This article describes a combination of experimental and mathematical methods for the determination of undissolved air content in hydraulic oil. The experimental part consists of the determination of the oil bulk modulus, considering the influence of undissolved air by means of a volume compression method in a steel pipe. A multiphase model of an oil/undissolved air mixture is subsequently defined using Matlab SimHydraulics software. The multiphase model permits the volume compression of oil

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... air bubbles independently of each other. Furthermore, time dependencies of pressures are mathematically simulated during the compression of the multiphase mixture of oil and undissolved air for different concentrations of the latter. The undissolved air content is determined by comparing the mathematically simulated and experimentally measured time dependencies of pressure increases. Highlights • Experimental determination of secant bulk modulus and tangent bulk modulus of oil by means of a compression method. • Multiphase mathematical model of compressibility of oil/undissolved air mixture. • Mathematical simulation and measurement of time dependence of pressure during compression of oil/undissolved air mixture in steel pipe. • Determination of undissolved air content in oil by comparing the mathematical model with the measurement. • The undissolved air concentrations in the measured hydraulic system were determined in the range of 0.22 % to 0.49 %.