There are two primary units that tell you how dense a fluid is: Specific Gravity and API Gravity.
Knowing these units and toggling between them can be important in deciding which equipment to use for your operation.
In this article, we’ll provide equations to calculate both specific gravity and API gravity.
Specific Gravity Equation
The specific gravity of a fluid is the ratio between its density and the density of water. You can calculate specific gravity with this simple equation:
Density of the fluid ÷ Density of water = Specific Gravity
Water always has a density of approximately 1,000 kilograms per cubic meter. At 60° F this ends up being 999.016 kg/m³, which is the measurement we will use in our upcoming example.
The specific gravity will tell us whether the fluid will float or sink in the water. If it is higher than 1 it will sink in water; lower than 1 and it will float.
When processing oil, producers, midstream companies, and refiners separate it into various products. These companies use the specific gravities as well as boiling points to efficiently separate these compounds.
API Gravity Equation
API stands for the American Petroleum Institute. This organization standardized a unit called API Gravity.
If you know the specific gravity of a fluid, you can use this equation to calculate its API Gravity:
141.5 ÷ Specific Gravity – 131.5 = API Gravity
API gravity can also tell us whether the fluid will float or sink in the water. If API gravity is less than 10 it will sink in water; higher than 10 and it will float.
Equations in Action
Now that we know how to calculate these measurements, let’s take a look at a real-life example with a sample of crude oil.
To start with, we measure the density of the fluid by dividing the mass (weight) by the volume (amount). In this case, our fluid density is 742.27 kg/m³. Let’s bring that into our equation.
742.27 kg/m³ ÷ 999.016 = .743
This brings us to our Specific Gravity of .743. Now we can bring this into the next equation to find out our API Gravity.
141.5 ÷ .743 – 131.5 = 58.94
With that, we have discovered both the Specific Gravity, and our API Gravity for our particular crude oil sample. If you’re in a position to use your cell phone or computer, you can also use calculation tools to quickly discover these figures.