What is API Gravity?
API gravity is short for American Petroleum Institute gravity, an inverse measure that is used to determine the weight of petroleum liquids in comparison to water. If a liquid has API gravity of more than 10 it is considered a light oil that floats on water. If the liquid’s API gravity is less than 10 it will sink and falls into the heavy oil category. While API gravity essentially measures the relative density of petroleum liquid and water it is primarily used to evaluate and contrast the relative densities of petroleum liquids.
In mathematical terms API gravity has no dimensions. However the measure is gradated in degrees using a purpose built hydrometer instrument. Thanks to a strategic API scale design most petroleum liquids will be categorised between 10 and 70 API gravity degrees.
The original technique used to measure the gravity of liquids was called the Baumé scale. It was developed in France in 1768 and officially accepted by the U.S. National Bureau of Standards in 1916. After encountering a series of errors and variations the American Petroleum Institute refined the scale and created API gravity. This is now widely used across the globe.
The official formula used to derive the gravity of petroleum liquids from the specific gravity (SG), as follows:
The relative density of petroleum liquids can also be uncovered by using API gravity value:
Using the following formula, API gravity can also be used to calculate how many barrels of crude oil can be produced per metric ton. Given that the weight of an oil plays an integral role in establishing its market value this formula is incredibly important!
In general oils with API gravity of 40 – 45 generate the highest market prices. Any oils with API gravity of 45 or over have shorter molecular chains which are less desirable to refineries. Below is an overview of the four major crude oil classifications:
Any crude oil with API gravity of over 31.1 degrees falls into the light crude oil category.
Oils with API gravity falling between 22.3 and 31.1 degrees are classed as medium crude oils.
Heavy crude oils have API gravity of under 22.3.
Also referred to as bitumen, extra heavy crude oils have API gravity of below 10.0 degrees.
While these are accurate classifications it’s important to note that exact differentiations between light, medium, heavy and extra heavy will vary depending on the region of origin. At the end of the day, fluctuations are largely based on current oil commodity trading.