Viscosity is one of the most critical properties to consider when selecting a flowmeter for oil applications. Viscosity describes a fluid's resistance to flow—thin oils like gasoline have very low viscosity, while heavy crude oil and bitumen can be extremely viscous. The viscosity of oil is not fixed; it changes significantly with temperature, decreasing as the oil is heated and increasing as it cools. A flowmeter selected for oil's viscosity at one temperature may not perform correctly at a different operating temperature, so the full range of viscosity expected over the operational year must be considered during the selection process.
Different flowmeter technologies respond to viscosity in different ways. Turbine flowmeters perform best with low-viscosity oils (below 10 cSt) and become less accurate as viscosity rises because the rotor experiences increased drag at low flow rates. Vortex and electromagnetic meters also have practical upper viscosity limits. Positive displacement meters—especially oval gear and rotary piston types—handle viscosity changes very well and maintain accuracy from very low to very high viscosities (up to 10,000 cSt or more in some designs), making them the preferred choice for medium to heavy oil. Coriolis meters are largely insensitive to viscosity and provide accurate measurements across the full viscosity spectrum, but they are more expensive.
When specifying a flowmeter for a wide-ranging viscosity application—such as a refinery that handles both light and heavy crude oils—the selected meter must be proven to maintain the required accuracy across the entire viscosity range. Manufacturers publish viscosity correction curves or factor tables that show how the meter's linearity changes with viscosity. Some flow computers can apply automatic viscosity correction if a real-time viscosity measurement is available. Consulting the manufacturer with a detailed fluid data sheet that includes the minimum and maximum viscosity expected at operating temperature is essential for making a reliable meter selection.