Elastic Distortion Calculations on a Special Piston Gauge (PG27) up to 28 MPa in Different Operational Modes

The elastic distortions of a special hydraulic piston gauge (PG27) operating at up to 28 MPa in simple, reentrant and controlled-clearance configurations were calculated by an iterative analytical technique. It was possible to determine the radial displacements of the piston and the cylinder at any pressure, and the pressure distribution in the clearance with different fluids, and to compute the pressure distortion coefficient of the piston-cylinder unit under conditions of use. Calculations show that the elastic distortion, the pressure distribution in the clearance and the pressure distortion coefficients are not fluid-dependent, at least for two of the most commonly used fluids. In the simple configuration, the radial distortion of the piston-cylinder unit at the maximum pressure is of the same magnitude as the undistorted radial clearance (0,527 μm). Agreement between the experimental and the calculated pressure distortion coefficients is about 1,4% and corresponds to less than 0,6 part per million difference in effective area at 28 MPa. In the re-entrant configuration, the clearance between the piston and cylinder decreases to a minimum value of 0,282 μm at 28 MPa. Agreement between the experimental and the calculated pressure distortion coefficients is within 14,3% and corresponds to 8,0 ppm difference in effective area at 28 MPa. In the controlled-clearance configuration, the distortions of the cylinder are, on average, equal to zero, as expected with controlled-clearance piston gauges, and the pressure distribution in the clearance is the most linear of the three operating modes. Agreement between the experimental and the calculated pressure distortion coefficients corresponds to less than 11,0 ppm difference in effective area at 14 MPa and 28 MPa.