Fuel for Thought
What is API Gravity?
Feb 08 2015
Have you heard of API gravity? If you don’t work in the petroleum industry, this term might have passed you by. Which is why we’ve put together this useful guide covering what it is, how it’s used and where it came from.
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 origins of API gravity
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:
API gravity = 141.5/SG – 131.5
The relative density of petroleum liquids can also be uncovered by using API gravity value:
RD at 60oF = 141.5 / (API gravity + 131.5)
A key formula for establishing barrels of crude oil per metric ton
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!
Barrels of crude oil per metric ton = 1 / [141.5 / (API gravity + 131.5) x 0.159]
API gravity classifications and grades
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:
Light crude oil
Any crude oil with API gravity of over 31.1 degrees falls into the light crude oil category.
Medium crude oil
Oils with API gravity falling between 22.3 and 31.1 degrees are classed as medium crude oils.
Heavy crude oil
Heavy crude oils have API gravity of under 22.3.
Extra heavy oil
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.
If you’d like to learn more, take a look at this article: Density Measurement in the Petroleum Industry. Density is one of the most prevalent physical property used to classify and characterise fluids, not only in the environmental, cosmetics, pharmaceuticals, and food and beverage industries, but also in hydrocarbon processing industry (HPI). Used in conjunction with the specific-industry API Gravity scale, its accurate and precise measurement is necessary for converting the measured volumes into volumes at reference temperatures (15°C; 20°C and 60°F).
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