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Feasibility of directly measuring piston-cylinder distortion

Piston-cylinder pressure balances are widely used as pressure standards. Under pressure the clearance between the piston and cylinder increases by an amount that varies along the piston-cylinder engagement length. Recently, finite element techniques have been specifically tailored to model this piston-cylinder distortion and the related pressure gradients and flow in the operating fluid. While the finite element techniques allow comparison of the effects of piston-cylinder design modifications, they still need to be "calibrated" with empirical data. In this study a number of novel ways are considered in which the distortion (or lateral displacement) of piston-cylinders might be measured as they are pressurized. For such data to be of value in refining and confirming the validity of finite element models, an uncertainty of about 4 nm (k = 2) is required in the physical measurement of the piston-cylinder distortion. Techniques considered range from discrete sensing of changes in the piston-cylinder gap, such as embedded capacitive, inductive or fibre-guided interferometric probes, to full-field sensing such as optical interferometry, X-ray or optical thin-film measurements and frustrated total internal reflection.