API gravity Totally Explained

Everything about Api Gravity totally explained

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it's lighter and floats on water; if less than 10, it's heavier and sinks. API gravity is thus a measure of the relative density of a petroleum liquid and the density of water, but it's used to compare the relative densities of petroleum liquids. For example, if one petroleum liquid floats on another and is therefore less dense, it has a greater API gravity. Although mathematically API gravity has no units (see the formula below), it's nevertheless referred to as being in “degrees”. API gravity is graduated in degrees on a hydrometer instrument

and was designed so that most values would fall between 10 and 70 API gravity degrees.

History of development

The U.S. National Bureau of Standards in 1916 established the Baumé scale (see degrees Baumé) as the standard for measuring specific gravity of liquids less dense than water. Investigation by the U.S. National Academy of Sciences found major errors in salinity and temperature controls that had caused serious variations in published values. Hydrometers in the U.S. had been manufactured and distributed widely with a modulus of 141.5 instead of the Baumé scale modulus of 140. The scale was so firmly established that by 1921 the remedy implemented by the American Petroleum Institute was to create the API Gravity scale recognizing the scale that was actually being used.

API gravity formulas

The formula used to obtain the API gravity of petroleum liquids is thus: ť

API~gravity = 	frac

Direct Measurement of API gravity (Hydrometer method)

This method of measurement is similar to the method above except that the hydrometer is graduated with API gravity units instead. This method gives the advantages of field testing and on-board conversion of measured volumes to volume correction. This method is detailed in ASTM D287.

Classifications or grades

Generally speaking 40 to 45 API gravity degree oils have a greatest commercial price and values outside this range have lower commercial price. Above 45 degrees API gravity the molecular chains become shorter and less valuable to a refinery. Crude oil is classified as light, medium or heavy, according to its measured API gravity.
   Light crude oil is defined as having an API gravity higher than 31.1 °API
   Medium oil is defined as having an API gravity between 22.3 °API and 31.1 °API
   Heavy oil is defined as having an API gravity below 22.3 °API.
   Not all parties use the same grading. The United States Geological Society uses slightly different definitions. Simply put, bitumen sinks in fresh water, while oil floats.
   Crude oil with API gravity less than 10 °API is referred to as extra heavy oil or bitumen. Bitumen derived from the oil sands deposits in the Alberta, Canada area has an API gravity of around 8 °API. It is 'upgraded' to an API gravity of 31 °API to 33 °API and the upgraded oil is known as synthetic crude.

Further Information

Get more info on 'Api Gravity'.

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