What is oil density?

Oil density is a crucial property not only in lubricants but in all fluids. For instance, as the density of a lubricant increases, the fluid becomes thicker. This leads to an increase in the amount of time it takes for particles to settle out of suspension. But before going forward we need to understand what density is?

The density 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 of Lubricants:

Density plays a critical role in how 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.

The density of most oils will range between 700 and 950 kilograms per cubic meter (kg/m3). In oils, it is usually indicated in the temperature of +15°C or +20°C, in units kg/m3. Water has a density of 1,000 kg/m3. This means that most oils will float on water as they are lighter by volume. If the density of an object is less than that of water, then that object will float. This is why if you have a moisture problem in your lube system that the water settles to the bottom of the sump and is drained out first whenever the plug is pulled, or the valve is opened. This is not always the case, as some Group IV base oils can have a higher density than water, effectively causing the oil to sink in the water.

Density Unit Conversion

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

Density Temperature Relation

Density 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 calculation density at the given temperature:

Density Pressure Relation

When the lubricating oil is compressed, density of the 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:

Density Test Standard

ASTM D5002-19: Standard Test Method for Density, 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 utilizing 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 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, 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 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 of oil density calculator. There is an option of embedding the calculator. You can use that calculator.

Densities of some common liquids