API gravity is a measure of how heavy or light a petroleum liquid is compared to water. If the API Gravity is greater than 10 it is lighter than water, if it is lower than 10 it will be heavier.
Light crude oils are more of a liquid flows freely at room temperature. This is because of its lower density, lower viscosity values, and low specific gravity. These characteristics are due to the presence of a high proportion of light hydrocarbon fractions and generally low paraffin (wax) content. These values correlate with a higher API value.
Heavy crude oils do not flow as easily. The density and specific gravity numbers are higher meaning lower API numbers.
API gravity numbers are determined by sampling a production stream of a certain volume during a defined time. In order to get an accurate measurement, a homogenous sample is needed to be sure it is a good representation of the fluid flowing through the pipeline. The sample should be retrieved from a point that is representative of the entire cross sectional area of the pipe.
The size of the sample needs to be proportional to the flow rates and the sample size must be consistent, within 2%, for each sample. The required volume of the collected sample will depend on the number and type of analyses performed. There should also be enough left over in the event that the analyses need to be repeated.
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