Editing API gravity (section)

{{Short description|Measure of how heavy or light a petroleum liquid is compared to water}}
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 is lighter and floats on water; if less than 10, it is heavier and sinks.

API gravity is thus an inverse measure of a petroleum liquid's [[density]] relative to that of water (also known as [[specific gravity]]). It is used to compare densities of [[List of crude oil products|petroleum liquids]]. For example, if one petroleum liquid is less dense than another, it has a greater API gravity. Although API gravity is mathematically a [[dimensionless quantity]] (see the formula below), it is referred to as being in 'degrees'. API gravity is graduated in degrees on a [[hydrometer]] instrument. API gravity values of most petroleum liquids  fall between 10 and 70 degrees.

In 1916, the U.S. [[National Bureau of Standards]] accepted the [[Baumé scale]], which had been developed in France in 1768, as the U.S. standard for measuring the [[specific gravity]] of liquids less dense than [[Density of water|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.<ref>[http://www.sizes.com/units/hydrometer_api.htm API Degree history]</ref>