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Effects of the O-ring used for sealing in high-pressure balances on measurements of pressure

An oil-operated pressure balance is standard equipment widely used in the field of pressure metrology. Most of the commonly used pressure balances are in a simple piston-cylinder configuration where the piston and the cylinder can deform freely under pressure. This simple piston-cylinder assembly has an O-ring chamber on the cylinder bottom to seal the cylinder. The effects of this O-ring seal on the effective area are not well known. This paper demonstrates by the numerical method using finite element method and the experimental method that, when performing precise measurements of pressure using a pressure balance, the cylinder will be deformed by the oil pressure exerted on the bottom of the cylinder surrounded by the O-ring, causing a non-linearity in the change of effective area, and that, when the piston-cylinder assembly is used in the body with different diameters of O-ring, the effective area, i.e. the pressure, can be significantly changed. The effects of O-ring seals on the effective area of a DH5306 oil-operated pressure balance were investigated using two piston-cylinder assemblies with nominal effective areas of 2 mm2 over the pressure range of 100-300 MPa. Three different sizes of O-ring, 4.8, 8.0 and 11.7 mm in diameter, were used. The maximum change of the effective area can reach 30 ppm at 300 MPa and 18 ppm at 100 MPa due to the change of O-ring. Therefore, for precise pressure measurement, the correct understanding of the O-ring under the cylinder is required along with checking of the size of the O-ring under the cylinder whenever calibrating the piston-cylinder assembly.