In closed conduit water systems, being able to accurately measure flow is absolutely essential. For many meter designs, including electromagnetic induction meters (also known as magnetic flow meters), the greatest accuracy is achieved when the device is calibrated correctly. Optimal meter accuracy often depends on the flow conditions associated with the upstream geometry in the pipe system. Manufacturers typically give standards for the length of straight pipe required upstream of the meter. These standards vary, however, and do not address every possible configuration that may be installed upstream of the straight pipe requirement.
An investigation on the effects of the length of straight pipe between a single 90° elbow and the upstream side of magnetic flow meters was completed in this research. Eleven 10-inch meters were chosen for testing. The procedure included a baseline test with more than forty diameters of straight pipe between the elbow and the meter. The accuracy of the meter was determined over a range of flow velocities typical for operation of this type and size of meter. Further tests were performed with the meter installed only three diameters downstream of the elbow. These tests constitute Phase I. In Phase II, four meters were tested with the upstream pipe length varying from a close-coupled installation to ten diameters of straight pipe between the elbow and the meter to observe variances in accuracy with distance from the elbow.
The intent of the research was to show whether manufacturer accuracy specifications are achievable in actual application. It was determined that very few meters meet the manufacturer’s specification for accuracy even when installation requirements were met. Post-factory calibrations and minimization of velocity profile disruption through consideration of upstream geometry is recommended
