Accurate flow measurement is one of the most important things that an operator can obtain. Without the correct flow, it is impossible to provide accurate data for the Discharge Monitoring Reports or process control sampling and analysis.
The most commonly used technique of measuring the rate of flow in open channels is by inserting a hydraulic structure into the channel that changes the level of liquid in or near the structure. The flow rate through or over the restriction will be related to the liquid level, and the flow rate can be derived by measurement of the liquid level. These hydraulic structures are known as primary measuring devices and can be divided into two broad categories: weirs and flumes.
Weirs are the simplest and least expensive type of primary measuring device. Weirs should only be used at effluent or clear water locations due to the tendency of solids to accumulate upstream of the weir. The most common weirs are the contracted rectangular weirs (with end contractions), suppressed rectangular weirs (without end contractions), triangular (or V-notch) weirs and trapezoidal (or Cipolleti) weirs. The V-notch weir is an accurate flow measuring device particularly suited for small regular flows.
Flumes are used to measure flow in open channels where the use of weirs are not feasible. Although weirs are generally more accurate than flumes, flumes can measure flow over a wider range than weirs, and they operate with a much smaller loss of head and advantage for many existing open channel applications where the available head is limited.
They are better suited to handling flows containing solids because high velocity though the flume tends to increase solids because the high velocity through the flume tends to make it self-cleaning. The major disadvantage is that a flume installation is typically more expensive than a weir. The Parshall flume is the primary flume used in wastewater plant installations.
Secondary measuring devices (or open channel flow meter) are used in conjunction with primary measuring devices to measure the rate of flow in an open channel. This device basically has two functions: to measure the liquid level in the primary measuring device and to convert this liquid level into an appropriate flow rate according to the known liquid level flow rate relationship of the primary device. This flow rate may then be integrated to obtain a totalized volume, transmitted to a recording device and/or used to pace an automatic sampler.
The following are some of the most used secondary methods of secondary flow measurement.
Floats are used in conjunction with either a cable and pulley or a pivoting arm to convert the liquid level measured by the float to an angular position of a shaft. An indicator pointer attached to the shaft indicates the liquid level and/or flow rate shown on a permanently mounted scale. One problem often encountered is the buildup of trash and scum on floats, which weighs them down and results in low-flow measurements.
Ultrasonic meters measure 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.
A bubbler tube is anchored in the flow stream to a fixed depth, and pressurized air bubbles are pumped through the tube at a constant rate. The pressure necessary to maintain a constant bubble rate is measured, and this pressure is directly proportional to the liquid level.
A sealed pressure transducer is submerged directly in the flow stream, and the pressure measured by the transducer is proportional to the liquid level.
Flow measurements must be accurate to + or – 10 percent and calibration of flow meters is necessary to ensure accurate data.
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