Liquid flowrate can be measured using two methods:
- Volumetric, where flow is directly measured (turbine meters), or calculated using other process variables (pressure differential in the case of orifice meters). In this case, the volume is multiplied by some form of density to get mass.
- Mass, which is measured using coriolis type meters. To get volumetric flowrates, the mass must be divided by density.
An example where confusion between the two types of methods can result in a financial or accounting problem is determining the amount of free water in a crude flowstream.
The volume fraction of water ismp typically determined on-site using a ‘shakeout’ method. A crude sample is extacted from the process and spun down in a centrifuge to get good separation between the crude and water. A de-emulsifier is typically added to break up the emulsion layer in the sample. The volume of water in the cylinder is the volumetric amount of water in the sample, say anything between 0.5 – 40%.
The details of the above method can be read up in the Manual of Petroleum Measurement Standards (MPMS), Chapter 10-Sediment and Water, Section 4-Determination of Water and/or Sediment
in Crude Oil by the Centrifuge Method (Field Procedure)
Posted by Wata 
Wata - Malaysian Oil and Gas Engineer 