Abstract. A comparison in the hydraulic pressure range between 10 MPa and 100 MPa was undertaken as EUROMET Project 389. The comparison was originally between five national metrology laboratories but was subsequently expanded to include a further ten participants. The pilot laboratory was the National Physical Laboratory. This report presents the results obtained by each laboratory for the effective area of the piston–cylinder assembly used in the comparison. The results of all participants were found to be consistent with each other and with the calculated reference value, within their claimed uncertainties, at all pressures. The results are therefore considered to be very satisfactory.

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