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E-Zine March 2008
Click here to review Part 1 The concept that these flowmeters are not able to measure at low flow rates is important because in many applications, significant amounts of fluid at low flow rates can pass through the flowmeter without being measured. Because the density of most liquids is in a relatively small range centered about the density of water, it is common to assume that the minimum velocity constraint for water (as determined by the supplier) is close to the actual minimum velocity constraint of the fluid. Due to differences in sensing systems, it is advisable to consult supplier literature to determine the minimum velocity constraint. In contrast, the minimum velocity constraint for free air applications can be over 2 meters per second (6.5 feet per second). If the density of the air increases (i.e., by being compressed), the minimum velocity constraint would be lower because the density of the compressed air is higher. Due to variation in the sensitivity of sensing system designs, the minimum velocity constraint for gas applications should be determined using supplier literature. In general, note that the minimum velocity constraint is dependent upon density --- not specific gravity. Notwithstanding this statement, when the effects of composition and liquid thermal expansion are neglected, density and specific gravity become essentially the same. However, the specific gravity of a gas will remain the same even when changing pressure and/or temperature cause large changes to its density. For example, free air and compressed air have a specific gravity of 1.00. However, the density of free air will increase over ten-fold when it is compressed to 10 bar (approximately 145 pounds per square inch) gauge. Stated differently, changes in fluid density affect the minimum velocity constraint of the flowmeter. However, fluid density changes can be caused by changes in composition, temperature, and/or pressure, especially in gas and vapor applications. These concepts should be applied to each application to understand the effect that operating conditions have on the minimum measurable flow rate. The following table contains data that illustrates how fluid density can significantly affect the minimum velocity.
Excerpted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters ISSN 1538-5280 |
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