Air dissolved in hydraulic fluid (not free air nor entrained air) is so elusive in the way it appears and disappears within the system that most hydraulic system designers are not familiar with dissolved air (or gases). Undoubtedly, however, they have experienced (to their dismay) the effects of entrained and/or free air in the system.

Dissolved air comes out of solution

Because air in solution in a hydraulic fluid has only a minor effect on hydraulic performance, why should the designers be concerned with it? The answer is that even if all the air bubbles and pockets were removed from a hydraulic system totally enclosed, after a short run-in period, air bubbles would begin to reappear. The source of these bubbles is the working fluid itself because all fluids (except fully deaerated ones) contain dissolved air that, under certain conditions, comes out of solution and plagues the system.

Air generating test

How hydraulic fluids “generate” air can be shown by the following test.

A conventional, double-acting actuator was connected to a 1-gpm, 3000-psi hydraulic system filled with MIL-H-5606 fluid, Figure 1. It was instrumented with a device, which measures the amount of entrained and free air in any air-oil mixture, Figure 2.  Figure 3 is a device, used in tests described later in the article, to measure the amount of dissolved air in a fluid.

Starting with an empty actuator, flushing began at low pressure, about 300 psig. The cylinder was cycled with a 4-way valve with flow passing through an .0032-in diameter orifice at a cylinder port. The purpose of the orifice was to stimulate the area of a .0032-in diameter valve opening. After every six cycles, the test stand was shut down and an air measurement taken. Figure 4a illustrates the low pressure flushing cycles.

In addition, the cylinder was periodically pressurized to 1000,2000, and 3000 psig. The amount of fluid required to achieve these pressures was measured and recorded, Figure 5.a. The effective bulk modulus at 3000 psig was then computed for various air contents using the values obtained, Figure 5b.

It was startling to discover that, with a content of only 0.17% of compressible air, the effective bulk modulus was only half of the theoretical bulk modulus. The first inclination is to take solace from the fact that careful flushing had brought the compressible air content down to 0.17%.

After flushing, test stand pressure was increased to 1000 psig. After three cycles, the air was measured and found to be .08%. After six cycles, it was 2.6% and so on, Figure 4b. In short, dissolved air came out of solution and collected in the actuator. It is this form of air that negates normal fill and flush techniques.

Dissolved air increases weight, not volume

It should be emphasized that neither the presence nor the absence of dissolved air affects the volume of the oil. And test data seems to indicate that there is no effect on bulk modulus, providing the air is in solution. These facts, at first, appear paradoxical. However, if one visualizes a container filled to the brim with marbles (which represent the oil molecules), it is possible to pour in fluid (representing air) around them, or remove the fluid with no change in volume. The weight of the container changes, not the volume.