API Gravity of Petroleum Products:

Before discussing API gravity in detail, lets understand that API gravity of petroleum products is not a measured value. Instead it is a calculated value derived from density and specific gravity of liquid petroleum products.

Another interesting fact about API gravity is that it is not a true value. Instead, it is an average value of the API gravity of every single hydrocarbon present in that product. Since naphtha, reformate, alkylate, gasoline, diesel, jet fuel, black oil and bunkers are all mixtures of hundreds of hydrocarbons, each hydrocarbon contributes its own API gravity and therefore the reported API gravity is always the average API gravity of a homogenous mixture. Further, API gravity is always reported @ 60°F. This is universally accepted to offset the effect of temperature. When API gravity of top, middle, bottom samples taken from a tank is not the same, the tank is said to be stratified, meaning not mixed enough to form a homogenous mixture.

API gravity value plays a very important role in calculating the barrels of product per metric ton. Since most products are bought and sold by metric tons, having a wrong API gravity can have a significant effect on the value of the cargo. The independent laboratories are tasked with the measurements of specific gravities and API gravity of a product being traded. A small difference in API gravity can mean thousands of dollars for ship load cargoes. Field personnel must draw representative samples and lab techs must run the test right to insure reporting of correct API gravity.

Density – specific gravity – API gravity

Density of a liquid is its mass/volume at a given temperature. Most common units are:

  • mg/ml
  • kg/l
  • kg/m3
  • lbs/gal
  • tons/bbl

Density is the only measured value. Density of the same liquid can be different if measured at different temperatures.

When density of a liquid is divided by density of water at the temperature, it becomes specific gravity of that liquid.

Basic difference between density and specific gravity is that density varies with temperature and has units, while specific gravity is independent of temperature and being a ratio has no units.

Calculation of API Gravity from its Density

In order to calculate API gravity from density, the density is first measured either by using a hydrometer or a densitometer in the laboratory. This density of a liquid is divided by density of water at the same temperature. This new value is called specific gravity.