Flow Metering
Flow measuring techniques will vary depending on the application flow type. There are two basic types of flow systems; closed channel, and open channel. A Closed Channel can be described as water flow through a completely filled pressurized pipe. Flow measurement is typically performed by inserting a mechanical meter, venture meter, magnetic meter within the pipe. A typical example of a closed channel flow is a city potable water line that is metered with a turbine meter.
 The second type of flow type, Open Channel, is best described as, water that flows with a “free surface” typically in a non-pressurized (atmospheric) pipe or channel. Examples are rivers, irrigation/drainage ditches, canals, and for sanitary sewer. The most practical method for Open channel flow measurement is accomplished by the use of a hydraulic structure; flumes and weirs. These hydraulic structures enable flow calculation by measuring the water depth at a single point. And by using the structure’s associated equation or table, the flow rate can be calculated.
A weir is a dam that spans across an open channel that liquid flows over. Weirs are classified according to the shape of the notch (the cross-sectional area that water flows through). The most common weir types are rectangular, trapezoidal (or Cipolletti), and triangular (or V-notch).
A flume is a specially shaped open channel structure that temporarily modifies the flow through the channel to enable the measurement of the flow rate. Specifically, the flume restricts the channel area and/or changes the channel slope, resulting in an increased velocity and a change in the level of the liquid flowing through the flume structure. The flow rate through the flume can be determined by measuring the head at a single point and using the flume’s associated equation (head-flow rate relationship).
Measurement of the flow rates in an open channel flume or weir can be performed manually by reading the head measurement and calculating, or by the means of an automatic flow meter. The most common open channel meters are the Ultrasonic, Bubbler, & Pressure Transducer. The Ultrasonic Meter measures the time required for an acoustic pulse to travel from a transmitter to the liquid surface (where it is reflected) and returned to a receiver.
The Bubbler Meter consists of a bubbler tube that is anchored in the flow stream at a fixed depth, then. The tube supplies a constant bubble rate of pressurized air. The air pressure required to maintain the bubble rate is measured; this pressure is proportional to the liquid level.
The Pressure Transducer consists of a sealed pressure transducer submerged in the flow stream at a fixed depth. The pressured measured by the tranducer is proportional to the liquid level.
Park® provides flow measuring solutions for wastewater applications. Contact us on you next application.
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