Density measurement is an important complement to volumetric oil flow measurement in applications where the mass or energy content of the oil must be determined accurately. The density of oil varies with temperature—becoming less dense (lighter) when heated and more dense (heavier) when cooled—and also varies between different oil grades and blends. When oil is purchased or sold by volume, temperature-corrected volume at standard conditions must be used to ensure fair comparison regardless of the temperature at which the oil was measured. This correction requires knowledge of the oil's density at the reference temperature, obtained from density tables or direct measurement.
Online densitometers—instruments that measure the density of the flowing oil in real time—are often installed in conjunction with flowmeters in custody transfer, blending, and quality control systems. The most common types are vibrating tube densitometers, nuclear densitometers, and Coriolis meters (which measure density as a byproduct of the flow measurement). Vibrating tube densitometers oscillate a small tube filled with the process fluid; the resonant frequency of the tube changes with the density of the contained fluid, providing a continuous density measurement. This real-time density data is fed into the flow computer, which uses it to calculate the oil's standard volume and, if required, its mass flow.
In blending operations, density measurement serves as a proxy for the blend quality. By monitoring the density of the product stream in real time, operators can verify that the blend ratio is on target and make immediate adjustments if the density deviates from the specification. In loading and unloading operations, density data helps verify that the correct product grade is being handled, preventing costly contamination of the wrong product grade into a storage tank or vessel. For flow measurement systems without inline Coriolis meters, pairing a volumetric meter with a densitometer provides a cost-effective way to achieve near-Coriolis-quality mass flow accuracy using separate, best-in-class instruments for each measurement function.