Measuring Relative Density of Lubricants

Noria Corporation
Tags: oil analysis

“What temperature is best to measure the relative density of a lubricant in order to calculate its volume?”

Density plays a critical role in how a lubricant functions as well as how machines perform. Most systems are designed to pump a fluid of a specific density, so as the density begins to change, the efficiency of the pump begins to change as well.

The ASTM D1298-12b Standard Test Method for Density, Relative Density, or API (American Petroleum Institute) Gravity of Crude Petroleum and Liquid Petroleum Products states that accurate determination of the API gravity, density or relative density (specific gravity) uses a standard temperature of 60 degrees F (15 degrees C).

In Layman’s terms, density is the mass of an object relative to the volume it occupies. Mathematically, density, mass and volume are related according to the following formula:

ρ=m/V   where ρ=density, m=mass and V=volume.

The density of most oils will range between 700 and 950 kilograms per cubic meter (kg/m3). By definition, water has a density of 1,000 kg/m3. What this means is that most oils will float on water as they are lighter by volume. This is not always the case, as some Group IV base oils can have a higher density than that of water, effectively causing the oil to sink in the water.

The API measurement of density is reported a little differently. This measurement uses a comparison to water on an inverse scale. Water is represented by a 10 on the scale. Anything greater than 10 has a lower density than the water and will float upon it. Anything less than 10 will be heavier and sink in the water. Below is a chart displaying how API relates to specific gravity and weight per volume.

Keep in mind that as density increases, so too does the erosive potential of the fluid. In high turbulence or high-velocity regions of a system, the fluid can begin to erode piping, valves or any other surface in its path.

Not only are solid particles affected by the density of a fluid, but so are contaminants such as air and water. Both of these contaminants have a marked impact on density. Oxidation influences the density of a fluid as well. As oxidation progresses, the density of the oil increases.

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