Improvement in precise estimation of the pressure distortion coefficient for controlled-clearance piston-cylinders at higher pressures

Pressure balance equipped with a controlled-clearance (CC) piston-cylinder has been developed with the aim of improving hydraulic pressure standards in the pressure range of up to 1 GPa. To estimate the pressure distortion coefficient of the CC piston-cylinder, we performed characterization experiments based on the Heydemann-Welch (HW) model. In this paper, we focused on the change in the piston fall-rate in response to the applied jacket pressure for estimating the zero clearance jacket pressure. Some piston-cylinders exhibited characteristics at high system pressures that differ from those predicted by the HW model. Specifically, deviations from the linear relationship were observed in the plot of jacket pressure against cubic root of the piston fall-rate. To address this problem, a method was investigated for altering deformational characteristics of the CC piston-cylinder. In particular, the area of the outer surface of the cylinder which is subjected to the jacket pressure was changed by using sleeves of different length. A 0.5 MPa/kg piston-cylinder with a shorter sleeve showed better linearity at 500 MPa, resulting in the estimation of the zero clearance jacket pressure with smaller error. It is found that modifying the sleeve length and its position in the housing is an effective method for obtaining deformational characteristics that are suitable for applying linear approximation to the relationship between the jacket pressure and cubic root of the piston fall-rate.