspace in the arrester is creating a shock absorber just like on your car but I will use a bicycle tire to bring it to the kid level.
With the pressure a few psi below the actual the piston can move and absorb the shock of the water hammer.
If the pressure is above the water pressure the piston will be locked in place and not absorb the water hammer it would be like riding a bike with 120 psi air in the tires - you feel every grain of sand or rock you ride over.
If the pressure is too low in the arrester it’s not going to respond on the opposite side of too full - kind of like riding a bike with a flat tire the tire rolling around on the rim and not absorbing the shock
With the air just right at 5 below the arrester absorbs the shock of the water hammer like a bike tire rolling over a stone - you barely notice it.