Clearance Control
The design of a piston-cylinder assembly where the measured pressure is applied to both the internal and external surfaces of the cylinder governs the clearance at high pressures. This configuration, known as a reentrant cylinder, counteracts the natural tendency of the cylinder to expand under internal pressure. By squeezing the cylinder inward, the clearance between the piston and cylinder is kept extremely small and stable.
This maintains the flow rate of the pressure medium within acceptable limits.
Elastic Deformation
Traditional cylinders expand under internal pressure, which increases the gap and speeds up the piston descent. In a reentrant cylinder, the outer pressure forces the cylinder to contract, offsetting this expansion. It ensures a stable effective area in calibrations above one hundred megapascals.
Pressure Compensation
The degree of compensation depends on the ratio of the internal and external surfaces of the cylinder exposed to pressure. When the pressure increases, the reduction in clearance prevents excessive leakage, allowing the piston to float for a longer period. This longer float time gives the metrologist enough time to perform high-precision measurements.
Without this feature, the oil would bypass the piston too quickly at high pressures.
Calibration Integrity
Calculating the effective area of a reentrant cylinder requires precise modeling of the elastic deformation of both the piston and the cylinder under pressure. This characterization is verified through cross-float calibration against a known primary standard. Any structural asymmetry or material defect in the cylinder can lead to uneven deformation, which alters the effective area and increases measurement uncertainty.
This calibration must be performed by a laboratory with low-uncertainty capabilities.
