Oceanography (2+1)

Measurement of currents

Measurement of currents:  Currents are measured by two methods they are

 A.Direct method: Currents can be measured directly by observing the motion of water relative to the solid earth or by observing the motion of the solid earth relative to the water.

Example: A floating device tied to a weight at the ocean bottom by a wire of appropriate length depending on the depth of the current to be measured. The water speed can be measured by the speed with which a propeller is turned, and the current direction might be detected by comparing the direction of a weather wave-type pointer to a compass needle.

An example for lager device would be a float of just the right buoyancy to float at the desired depth. It should be deposited one day and then retrieved several weeks or months later and it should be attached with signal generating device to identify. The ocean current would then be known by how far and in what direction the float had gone.

 B.Indirect method:

Geostrophic technique in which the current can be measured by measuring the slope of density contours due to current flow (cf) by knowing the slope of the density gradients and knowing the latitude.

In the second method the principle of electricity and magnetism is involved. When charged particles move through a magnetic field they get deflected, opposite charges in opposite direction. Ocean currents carry many charged particles-the salt ions that are moving through the earth’s magnetic field. When two metal plates are put in to this current the positive ions will go toward one plate and the negative ions towards the other. By measuring the electrical voltage thus generated between these two plates, and the orientation of the plates for maximum voltage, we can predict the speed and direction of current. All insturments dicussed so far produce information about oceanic property irrrespective of the dynamic state of the ocean.

An elementary way of observing oceanic movement is the use of drifters.  Drifters are platforms designed to carry instruments. But all measurements obtained from drifters are of little use unless they can be related to positions in space. A geolocation (GPS) device which transmits the drifter location to a satellite link is therefore an essential instrument on any drifter, and this turns the drifter into an instrument for the measurement of ocean currents.

Ocean currents can be measured in two ways. An instrument can record the speed and direction of the current, or it can record the east-west and north-south components of the current. Both methods require directional information. All currents meters therefore incorporate a magnetic compass to determine the orientation of the instrument with respect to magnetic north. Four classes of current meters can be distinguished based on the method used for measuring current magnitude.

Mechanical current: Meters use a propeller-type device. A  Savoniur rotor  or a paddle-wheel rotor to measure the current speed, and a vane to determine current direction. Propeller sensors often measures speed correctly only if they point into the current and have to be oriented to face the current all the time. Such instruments are therefore fitted with a large vane, which turns the entire instrument and with it the propeller into the current.

Propellers can be designed to have response with the angle of incidence of the flow. Two such propellers arranged at 90⁰ will resolve current vectors and do not require an orienting vane.

The advantage of the Savonius rotor is that its rotation rate is independent of the direction of exposure to the current. A Savonius rotor current meter therefore does not have to face the current in any particular way, and its vane can rotate independently and be quite small, just large enough to follow the current direction reliably.

With the exception of the current meter that uses two propellers with cosine response set at 90⁰ to each other, mechanical current meters measure current speed by counting propeller or rotor revolutions per unit time and current direction by determining the vane orientation at fixed intervals. In other words, these current meters combine a time integral or mean speed over a set time interval (the number of revolutions between recordings) with an instantaneous reading of current direction (the vane orientation at the time of recording). This gives only a reliable recording of the ocean current if the current changes slowly in time. Such mechanical current meters are therefore not suitable for current measurement in the oceanic surface layer where most of the oceanic movement is due to waves.

The Savonius rotor is particularly problematic in this regard. Suppose that the current meter is in a situation where the only water movement is from waves. The current that alternates back and forth, but the mean current is zero. A Savonius rotor will pick up the wave current irrespective of its direction, and the rotation count will give the impression of a strong mean current. The paddle-wheel rotor is designed to rectify this; the paddle wheel rotates back and forth with the wave current, so that its count represents the true mean  current.

Mechanical current meters are robust, reliable and comparatively low in cost. They are therefore widely used where conditions are suitable, for example at depths out of reach of surface waves.

Electromagnetic current: Meters exploit the fact that an electrical conductor moving through a magnetic field induces an electrical current. Sea water is a very good conductor, and if it is moved between two electrodes the induced electrical current is proportional to the ocean current velocity between the electrodes. An electromagnetic current meter has a coil to produce a magnetic field and two sets of electrodes, set at right angle to each other, and determines the rate at which the water passes between both sets. By combining the two components the instrument determines speed and direction of the ocean current.

Acoustic current: Meters are based on the principle that sound is a compression wave that travels with the medium. Assume an arrangement with a sound transmitter, and let receiver B located downstream. If a burst of sound is generated at the transmitter it will arrive at receiver B earlier than at receiver A, having been carried by the ocean current.

A typical acoustic current meter will have two orthogonal sound paths of approximately 100mm length with a receiver/transmitter at each end. A high frequency sond pulse is transmitted simultaneously from each transducer and the difference in arrival time for the sound travelling in opposite directions gives the water velocity along the path.

Electromagnetic and acoustic current meters have no moving parts and can therefore take measurements at a very high sampling rate (up to tens of readings per second ). This makes them useful not only for the measurement of ocean currents but also for wave current and turbulence measurements.