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Gases are soluble
in mineral oils to a limited amount.
The amount varies with the type
of gas and oil temperature. For
example, 8 to 9% of air, by volume,
is soluble in mineral oil at room
temperature and is invisible. Dissolved
gases affect oil viscosity, bulk
modulus, heat transfer, oil and
metal oxidation, boundary lubrication,
foaming and cavitation. Boundary
lubrication is improved by the oxygen
in dissolved air because it continuously
repairs the protective oxide films
on metals. Dissolved oxygen is considered
an important anti-scuff component.
The amount of dissolved gas become
evident when gases come out of solution
vigorously when the oil is subjected
to low pressures. |
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The amount
of soluble gas is measured by ASTM
D 2780 "Solubility of Fixed
Gases In Liquid Test". This
method physically separates the
gas through an extraction process
and measures the quantity volumetrically.
This method allows for subsequent
qualitative analysis of the extracted
gas by any appropriate technique. |
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If the amount of a gas in oil exceeds saturation, small bubbles will form, remain suspended, and the oil will appear hazy. This is called entrained gas. The bubbles slowly rise to the surface. Bubbles of a gas, such as air, in an oil film cause holes that reduce oil film continuity and decrease the film's ability to prevent solid-to-solid contact. |
The relative tendency of various oils to release entrained gas is measured by a gas bubble separation method ASTM D 3427. The method uses a cylinder-like test vessel with gas inlet and outlet ports. Air, or another gas (if of interest), is introduced into the bottom of the vessel at a specified temperature and flow rate. At the end of seven minutes the gas flow is stopped and the change in density as measured by a densitometer is recorded. The test is complete when the total volume of entrained air is reduced to 0.20% by volume. The results are reported as the time it took for the oil to attain this value. |
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Foaming is defined as the production and coalescence of gas bubbles on a lubricant surface. Foam may be a result of a variety of problems including air leaks, contamination, and over filling of sumps. Foaming can cause loss of oil out of a vent and serious operational problems in most lubricated systems. Excessive foam can starve bearings and pumps of liquid lubricant (pump cavitation) causing failure, and cause poor performance in hydraulic systems. The foaming characteristics of an oil are measured by ASTM D-892. Using a calibrated porous stone, air is blown into the bottom of a graduated cylinder for a specified time. Immediately upon completion of the blowing period, the foam that has formed on the top of the oil is measured. Ten minutes after the completion of the blowing period, an additional measurement is made of the remaining foam as the foam retention characteristics of the oil. The results are reported in milliliters. |
Most commercial lubricants contain foam inhibitors, as these inhibitors reduce foaming. A few ppm of silicone is commonly used
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