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Flow Metering

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.
Open channels are used to conduct liquids in most sewer systems, sewage treatment plants, industrial waste applications, and irrigation systems.

There are three methods for automatically measuring open channel flow:

  • Hydraulic Structures 
  • Area Velocity 
  • Slope-Hydraulic Radius 

The most common method of measuring open channel flow is the hydraulic structures method. A calibrated restriction inserted into the channel controls the shape and velocity of the flow. The flow rate is then determined by measuring the liquid level in or near the restriction.
The restricting structures are called primary measuring devices. They may be divided into two broad categories–weirs and flumes.
A weir is an obstruction or dam built across an open channel over which the liquid flows, often through a specially shaped opening. Weirs are classified according to the shape of this opening. The most common types of weirs are the triangular (or V-notch) weir, the rectangular weir, and the trapezoidal (or Cipolletti) weir. The flow rate over a weir is determined by measuring the liquid depth in the pool upstream from the

Weirs can be simple and inexpensive to build and install. Common materials of construction include metal, fiberglass and wood. However, they represent a significant loss of head, and are not suitable for measuring flows with solids that may cling to the weir or accumulate upstream from it.
Automatic measurement of the flow rate in an open channel flume or weir can be performed manually by reading a single level 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 transducer is proportional to the liquid level.

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