See this Application Note Accounting for Gravity When Using a Deadweight Tester or Piston Gauge...
To establish pressures using a deadweight tester (DWT) or piston gauge, the user must account for the acceleration of gravity at the location where the instrument is operated. It is important to note that the acceleration of gravity varies substantially around the world. With a range of approximately 0.5 %, the local gravity correction is typically the largest environmental influence acting on the deadweight tester.
Various methods of accounting for, and determining, gravity are in the document.