Flow Meter Manholes

Flow Metering Manholes can be grouped into two types: open pipe and closed pipe.

Closed pipe flow metering manholes use mag meters, clamp on meters, and insertion meters. Open channel manholes incorporate flumes, weirs, or direct sensing (radar, laser, or area velocity) meters.

Closed Pipe Flow Manholes

For closed pipe manholes, mag meters are the most common. Mag meters provide excellent accuracies, forward / reverse flow calculation, and are cost competitive. One drawback to using them is the need for isolation valves or a toads structure to allow the meter to be serviced. This takes extra room - requiring either large diameter manholes (or vaults). Without a isolation valves or a bypass, should the meter ever fail or require servicing, there is no practical way to access the meter without affecting the entire line.

interior of a closed pipe magnetic flow meter manhole

Open Channel Flow Manholes

For open channel flows flumes are the most common. Flumes require low head losses (in comparison to weirs), are not proprietary (as are direct sensing devices - and are thus readily verifiable), are available in a wide array of styles and sizes, and can be (depending upon the application) integrated into new or existing concrete / brick manholes or can be factory integrated into fiberglass Packaged Metering Manholes.

parshall flume integrated in a manhole to meter open channel flows

While useful, flumes are limited when it comes to line size. Rarely are flumes used to measure flows through manholes where the line size is above 36 / 42". At these sizes direct sensing meters make better economic sense are most commonly used.

large Cutthroat flume in a Packaged Metering Manhole - PMM - for open channel flow metering

Weirs, while accurate, are not commonly used in flow metering manholes. The high head loss required and sediment capturing nature of weirs limits their usefulness. One exception, though, is dam seepage. Here sediment capture is important and weirs are much more common.