Measurement of Effective Bulk Modulus for Hydraulic Oil at Low Pressure

Oil properties are very important input parameters for the simulation of hydraulic components. Precise values of effective bulk modulus at low pressures are especially required to improve the simulation accuracy of the pumps suction side or of cavitation in pumps or valves. So far, theoretical equations to compute the effective bulk modulus of hydraulic oil have not been experimentally verified, and only poor measured data are available to calculate the effective bulk modulus at low pressure. Therefore in this paper, the theoretical equation was verified for effective bulk moduli based on measurements of pressure change as a function of volume change at low pressures, varying temperature, entrained air content, and type of state change. Furthermore, the comparison of effective bulk moduli calculated with three different methods (mass-change, volume-change, and sound-speed method) shows that the effective bulk modulus can be calculated well from the measurement results of all three methods. The calculated effective bulk moduli values show little variation among the methods. Additionally, the release pressure of dissolved air in oil and the change of the polytropic gas constant depending on the speed of volume change rate were identified in this study.