Hydraulic fluid is the most vital element in a hydraulic system, performing various functions. Among them, power transmission is the primary one. The basic requirements for satisfactorily performing this primary function include consistent response, optimal efficiency, and safety. To ensure responsive, efficient, and safe power transmission, a hydraulic system needs to be sufficiently stiff. From a hydraulic fluid aspect, this requirement means that the fluid is little compressible over the entire operating pressure range. The commonly used commercial hydraulic fluids, are normally said to be incompressible fluids. However, like almost all liquid fluids, hydraulic fluids do present some very small compressibility in proportion to the operating pressure: about 0.3% at 1 MPa to a little over 1.3% when the pressure increases to 25 MPa. While those compressibility changes of hydraulic fluids may delay the rate of response, especially with the dynamics of the external load commonly existing on mobile hydraulic systems, such a slight change in the response rate is often not a concern. However, such a satisfactory response rate may easily be revoked if there is even a small amount of air being dissolved in the fluids because of its high compressibility. Therefore, one critical measure of the quality of hydraulic fluids is the percentage of gases that can be dissolved in the fluids. The less dissolved gases in the fluids the better.