14
On a smaller scale, geometric arrangements of electrodes or induction coils in
field instruments achieve similar results. Single-point electromagnetic field induction instruments for field deployment respond to the movement of ions in solution
swept along in the ocean current. A durable solution has been found using orthogonal titanium electrode pairs embedded on the surface of a spherical pressure housing
to sense horizontal ocean currents. Such instruments reliably measure currents
ranging to 750 cm.s
−1
(about 50 knots) and can achieve resolution thresholds below
0.05 cm.s
−1
.
Acoustic Current Meters and Profilers Acoustic instruments make use of the
piezoelectric effect undergone by materials such as ceramics and quartz crystals that
generate an electrical potential upon mechanical stimulus or, vice versa, suffer
mechanical deformation upon electrical stimulus. Electrically triggered pulsed
deformation of a transducer immersed in an aqueous medium produces an acoustic
wave train. In practice, such mechanical transducers are the basis for acoustic communications but have also provided the basis for a wide range of acoustic Doppler
instruments. Piezoceramic transducers which may be fabricated in various dimensions and designs are widely used for acoustic applications.
Single-point acoustic current meters sample small volumes of seawater using
high frequency (up to 6 MHz) transducer arrays. Several models in field use incorporate orthogonal pairs or other transducer arrays to provide detailed, extended 3D
data time series of current speed and direction. Rapid alternating operation of a
transducer pair to either receive or transmit an acoustic pulse allows determination
of the velocity vector component parallel to the pair axis by measuring the acoustic
phase shift caused by along-axis water transport. Additional orthogonal pairs allow
2D and 3D resolution of the current field. Such instruments can achieve threshold
measurements down to less than 0.05 cm.s
−1
sampling at rates up to 10 Hz. Operating
potentials are in the range of 5–32 V DC with typical current drain below
50 mA. Acoustic Doppler velocimeters focus four angled orthogonal transducers
sampling a volume of 6 mm diameter at 5 cm from the transducer plane (Fig. 2.2).
Resolution of such instruments is on the order of 1 mm. s
−1
.
As described above for single depth acoustic current velocimeters, acoustic
Doppler current profilers (ADCP) rely on return echoes from suspended matter
throughout the water column. These ADCPs have found the widest acceptance for
extended operational deployment since a single stationary downward- or upwardFig. 2.2 Acoustic velocimeter for single-point current measurement. The instrument measures
current velocity in a small volume of water at the focus point forward of the transducer array
2 Electronic Sensors and Instruments for Coastal Ocean Observing
On a smaller scale, geometric arrangements of electrodes or induction coils in
field instruments achieve similar results. Single-point electromagnetic field induction instruments for field deployment respond to the movement of ions in solution
swept along in the ocean current. A durable solution has been found using orthogonal titanium electrode pairs embedded on the surface of a spherical pressure housing
to sense horizontal ocean currents. Such instruments reliably measure currents
ranging to 750 cm.s
−1
(about 50 knots) and can achieve resolution thresholds below
0.05 cm.s
−1
.
Acoustic Current Meters and Profilers Acoustic instruments make use of the
piezoelectric effect undergone by materials such as ceramics and quartz crystals that
generate an electrical potential upon mechanical stimulus or, vice versa, suffer
mechanical deformation upon electrical stimulus. Electrically triggered pulsed
deformation of a transducer immersed in an aqueous medium produces an acoustic
wave train. In practice, such mechanical transducers are the basis for acoustic communications but have also provided the basis for a wide range of acoustic Doppler
instruments. Piezoceramic transducers which may be fabricated in various dimensions and designs are widely used for acoustic applications.
Single-point acoustic current meters sample small volumes of seawater using
high frequency (up to 6 MHz) transducer arrays. Several models in field use incorporate orthogonal pairs or other transducer arrays to provide detailed, extended 3D
data time series of current speed and direction. Rapid alternating operation of a
transducer pair to either receive or transmit an acoustic pulse allows determination
of the velocity vector component parallel to the pair axis by measuring the acoustic
phase shift caused by along-axis water transport. Additional orthogonal pairs allow
2D and 3D resolution of the current field. Such instruments can achieve threshold
measurements down to less than 0.05 cm.s
−1
sampling at rates up to 10 Hz. Operating
potentials are in the range of 5–32 V DC with typical current drain below
50 mA. Acoustic Doppler velocimeters focus four angled orthogonal transducers
sampling a volume of 6 mm diameter at 5 cm from the transducer plane (Fig. 2.2).
Resolution of such instruments is on the order of 1 mm. s
−1
.
As described above for single depth acoustic current velocimeters, acoustic
Doppler current profilers (ADCP) rely on return echoes from suspended matter
throughout the water column. These ADCPs have found the widest acceptance for
extended operational deployment since a single stationary downward- or upwardFig. 2.2 Acoustic velocimeter for single-point current measurement. The instrument measures
current velocity in a small volume of water at the focus point forward of the transducer array
2 Electronic Sensors and Instruments for Coastal Ocean Observing