![]() Measurements were made at 3 depths to confirm that the velocity was increasing with depth with the following results explaining how the error was greater during higher flows. Flume delta pattern was established, we measured the approach velocity within the vault with an electromagnetic point velocity sensor both ahead of and in multiple locations within the flume. Real time flow data was monitored using 2105CI modems installed in the field which pushed data to a remote server.Īlmost immediately, we noticed that the Parshall flume reported flow rates consistently lower than the LaserFlow and during periods of high flow, the delta was often in excess of +/- 10%.ĭuring the LaserFlow installation, it was noted that the approach velocity seemed very high however, we did not inspect the downstream vault to see if velocity remained high entering the flume. ![]() The LaserFlow Flow Meter installation was about 200 feet upstream of a metering vault with an 18 inch Parshall Flume which would serve as a comparison flow measurement. We were originally called to demonstrate the accuracy of the Teledyne Isco LaserFlow Non-Contact Flow Meter. Recently, we encountered a flow measurement application in a wastewater collection system with excessive approach velocity at a Parshall flume. However, excessive approach velocities diminishes the designed purpose of this restriction point in the flume pushing through the flume faster than it should, and as a result registering a lower depth at the Ha metering location. This allows for a single point head measurement to be made upstream of the throat at Ha in order to calculate the flow rate. The geometry of a Parshall flume is such that it induces supercritical flow by means of a restriction in the flume’s sidewalls and a drop in the flume floor. In this state gravitational forces are predominant and the flow has a low velocity which can be described as tranquil and streaming. Subcritical Flow – a hydraulic condition in which inertia forces are less than gravitational forces.Parshall flumes provide a very repeatable relationship between water level (Ha) and flow rate but, only so long as the approaching flow conditions are appropriate.įor any flume design, the hydraulics must cause a condition of subcritical flow with as much energy, velocity, and speed removed before entering the flume. Since the 1930s, wastewater professionals have relied on the Parshall flume as the primary device for open channel flow measurement applications.
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