| Vol 1 No. 8 Date: July 8, 1999 Subject: Flow Measurement News From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com ........ ******************************************* This newsletter is e-mailed by McCrometer monthly to subscribers worldwide. It consists of flow measurement information of interest for professionals in this field. For More information on our complete line of flowmeters go to: http://www.mccrometer.com Ask the Experts... REGARDING GAS ADIABATIC EXPANSION FACTOR Flow equations for differential pressure meters such as the V-Cone, are derived from Daniel Bernoulli's theorem of the conservation of momentum in a pipe. This theorem states that for a fluid moving through a closed pipe, as velocity increases, pressure decreases. The derived equation for differential pressure meters assumes the density of a fluid will not change within the meter. This assumption applies only to incompressible flows. In the case of compressible flows, a correction must be made. In a gas, a change in pressure will change the density of the gas. Since the V-Cone is a differential pressure meter, the line pressure will be higher upstream of the meter than downstream. Therefore, the density upstream will also be higher than the density downstream. This difference in density will create a measurement error if not corrected. It is corrected by a term called the "gas expansion factor." This term correctly identifies that the gas expands in the low pressure downstream of the meter. This, however, should not be confused with the "compressibility factor" of a gas, which is distinguished as a physical property of the gas not the meter. McCrometer uses the letter "Y" to symbolize the gas expansion factor for the V-Cone. The value of Y is always equal to or less than one. To use the factor, we simply multiply the calculated value directly into the flow equation. The expansion factor is a function of differential pressure (DP) and line pressure in the meter. As the amount of DP goes up in ratio to the line pressure, the Y factor goes down. This means the Y factor changes with any change in DP or pressure and must be calculated with each flowrate measurement. The instrumentation used to calculate flowrates through a DP meter must be capable of doing these calculations and programmed properly for gas applications. If you would like more information regarding the gas expansion factor, please contact McCrometer and ask for the following literature: If you have suggestions for newsletter topics you would like to see, please contact us at info@mccrometer.com ******************************************* |