Telemetry mfmr transducer—
Container for clock, magnetic tape, circuitry and batteries
Figure 3.3 The Aanderaa™ recording current meter.
using solid-state electronics. Intervals at which readings are taken can be varied. Other sensors attached to the device measure water temperature, pressure and conductivity. As a recording instrument this meter can be used at virtually any depth and has been used for many years to measure deep ocean currents.
Other types of current meters are now available which avoid the use of rotors. Acoustic Doppler Current Profilers (ADCP) are used to look at current patterns in the upper parts of the water column and are either ship mounted or moored. They work on the basis of transmitting pulses of high frequency sound (mostly 150 or 300 kHz) and measuring the frequency shift caused by the back-scattering particles moving in the currents. The back-scatterers have to be half a wavelength across to be seen by the acoustics, which is about 1 cm for 150 kHz. A dense cloud of smaller particles can also reflect the sound. The 'particles' are, of course, often planktonic animals, and ADCP is now being used quite extensively by biologists to carry out synoptic surveys of pelagic communities. The technique is non-invasive and extremely rapid but has the disadvantage that you cannot tell what sort of animals the acoustics are seeing.
The direct measurement of water currents by meters suspended in the water involves difficulties connected with obtaining a fixed reference point, particularly in deep water. An anchored vessel is by no means stationary, and movements of the ship may falsify the measurements. However, recent developments using satellite position-fixing equipment allow ships to maintain their position relative to a fixed point on the sea-bed within a few metres. Alternatively a buoy attached to a taut anchor wire can be used as the reference point, movements of the ship relative to the buoy being allowed for in the interpretation of the meter readings. Or recording instruments may be attached to a submerged buoy system anchored to the sea bottom. The submerged floats with their attached meters can be retrieved via a special acoustical link that breaks away from the anchor when it receives the appropriate signal. Meters may also be attached to data-recording buoys (see page 55).
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.