Heavy oils are characterised depending on their density rather than their viscosity . Although there are various definitions for what constitutes heavy oil it is commonly agreed that the majority contain impurities such as asphaltenes, waxes and carbon residue. The API gravity definition, a common definition used in the oil and gas industry worldwide, states that heavy oil has an upper limit of 22°. Figure 1 shows the upper and lower limits of various categories of oil as stated in the API gravity definition.
Confirmed world oil reserves are split approximately into 70% high viscosity and 30% (low viscosity) conventional light oils. High viscosity oils are regarded as a vital energy resource for the foreseeable future, with significant yields forecast at 100 years or more.
A literature review conducted by NEL and Oxford University highlighted the issues facing application of conventional flow meters to high viscosity fluids . Following on from the review, an initial experimental test programme  was instigated using a selection of conventional flow meters applied in viscous fluids. The overall conclusion from this work reinforced the notion that liquid flow meters cannot simply be relocated from low to high viscosity service without suitable characterisation or modification, nor can calibrations conducted in a low viscosity medium necessarily be applied to heavier crudes without appropriate compensation.