Density of Lubricating Oil: 2 Key Facts You Must Know

What Is the Density of Lubricating Oil?

Density of Lubricating Oil is a crucial property in all lubricant applications. As temperature, pressure, and composition vary, the density of lubricating oil changes significantly, affecting performance, pumping efficiency, and compatibility. But before going forward, we need to understand what density is?

The density of lubricating oil is also known as the specific mass, is its mass per unit volume. Mathematically, density is defined as mass divided by volume.

The formula for density is d = M/V, where d is density, M is mass, and V is volume.

Density offers a convenient means of obtaining the mass of a body from its volume or vice versa; the mass is equal to the volume multiplied by the density (M = Vd), while the volume is equal to the mass divided by the density (V = M/d). The weight of a body, which is usually of more practical interest than its mass, can be obtained by multiplying the mass by the acceleration of gravity.

Measuring Density Lubricating oils:

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

Most oils have a density between 700 and 950 kg/m³, usually measured at +15°C or +20°C. Since water’s density is 1,000 kg/m³, most oils float on water. This allows water in a lube system to settle at the bottom and be drained first. However, some Group IV base oils can be denser than water, causing them to sink instead.

Density Unit Conversion

Here is a simple density (and viscosity) units conversion tool:

How Temperature Affects Density of Lubricating Oil

The density of lubricating oil depends on temperature, even though the dependency is relatively small compared to lubricant viscosity. Here is an empirical formula that can be used to calculate the change of density with temperature (Grease Lubrication in Rolling Bearings):

(1)  

where for and for . As it can be seen, this empirical relation applies only for the oils having the densities in a specified range, however, this range covers most commonly used lubricating oils (860-980 )

Here is a simple calculator that uses this equation to calculate the density of lubricating oil at the given temperature:

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3 + 7 =

Pressure Effects on Lubricating Oil Density

When the lubricating oil is compressed, density of lubricating oil increases. This increase starts to be noticeable at relatively high pressures (>0.1GPa), which is however quite common for Elastohydrodynamic conditions (EHL). In EHL, the most widely used formulat to describe the change of oil density with pressure is known as Dowson an Higginson density equation:

(2)  

Here is a simple density calculator based on pressure:

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Standard Test Methods for Density of Lubricating Oil

ASTM D5002-19: Standard Test Method for Density of Lubricating Oil, Relative Density, and API Gravity Of Crude Oils By Digital Density Analyser. This test method covers the determination of the density, relative density, and API gravity of crude oils that may be handled generally as liquids at test temperatures between 15 °C and 35 °C utilising either manual or automated sample injection equipment. This test method applies to crude oils with high vapour pressures, provided appropriate precautions are taken to prevent vapour loss during the transfer of the sample to the density analyser.

This test method was evaluated in an interlaboratory study testing using crude oils in the range of 0.75 g/mL to 0.95 g/mL. Lighter crude oil may require special handling to prevent vapour losses.

ASTM D1298-12: Standard Test Method for Density of lubricating oil, Relative Density (Specific Gravity), or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method. This test method covers the laboratory determination using a glass hydrometer in conjunction with a series of calculations of the density, relative density, or API gravity of crude petroleum, petroleum products, or mixtures of oil and nonpetroleum products generally handled as liquids, and having a Reid vapour pressure of 101.325 kPa (14.696 psi) or less. Values are determined at existing temperatures and corrected to 15°C or 60°F by means of a series of calculations and international standard tables.

Density of Oil Calculator:

The recalculation of the density of lubricating oil for different temperature and pressure values. Formulas are taken from Russia’s GOST R 8.610-2004. “State system for ensuring the uniformity of measurements for the density of oil. The tables for recalculation” standard. Used formulas are listed below the calculator.

Note: https://planetcalc.com/2834/ Link for density of lubricating oil calculator. There is an option of embedding the calculator. You can use that calculator.

This table lists the density of lubricating oil at different temperatures.

Check What is Lubrication

https://youtu.be/TROM4gXPIdk?si=hs6Lw1IS73ljz7rI

References

1. https://www.astm.org/Standards/D4052
2. https://www.britannica.com/science/density
3. DOWSON, D., JONES, D. Lubricant Entrapment between Approaching Elastic Solids. Nature 214, 947–948 (1967). https://doi.org/10.1038/214947b0