The presence of air in the fluid of a working hydraulic system can cause significant performance problems. Furthermore, the air may be entrained or dissolved, which is difficult to detect. Only the most experienced hydraulic engineers can attribute lackluster system performance to the presence of air in the working fluid. Some of the adverse effects of air in oil are illustrated in Figure 1.

Figure 1. Problems Associated with Air in Hydraulic Oil

Part of the difficulty in the diagnosis of air is that a reliable and practical air measurement device has never been made available outside of academia. A practical air measurement device would help hydraulic system developers and users assess and determine performance problems due to aeration. Hydraulic system modelers might also benefit from this type of information by incorporating aeration data into their analytical models.

Out With the Old . . .
Researchers used methods such as sonic velocity and turbidity measurements in the past to measure aeration. However, none of these methods were developed for commercial use. Sonic velocity and turbidity are applicable only to the measurement of entrained air (bubbles) and are not effective at assessing dissolved air. In the absence of practical devices, hydraulic engineers were forced to rely on intuition and qualitative visual inspections. Figure 2 provides a typical qualitative correlation between visual appearance and the percent of entrained air by volume for a typical hydraulic oil

Figure 2. Qualitative Appearance of MIL-L-2104B
With Varying Amounts of Entrained Air

. . . . In With the New
The Aeration Measuring Device (AMD) has the unique ability to sample and evaluate oil from pressurized lines in an operating hydraulic system (Figure 3).

Figure 3. The FES Aeration Measuring Device (AMD)

The AMD utilizes a mini slip-on gauge probe, which works in conjunction with a mini slip-on gauge plug that can be installed at any pressurized location (up to 2,200 psig). Figure 4 illustrates the AMD about to be attached to a gauge plug that is installed in a typical hydraulic tee fitting.

Figure 4. The AMD Ready for Attachment to Hydraulic System

A screw-on cap is provided with the gauge plug to protect it when not in use. An optional high-pressure probe and plug, which extends the sampling pressure capability to 5,000 psi, is now available. Attaching the AMD to the plug requires only 12 pounds of force for every 1,000 psi of system pressure.

How it Works
The primary components of the AMD are a barrel and plunger, manual crank, digital scale, manually adjustable metering valve, pressure gauge for monitoring barrel pressure (140 psig max) and a slip-on gauge probe. The digital scale monitors the plunger position and the manual crank is used to purge the translucent barrel of sample fluid so that the sampling procedure may be repeated. The sampling procedure requires the AMD to initially be purged of all air.

The digital display is set to zero and the metering valve is carefully opened. When the desired amount of sample is in the AMD, the valve is closed, the probe is removed from the gauge plug and the initial digital display reading is recorded. A vacuum is then created in the barrel by pulling back on the plunger using the crank handle. The vacuum separates the entrained and dissolved air from the oil. The operator then tilts the probe upward, opens the bleed valve and expels all air from the device. The final volumetric digital display reading is recorded and compared to the initial reading for determining the amount of air present. The procedure is virtually the same for reservoir sampling.

The AMD created by FES Inc., has been evaluated using a special aeration test stand. The test involved injecting a known amount of air into the system fluid and the fluid circulated until the conditions became uniform. The test was repeated for different aeration levels, with a deaeration period between each test run. Table 1 shows the average results of four measurements taken at each aeration level to insure reliability of the readings. As the fluid became more aerated, a reduction in sample size was necessary in order to be able to pull an adequate vacuum on the sample.

Table 1. Average Results of Four Measurements Taken at Each Aeration Level to Ensure Readings Reliability