How Open Channel Flowmeters Work
Open channel flowmeters measure the flow of liquids that are open to the atmosphere at some point in the measurement system. The liquid may be entirely open to the atmosphere, or may be contained within a closed pipe that is not full of liquid and only open to the atmosphere at the flowmeter itself.
Regardless of which type of open channel measurement techniques are used, level measurement must be used in combination of velocity measurement, and a flow computer using the manning equation of open channel flow to calculate the true flow rate of any wetted channel (pipes, streams, etc).
Traditional Methods
There are several types of open channel flow meters, the traditional methods using primary devices such as flumes or weirs. These consist of a primary device, transducer, and transmitter. The wetted primary device restricts the liquid flow stream. Under flowing conditions, this restriction causes a rise in liquid level at a location either upstream or within the flowmeter. When the flow increases, the level rises higher. A transducer is mounted on or near the primary device to sense the level. The electronic transmitter uses the signal from the transducer to measure the level to determine liquid flow.
Different geometries are used for open channel measurement, including flumes that make the channel narrower, weirs that force the liquid over a dam-like obstruction and nozzles that restrict the flow of liquid before it freefalls from the nozzle.
Due the limited applicability of open channel flowmeters, material of construction are typically limited to those that target these applications. Sizes range from a few inches to tens of feet. It is generally less expensive to purchase smaller open channel flowmeters and field construct larger ones (typically with concrete).
Problems With the Traditional Method of Using Flumes and Weirs
Since these methods use a primary device, sedimentation, dirt, etc. often times accumulate on the bottom of these devices, level measurement becomes highly inaccurate therefore jeopardizing overall measurement. Moreover, the original shape of these primary devices is subject to wear and tear over time which further introduces inaccuracies in the overall measurement system. It is for this reason that these devices are becoming obsolete. Smartmeasurement will not offer these types of devices for the simple reason that there are better methods available in the market.
Area Velocity Methods
There are two types of Area Velocity (AV) method of measuring open channel flow. Both type uses either Transit time or Doppler ultrasonic technologies (make a link to the technology section of SMC ultrasonic). The area velocity method calculates flow rate by multiplying the area of the flow by its average velocity. This is often referred to as the continuity equation, Q=AxV, where Q is flow, Ax is the wetted area of flow times V, the average velocity of the flow stream.
Doppler Ultrasonic Techniques
Doppler measurement (link to ultrasonic technology Doppler section) for open channel flow is designed to record instantaneous velocity components at a single point with a relatively high frequency. Measurements are performed by measuring the velocity of particles in a remote sampling volume based upon the Doppler shift effect. The velocity components using this technique are combined with level measurement and an open channel flow computer using the manning equation to calculate true open channel flow.
The Doppler flow meter is placed at the bottom of the channel which often times gets dirty thereby causing in accurate measurement. Since Doppler techniques measure the speed of particles coming towards the sensor, it only measures what it sees. Particles in a stream move at different speeds and it is highly unlikely that it can measure all the particles in the stream. This causes further uncertainty in overall open channel flow measurement.
Traditional Transit Time (TT) Ultrasonic Techniques
Differential travel time method in which an acoustic pulse travels downstream faster than a pulse travels upstream.lso referred to as ‘time of flight’: An acoustic pulse of traveling diagonally across the flow in the downstream direction will be accelerated with the velocity component of the water while, conversely, an acoustic pulse traveling diagonally upstream will be decelerated by the water velocity. The times of flight between the two transducers are directly proportional to its velocity. A series of flow transducers are paired across the open channel and the combined velocities of the paired transducers are averaged to provide a true profile of the total flow path. The velocity components using this technique are combined with level measurement and an open channel flow computer using the manning equation to calculate true open channel flow.
Traditional TT techniques although far superior to the above measurement techniques, still offer inaccuracies in overall measurement. Signal path between the paired transducers can be reflected if there are a lot of particulates, bubbles etc. in the flow stream. Moreover a fully developed flow profile must be obtained at the measurement point by having the required upstream and downstream diameters, usually 8-10 diameters up stream and 2-5 diameters downstream. Inaccuracies can also occur if the matching pair of transducers are not aligned properly across the open channel.
Smartmeasurement Transit Time DSP Ultrasonic Techniques. Alsonic-AVM
Smart Measurement’s ALSONIC-AVM system is an area-velocity meter that is used in conjunction with a user-supplied level transmitter to measure flow rates in open channels. The ALSONIC-AVM, which consists of an advanced DSP-based flow computer and four transducers, uses the transit time difference of ultrasonic sound pulses to measure the open channel flow velocity. The ultrasonic pulses are transmitted upstream and downstream across the channel at an angle α between the flow direction and the sonic wave path, with the difference in the sonic wave’s transit time being directly proportional to the liquid velocity.
The ALSONIC-AVM may be used in rectangular, circular, trapezoidal or other shaped channels. Since the transducers create almost no restriction, virtually no head loss is created. The advanced DSP-based flow computer with cross-correlation and FFT technology allows this system to work in the most difficult applications, including those involving liquids with high concentrations of suspended solids & air or a large noise component. Our unique TT technologies will enable accurate measurement even at less than idea flow profiles. The ALSONIC-AVM also comes with an option Laser Pointer Alignment (Patent) , so alignment of each paired transducers.
