Have you considered using some gym weight equipment to verify yourself? If cost effective to ship, my mad scientist home made tounge scale is accurate to about 1lb (verified with olympic weight plates) and I would also be willing to verify accuracy.
@2kwik4u is correct in that no way to really calibrate beyond ensuring the hydraulic fluid is full or swapping out the gauge. The way those things work is by measuring the psi on a known bore size and marking that out on the gauge face as a weight scale instead of pressure. The pressures on a small bore like those are huge so they have some inherent percision constraints of about 10 percent depending on how big and how high quality the gauge is.
The relatively giant bore on my panacke cylinder means I am measuring far less psi and I am doing it with a digital air pressure gauge accurate to .5 psi. I use a bicycle pump to pump up the cyclinder and once the rod on the piston starts to raise but before it hits the end of the stroke I take the reading off the permantly attached air gauge in psi and comapre to the pre-printed cross reference chart I made to convert the psi to lbs. This works anywhere in the middle of the stroke because pressure is staying the same and only the volume is changing (top of cylinder has an air outlet). This is slightly different than the one
@Betik has as the volume is constant (should be if not leaking) and only the pressure changes. Still the same principle of pressure over surface area.
