A high resolution gauge is used to determine the change in effective area of a 250 MPa piston-cylinder unit as a function of jacket pressure. These changes are compared with values calculated using a finite element method. The results of the comparison revealed that the jacket pressure seals were applying unexpected forces to the cylinder outer diameter. These forces were modelled in the finite element calculations giving reasonable agreement. Further measurements were made after modifying the jacket seal mounting. The new results showed reasonable agreement at low operating pressures but large disagreement between theory and experiment as the operating pressure approached 250 MPa.
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