Oil and gas production handbook An introduction to oil and gas production

Partners, authorities and customers all calculate invoices, taxes and 
payments based on the actual product shipped out. Often, custody transfer 
also takes place at this point, which means transfer of responsibility or title 
from the producer to a customer, shuttle tanker operator or pipeline operator. 
Although some small installations are still operated with a dipstick and 
manual records, larger installations have analysis and metering equipment.  
 
To make sure readings are accurate, a fixed or movable prover loop for 
calibration is also installed. The illustration shows a full liquid hydrocarbon 
(oil and condensate) metering system. The analyzer instruments on the left 
provide product data such as density, viscosity and water content. Pressure 
and temperature compensation is also included.