| Vol.1 No.3 Date: Jan. 5, 1999 Subject: Flow Measurement News From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com ........ ******************************************* For more information in our complete line of flowmeters go to: http://www.mccrometer.com ******************************************* Ask The Experts... CALIBRATING USING REYNOLDS NUMBER CORRELATION Differential pressure meters have the ability to measure a variety of fluids. A DP meter, such as McCrometer’s V-Cone® Flowmeter, can measure the flow of anything including water, air, steam and natural gas. However, when customers want these meters calibrated, it would be nearly impossible to calibrate with the exact fluid as the meter’s application. This can be overcome by the use of a Reynolds number. As stated in the second newsletter, a Reynolds number is a dimensionless number that can be used to describe flow. It is calculated by using the inertial and viscous forces of the flow. The inertial force is mass times velocity (or density times pipe diameter times velocity). The viscous force is simply the viscosity of the fluid. Since the Reynolds number is made up of four different variables, one Reynolds number can represent many different combinations of flowrates and pipelines. During the calibration process, we look at the Reynolds number range of the application. We match that Reynolds number using our calibration fluid, either water or air. This is called a Reynolds number correlation. For instance, a meter’s application may be natural gas. Since we are not able to calibrate on natural gas, we can match the Reynolds number of the natural gas application with an air flowrate. The air data is then used in the application. In other words, the correlation assumes that for a given Reynolds number, the flow characteristics are constant. Additionally, we tested a meter on air at a Reynolds number of 100,000 and obtained a certain meter factor. If we were to test this meter on natural gas at the same Reynolds number, we should obtain the same meter factor. The Reynolds number correlation states that at matching Reynolds numbers, the meter factors will match. Since we are using different fluids, though, the differential pressure, the velocity, and the flowrate will be different. This is very important to understand. The only calibration data that is used directly is the meter factor. With the meter factor, the differential pressure/flowrate relationship can be calculated for the meter's application. ******************************************* Copyright 1999 McCrometer |