We use value of 141.5 and 131.5 in the calculation of API gravity. How these values have determined?
Answer:
*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.
*API gravity formulas:
The formula used to obtain the API gravity of petroleum liquids is thus:
API gravity = (141.5/SG at 60 °F) - 131.5
Conversely, the specific gravity of petroleum liquids can be derived from the API gravity value as
SG at 60 °F = 141.5/(API gravity + 131.5)
60°F (or 15 5/9 °C) is used as the normal value for measurements and further tables give adjustments for temperature.
(See ASTM D1298)
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:
(141.5/1.0) - 131.5 = 10.0 degrees API.
*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 US Geological Society uses slightly different definitions at Simply put, bitumen sinks in fresh water, while oil floats.
Oil which will not flow at normal temperatures or without dilution is named bitumen and the API gravity is generally less than 10 °API. 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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