The API Manual of Petroleum Measurement Standards (MPMS) is the authoritative document on crude measurement for measurement professionals.
The document outlines the important elements throughout the process of moving crude oil from the wellhead to the refinery. Understanding these elements ensures accurate and reliable measurements during each phase of custody transfer.
There are many important elements that measurement professionals need to understand. Two of the most important elements are understanding the API Gravity calculation and how to use a tool to generate accurate measurements throughout the chain of custody.
What is the API Gravity Calculation?
API developed a specific gravity scale called the API Gravity that determines the heaviness of crude oil.
The formula is … API Gravity = (141.5/SG) – 131.5.
In the formula, SG refers to the specific gravity of the substance referenced to another substance. In crude measurement, the reference substance is pure water. If the crude oil had the same density as water, then the SG would be 1.0.
The resulting value is expressed in degrees between 10 and 70. In this example, the formula result of “(141.5/1.0) – 131.5” would be 10 degrees.
Once this calculation is determined, the API Gravity determines the classification of the substance:
- Light crude oil: API Gravity greater than 31.1 degrees
- Medium crude oil: API Gravity between 22.3 and 31.1 degrees
- Heavy crude oil: API Gravity between 10.0 and 22.3 degrees
- Extra heavy oil: API Gravity below 10.0 degrees
Tool to Calculate the API Gravity
As you can see in the API Formula, a critical piece of the puzzle is accurately calculating the Specific Gravity.
A hydrometer is an instrument used for measuring the relative density of liquids based on the concept of buoyancy.
Hydrometers are typically calibrated and graduated with one or more scales such as specific gravity. Hydrometers are a key tool in establishing specific gravity of crude oil using water as a reference.
As the density of liquids varies by temperature, API establishes a base condition of 60 degrees Fahrenheit for calculating SG. Therefore, a hydrometer must be used when the instrument and all liquids are at a temperature of 60 degrees.
When the substance settles on 60 degrees, a reading is taken from the hydrometer scale. When performing these measurements, the hydrometer should be cleaned, and a second reading should be taken to ensure there is not a discrepancy caused by the temperature of the environment or other external factors.
Completing the Measurement Process
Once the Specific Gravity is determined and plugged into the API Gravity formula, you can generate an accurate measurement of the API Gravity.
This measurement process is used throughout the chain of custody to determine and confirm the amount of crude oil transferred from the beginning to the end of the process. An error in measurement could have significant ramifications as the substance is transferred.
That’s why Gas Certification Institute (GCI) provides Crude Measurement training for measurement professionals and support personnel to understand how to perform accurate measurements.
Our Fundamentals of Crude Measurement class is based on the API Manual of Petroleum Measurement Standards to ensure that measurement professionals meet the API requirements for crude measurement.
Measurement professionals learn the fundamentals of crude measurement, including how to perform the API Gravity calculation and how to use tools that determine the physical properties of the substance.
To register yourself or your team for our next Fundamentals of Crude Measurement course, view the GCI class schedule. Contact us for assistance on completing the registration process.