A free surface gas/oil interface has been constructed to aid in the comparison of gas piston gauges with oil piston gauges. A coaxial three‐terminal capacitor partially immersed in the oil and partially in the gas permits the determination of hydrostatic heads and differential pressures between the piston gauges. The interface has been used in the comparison of primary standard gas and oil piston gauges with an average standard deviation about mean pressures of 4.5 Pa (6.5×10−4psi) over the range of 0.4 to 4 MPa (60 to 600 psi).