Current Meter For Flow Measurement

[Micol Cohn]

Inphysics, one distinguishes different reference frames depending on where the observer is located, this is the basics forthe Lagrangian and Eulerian specification of the flow field in fluid dynamics: The observer can be either in the Moving frame (as for a Lagrangian drifter) or in a resting frame.

Mechanical current meters are mostly based on counting the rotations of a propeller and are thus rotor current meters. A mid-20th-century realization is the Ekman current meter which drops balls into a container to count the number of rotations. The Roberts radio current meter is a device mounted on a moored buoy and transmits its findings via radio to a servicing vessel. Savonius current meters rotate around a vertical axis in order to minimize error introduced by vertical motion.[1]

Doppler instruments are more common. An instrument of this type is the Acoustic Doppler current profiler (ADCP), which measures the water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. The ADCPs use the traveling time of the sound to determine the position of the moving particles. Single-point devices use again the Doppler shift, but ignoring the traveling times. Such a single-point Doppler Current Sensor (DCS) has a typical velocity range of 0 to 300 cm/s. The devices are usually equipped with additional optional sensors.

Travel time instruments determine water velocity by at least two acoustic signals, one up stream and one down stream. By precisely measuring the time to travel from the emitter to the receiver, in both directions, the average water speed can be determined between the two points. By using multiple paths, the water velocity can be determined in three dimensions.

Travel time meters are generally more accurate than Doppler meters, but only record the velocity between the transducers. Doppler meters have the advantage that they can determine the water velocity at a considerable range, and in the case of an ADCP, at multiple ranges.

This novel approach is for instance employed in the Florida Strait where electromagnetic induction in submerged telephone cable is used to estimate the through-flow through the gateway[2] and the complete setup can be seen as one huge current meter. The physics behind: Charged particles (the ions in seawater) are moving with the ocean currents in the magnetic field of the Earth which is perpendicular to the movement. Using Faraday's law of induction (the third of Maxwell's equations), it is possible to evaluate the variability of the averaged horizontal flow by measuring the induced electric currents. The method has a minor vertical weighting effect due to small conductivity changes at different depths.[3]

Tilt current meters operate under the drag-tilt principle and are designed to either float or sink depending on the type. A floating tilt current meter typically consists of a sub-surface buoyant housing that is anchored to the sea floor with a flexible line or tether. A sinking tilt current is similar, but the housing is designed such that the meter hangs from the attachment point. In either case, the housing tilts as a function of its shape, buoyancy (negative or positive) and the water velocity. Once the characteristics of a housing is known, the velocity can be determined by measuring the angle of the housing and direction of tilt.[4] The housing contains a data logger that records the orientation (angle from vertical and compass bearing) of the Tilt Current Meter. Floating tilt current meters are typically deployed on the bottom with a lead or concrete anchor but may be deployed on lobster traps or other convenient anchors of opportunity.[5] Sinking tilt current meters may be attached to an oceanographic mooring, floating dock or fish pen. Tilt current meters have the advantage over other methods of measuring current in that they are generally relatively low-cost instruments and the design and operation is relatively simple.[6] The low-cost of the instrument may allow researchers to use the meters in greater numbers (thereby increasing spatial density) and/or in locations where there is a risk of instrument loss.[7]

Current meters are usually deployed within an oceanographic mooring consisting of an anchor weight on the ground, a mooring line with the instrument(s) connected to it and a floating device to keep the mooring line more or less vertical. Like a kite in the wind, the actual shape of the mooring line will not be completely straight, but following a so-called (half-)catenary.Under the influence of water currents (and wind if the top buoy is above the sea surface) the shape of the mooring line can be determined and by this the actual depth of the instruments.[8][9] If the currents are strong (above 0.1 m/s) and the mooring lines are long (more than 1 km), the instrument position may vary up to 50 m.

