Modern Physics

API Gravity: Crude Oil Scale and Classification

The API gravity is a specific gravity scale used by the American Petroleum Institute (API) to express the relative density of the crude oil and other petroleum products. Although by definition it is a dimensionless quantity, as we’ll see shortly, it is always expressed in API degrees.

On this scale, crude oils are classified as light, medium and heavy. This is very important in determining the market value of crude oil, as a light oil requires less refinement than a heavy one.

Oil quality depends on the composition and proportions of the mixture of hydrocarbons, which have a wide range of molecular weights. Although API gravity does not take into account the actual chemical composition of crude oil, but classifies it according to ease of transport and refining.

For classification, the following empirical formula is used:

ºAPI = (141.5 / γ T ) – 131.5 (fluids lighter than water)

γ T is the specific gravity of the fluid under standard temperature conditions, ie at 15,556 °C or 60 °F.

This is the standard temperature for any operation with crude oils and their derivatives, whether chemical or commercial. For other fluids, a temperature of 4°C is considered standard.

The specific gravity of a fluid

Specific gravity, also called specific gravity or relative density, is a dimensionless quantity that compares the density of a fluid with that of water.

Density depends on temperature and, as the oil is sold in volume, temperature has a decisive influence, as it is capable of producing appreciable variations, which are reflected in the price.

Therefore, the petroleum industry has set a standard temperature of 60°F to determine density and all related quantities in some way.

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Specific gravity γ T is defined mathematically as:

γ T = fluid density / water density at reference temperature (60°F)

Both the fluid and water density must be in the same system of units and were measured under the same pressure and temperature conditions. In this way, specific gravity lacks units.


The hydrometer (in English, hydrometer , erroneously translated as hydrometer, a device used to measure the flow), is a device to measure the relative density or specific gravity of a liquid.

His invention is attributed to the mathematician Hypatia of Alexandria (315-345). The apparatus consists of a hollow glass cylinder with a weight or ballast at one end, which helps to float vertically.

It is carefully introduced into the liquid whose density must be measured and left to rest until it stabilizes.

The density of the liquid depends on how far the hydrometer sinks into it. In light liquids, the instrument sinks more than in denser liquids. Therefore, the instrument provides a scale for reading.

There are hydrometers designed specifically for certain fluids, such as milk (lactometers) or wine (breath meters), to name a few. For raw products, the API densimeter is used, whose scale is already graduated in API degrees to avoid conversion. For example, a reading greater than 10 means an oil lighter than water, which floats in it.

And as temperature is a very important parameter, there are hydrometers that already come with a built-in mercury thermometer. They are known as thermo-hydrometers and are very suitable for field testing mainly.

The brute and its API gravity

API grades in oils range from 10 to 80, with most in the 20 to 70 API range.

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From the formula given at the beginning, the specific gravity is eliminated as:

γ T = 141.5 / (131.5 + °API)

The most usual ranges from 0.93 to 0.70. Note that heavier oils correspond to lower API values. On the other hand, higher API gravity values ​​indicate oils of greater commercial value because they are easier to process and transform.

This is the norm of up to 45 degrees API, because in addition to this value, in crude oil there is a preponderance of short hydrocarbon chains, which makes processing difficult.

Gross Rating

According to the value it generates on the API scale, oil is classified as:

– Light or light , API grade greater than 31.1, in which low molecular weight hydrocarbons predominate. It is a crude oil that is easy to transport and is in great demand for the manufacture of gasoline, diesel and kerosene.

– Medium or medium , whose API class is in the range of 29.9 to 22, has a good concentration of low molecular weight hydrocarbons, which also facilitates transport.

– Heavy , API grade between 21.9 and 10, it is abundant in medium molecular weight hydrocarbons, which makes transport difficult. Customs, oils of various types, paraffins and polymers are obtained from this type of crude oil.

-Overweighted , whose API is 10 or less, is more complicated to transport and process and therefore has less commercial value.

API severity varies by region, for example Latin American oil averages 25.1º API, while Middle East oil is light at 34º API.

Experimental determination of API gravity using the hydrometer

These are the main steps to take to obtain the API grades of a sample using the hydrometer :

– The temperature is adjusted according to the nature of the sample, important if they are volatile substances.

– The temperature of the sample must be the same as that of the sample.

– Transfer the sample carefully to the clean, unwashed sample. It is necessary to avoid the formation of air bubbles and minimize evaporation in case of volatile liquids.

– If bubbles appear, they can be removed by gently touching the surface with a filter paper.

– It must be ensured that the ambient temperature does not undergo considerable changes, not exceeding 2°C.

– The hydrometer is slowly submerged; when it is well centered, it sinks a little deeper and is released, trying to keep the rest of the tube dry. It must be kept away from the walls of the test tube.

– Wait a while for all air bubbles to reach the surface, especially if the sample is very viscous.

– Make a note of the temperature before reading. To do this, carefully stir with the thermometer, completely submerging the column of mercury. If the thermo-hydrometer is used, it is also shaken with the instrument by gentle vertical movements.

– The nearest mark can now be read on the device, where the liquid surface crosses the scale.

– Record temperature immediately after reading. If there was a noticeable variation, the measurement should be repeated.

– In opaque liquids, a correction must be introduced. The corrected reading is the severity of the API sought.

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