What is the API gravity of oil and how is it calculated?

The API gravity of the oil is a measure made to the oil that was created by the American Petroleum Institute to classify the oil in the knowledge to know how heavy or light it can be, for this it is important to know that the American Petroleum Institute is based on being able to compare oil with water as that universal liquid fluid.

Based on this comparison, the value of the API degrees of an oil that is >10 is because it is light and can float on water, on the contrary if it is <10 is because it is a heavy oil and sinks in water.

The API gravity of petroleum can be conceptualized in an order of ideas where it can be defined as the inverse measure of the density of petroleum when related to the inverse density of water.

The API gravity is extended to petroleum derived liquids, thus allowing to evaluate the quality of petroleum by-products such as gasoline and diesel engine oils.

As mentioned before, being the inverse of density, this helps us to affirm that oil or any of its derivative liquid fluids when comparing one with the other and seeing that one has a lower density than the other, it is because it also has a higher API gravity than the other.

Now that we know what the American Petroleum Institute actually seeks to calculate the API degrees of oil, it would be very convenient to know what is the mathematical equation that allows us to calculate the API degrees of oil:

As can be seen in the above equation, we need to know the specific gravity of petroleum (SG petroleum), for this we have to focus our learning of fluid mechanics, in particular in that subject when I studied the petroleum engineering program I was taught that the specific gravity of a fluid is calculated by dividing the density of oil by the density of water.

Regarding the density of water at normal conditions of 1 atmosphere of pressure and 4 degrees Celsius of temperature, we find that the density of water is 1000 kg / cubic meter, the variation of the density of water when the temperature varies is not much, for example when it is at 60 degrees fahrenheit (15.5 degrees Celsius) the density is 999.2 kg / cubic meter, which is why we can consider the density of water at normal conditions of pressure and temperature to 1000 kg / cubic meter.

If we follow the same normal conditions of pressure and temperature where we can estimate 60ºF (15ºC), for this range we can find oil ranging from 800 Kg/cubic meter to 980 Kg/cubic meter.

Example in the calculation of the API gravity of oil at densities of 820 kg/cubic meter.

• Calculation of the °API of the oil from 820 Kg/cubic meter:

The first thing we do is to calculate the specific gravity (SG petroleum)

Having obtained the specific gravity of this oil, which gave a value of 0.82, we can then calculate its specific gravity:

Conclusion

With the value of 41.06 ºAPI, we can classify this oil of 820 Kg/cubic meter as a light oil, since it has an API gravity greater than 31.1º, so we would be talking about an oil of high commercial value for being in the range of 40-45 ºPI.

In other cases, as the density of the oil increases and equals the density of water of 1000 Kg/cubic meter or even exceeds it, then we would be talking about an oil that is in the range of heavy to extra heavy oil, which although it has little commercial value, it is still oil, and with state-of-the-art technologies high quality derivatives can be obtained.

References

API Manual of Petroleum Measurement Standards, Chapter 11.1 – 1980,Volume XI/XII, Adjunct to: ASTM D1250-80 and IP 200/80.

What is API Gravity?

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