The air content in hydraulic transmission fluids significantly reduces bulk modulus of the fluid and causes a drop in the stiffness and response of the hydraulic system. It is consequently very important to monitor the air content in hydraulic fluid for ensuring the hydraulic works in good condition. In this paper, a novel method is presented in which the sampled fluid flows slowly into a vacuum chamber and the pressure of separated air is measured. A model of pressure-time characteristics is established, with moisture content taken into account as well, since moisture is volatile in vacuum and its content in tributyl phosphate (TBP) based fluid is usually too high to be neglected. The model can be simplified, which turned out to be a nonlinear least square problem. Comparison between the measured and calculated value shows that the model matches well with the experimental data.

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