How Open Channel Flowmeters Work

Open channel flowmeters perform flow measurement of liquids that are open to the atmosphere at some point in the flow measurement path. 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 installation point of the flowmeter itself.

Regardless of which type of open channel flow measurement techniques are used, level measurement must be used in combination with 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 flow 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 flow transmitter uses the signal from the transducer to measure the level and determine liquid flow. Different geometries are used for open channel flow 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, materials 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 Open Channel flowmeters using Flumes and Weirs

Since these methods use a primary device, sedimentation, dirt, and other debris often times accumulate on the bottom of these devices, making level measurement highly inaccurate and thereby jeopardizing overall flow measurement accuracy.  Moreover, the original shape of these primary devices is subject to wear and tear over time which further introduces inaccuracies in the overall flow measurement system. For these reasons, these devices are becoming obsolete. Smart Measurement will not offer these types of devices for the simple reason that there are better methods available in the market.

Area Velocity Flow Measurement Methods

There are two types of Area Velocity (AV) methods for measuring open channel flow. Both types use either Transit time or Doppler ultrasonic technologies. 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 Flow Measurement Techniques

Doppler Flow Measurement for open channel flow is designed to record instantaneous velocity components at a single point with a relatively high frequency.Flow measurement is 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 measurement.

The Doppler flow meter is placed at the bottom of the channel which often times gets dirty thereby causing inaccurate flow 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, hence not representing the average flow in the stream and causing uncertainty in overall open channel flow measurement.

Traditional Transit Time (TT) Ultrasonic Flow Measurement Techniques

This is the differential travel time method in which an acoustic pulse travels downstream faster than a pulse travels upstream.  This is also 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 difference between the times of flight between the two transducers is directly proportional to the fluid 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 measurement.

Traditional TT techniques although far superior to other flow 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, or  debris 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 is not aligned properly across the open channel.

SmartMeasurement™ Transit Time DSP Ultrasonic Flow Measurement Techniques

The Smart Measurement ALSONIC-AVM open channel flow measurement 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, open channel flow meter consists of an advanced DSP-based flow computer and four transducers and 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 uses patented “fine time measurement technology” where the meter’s ultrasonic beams are capable of measuring the fluid velocity at Pico-seconds time intervals.  This rapid succession of velocity measurements enables accurate, drift-free flow measurement of ultraclean to moving particles which are always present in an open channel system. The ALSONIC-AVM open channel flowmeters can measure up to up 30% of particles of various sizes in a flow stream.  All other competing systems measure at micro-second time intervals, thereby increasing the chance of errors occurring due to moving particles in a flow stream.

The ALSONIC-AVM open channel flowmeters 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 flow measurement even at less than idea flow profiles. The ALSONIC-AVM also comes with an option Laser Pointer Alignment (Patented), so accurate alignment of each paired transducers is assured.