The USGS Type AA current meter is commonly known as the Price-type current meter. This current meter is suspended in the water using a cable with sounding weight or wading rod (taking the tail section off) and will accurately measure streamflow velocities from 0.1 to 25 feet per second (0.025 to 7.6 meters per second). The main features of this meter are the uniquely designed bucket wheel shaft bearings and the two post contact chamber. The bucket wheel has six conical shaped cups, is five inches (12.7 cm) in diameter and rotates on a vertical axis inside the yoke. The tungsten carbide bearings for the bucket wheel shaft are located in deeply recessed inverted cups. When the meter is in use, these cups become air chambers and the entrapped air effectively excludes water and silt from the bearing surfaces giving extremely low starting velocities and minimal friction in the bearings.

The contact chamber houses a penta gear and two binding posts, each having a fine platinum alloy contact wire. One wire makes contact with the bucket wheel shaft once during every revolution; the other is used when fast velocities are encountered, and makes contact with the penta gear once during every five revolutions of the bucket wheel.

Each current meter is provided with a U.S. Geological Survey approved standard rating table to convert bucket revolutions to stream velocity in either English units (feet per second) or metric units (meters per second), spare parts, instrument oil, cleaning cloth, screwdriver and an instrument case with a water tight o-ring seal that floats if dropped in the water and provides proper protection of the meter during transportation and storage.

The meter is made from brass and stainless steel and all exposed surfaces are plated for corrosion-free service. The standard Type AA was designed for use with all of the counters as well as the AquaCalc 5000 Digital Flow Computer. No conversion kits or replacement contact chambers are necessary to use the latest digital technology with this meter.

SonTek manufactures affordable, reliable Acoustic Doppler Current Profiling (ADCP) instrumentation for water velocity measurement in oceans, rivers, lakes, harbors, estuaries, and laboratories. Our instruments use sound waves to tell you how fast water is moving, where it is moving, and even if it is not. Our customers are scientists, engineers, hydrologists, research associates, water resource planners, and anyone needing to collect velocity, flow, or discharge data in every body of water imaginable.

An Instantaneous Measurement is taken out into the field for a measurement of the current velocity and/or flow conditions. Select any one of these instruments to learn more about their ideal applications and technical specifications.

Looking for a long-term data collection solution? Our Continuous Monitoring instruments allow for permanent to semi-permanent installations for collecting water velocity and/or flow measurements on a regular, user programable time interval.

Hydrographers, Hydrologists, Research Scientists (and more) who work to understand better water properties and distribution are paramount to ensuring the sustainability of this resource well into the future.

Please look at a day in the life of Application Engineer Kevin Labbe. He helps researchers from the University of Georgia learn how to best use the SonTek-RS5, YSI's EXO2, and other instrumentation in the estuaries of Little St. Simons, Georgia.

Flood events on rivers can cause erosion of the soil around a bridge foundation, a process known as bridge scour. Over time, scour can cause dangerous foundation instability and is the leading cause of bridge failure. Acoustic technology makes railways safer.

A multidisciplinary team of hydrologists, climate scientists, and modelers led by UCLA geographer and geologist Laurence C. Smith uses the SonTek-M9 in Greenland Ice Sheet rivers to ground-truth runoff model predictions.

Before Dr. Cliff Kapono shifts his research to the stage of sampling and analyzing microbiota from the reef, he characterizes the flow of water and sediment throughout the water column. His tool is a SonTek CastAway-CTD, which measures conductivity, temperature, and depth at a rate of 5 Hz and then calculates salinity, sound speed, and other parameters.

Find out why not every flow meter is created equally. Take a comprehensive look at the functional and technical differences between continuous wave vs Multi-cell pulsed Doppler technology and the advantages and disadvantages of each including:

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Streamflow discharge is defined as the volume rate of flow of water that includes any substances dissolved or suspended in the water. Discharge is usually expressed in units of cubic feet per second (cfs or ft3/sec). With rare exception, stream discharge is not measured directly, but is computed indirectly from velocity and water level (stage) measurements. If the mean water velocity normal to the direction of flow (V) and the crossectional area (A) of water flow is known, then the discharge (Q) can be computed as Q=VA. As previously discussed in Lesson 1, determining the mean stream velocity is a labor intensive activity, and usually only performed to establish or refine a relationship between stage, which is easy to measure, and discharge. The discharge rating is a relationship between the stage and discharge or between stage, discharge, slope, rate of change of state, or other factors. These relationships are presented in Lessons 7, 8, and 9.

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