Compressibility Measurements of Hydraulic Fluids in the Low Pressure Range

This paper presents a test rig for compressibility measurements of hydraulic fluids. Today a lot of effort is put on simulation of low pressure lines in hydraulic systems. These simulations are important for increasing efficiency of fast switching applications like hydraulic buck converters or for the reduction of pressure pulsations in suction pipes of pumps. Simulation of high pressure systems is quite straight forward. The fluid parameters like density and bulk modulus can be assumed to be constant and as a result of this the flow equations are very simple e.g. the wave speed in a pipe is constant. For low pressure simulations these assumptions are not true any more. For example, air release in low pressure regions generates small air bubbles which cause dramatic changes in the fluid parameters like density and compressibility. As a result, simulations are only accurate for a special operating point with small variations. In the more general case of large pressure variations around a low pressure operating point, wave speed and the shape of the pressure pulses change as the previously discussed parameters do. The aim of this paper is to present a test rig and first measurements for compressibility identification of hydraulic oil in the low pressure range. Because of the importance of the initial oil-air ratio, the test rig has the capability to evacuate the fluid chamber and a device to incorporate an exactly defined volume of air. As a result pressure dependent compressibility graphs, for a defined value of incorporated air, can be recorded. Finally all problems and results are discussed; applications for simulations and further measurements will be stated.