gravity scale

API gravity

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 a measure of the relative density of a petroleum liquid and the density of water, but it is 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 is 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:

API~gravity = frac{141.5}{SG} - 131.5

Conversely, the specific gravity of petroleum liquids can be derived from the API gravity value as

SG mbox{at}~60^circ mbox{F} = frac{141.5}{API~gravity + 131.5}

Thus, a heavy oil with a specific gravity of 1.0 (i.e., with the same density as pure water at 60°F) would have an API gravity of:

frac{141.5}{1.0} - 131.5 = 10.0^circ mbox{API}

Measurement of API gravity from its density

To derive the API gravity from the density, the density is first measured using either the hydrometer, detailed in ASTM D1298 or with the oscillating U-tube method detailed in ASTM D4052. Density adjustments at different temperatures, corrections for soda-lime glass expansion and contraction and meniscus corrections for opaque oils are detailed in the Petroleum Measurement Tables, details of usage specified in ASTM D1250. The specific gravity is then calculated from the formula below and the API gravity calculated from the first formula above.

mbox{SG oil} = frac{rho_mathrm{oil}}{rho_{mathrm{H}_2mathrm{O}}}

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, oil with an API gravity between 40 and 45 commands the highest prices. Above 45 degrees the molecular chains become shorter and less valuable to refineries.

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 Survey 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.

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