E-Zine January 2014
Density of liquids and slurries is one of the important parameters to measure, and doing it accurately is a difficult problem. Density is defined as the mass per unit volume of a liquid, and is usually expressed in “mass-over-volume” units like g/cc. For measurement purposes, we often want to know relative density rather than density itself. This measurement is the ratio of the mass of a volume of liquid to that of water at a specified temperature (usually 4°C), with the relative density of water defined as 1.0. This type of density uses units like SGU. Density measurements are temperature dependent, and temperature compensation is required for accurate measurement.
Density of liquids and slurries in tanks is relatively simple, since the fluid is static. You can weigh the liquid and measure the level. You can use a displacer type device and measure buoyancy. You can measure hydrostatic head using a pressure transducer. There are numerous alternatives.
The problem comes when you try to measure the density of a flowing fluid. Worse, let’s suppose that the fluid is a slurry. Now, suppose that the density of one or both slurry components is changing.
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From Flow Control (September 2002)