Effective fluid bulk modulus has significant influence on the stiffness and dynamic performance of hydraulic systems. It is always varied with different working parameters as air content, fluid pressure, and temperature. To estimating effective bulk modulus accurately is very important when designing high performance hydraulic systems. In this study, a measuring device was developed based on the definition of bulk modulus, and the measurement accuracy was estimated. Experiments were carried out using L-HM46 antiwear hydraulic fluid at approximately constant temperature, the results show that the good repeatability and pressure resolution can be achieved. Results also show that when pressure is low, the bulk modulus increases strongly as pressure increases; pressures over 8.5 MPa have slight influence on the bulk modulus. Effective fluid bulk modulus can be expressed as a function of working pressure when the air content is certain.

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