^4fPISTON FALL RATE IN PRESSURE BALANCES: IMPROVEMENTS OF THE STOKES MODEL

摘要

^7fPiston fall rates for three pressure balances operating in liquid media and having different full scale values up to 20, 100 and 500 MPa are analyzed. Piston fall rates measurements were experimentally made taking great care for avoiding micro leaks in the apparatus and reducing the uncertainty to a few percent level. Fall rates have been calculated by FEM methods using at first Navier-Stokes fluid model. Discussions about the validity of the Navier-Stokes model for micro channels is considered in order to explain differences between experimental and FEM calculated fall rates. The clearance between the piston and cylinder has been considered as a micro channel, with high pressure axial gradients and fluid flowing in the clearance. Experimental and calculated piston fall rates results are compared and the main findings are discussed with the purpose of reducing their differences.