operation is to channel flow across an impeller mounted in a duct (Fig. 9). 

 Magnets embedded in the blades of the impeller close reed switches, and in 

 a bidirectional sensor the switching sequence (Fig. 10) determines the 

 direction of flow. 



Ducted, impeller current meters by Bendix were used in experiments 

 during the last 3 years. The first set of meters had four-blade impellers 

 in a 7.62-centimeter (3 inches) duct (Bendix Model B-7) modified for 

 bidirectional response (Fig. 11); the second set of four meters (Bendix 

 Model B-10) was designed to be bidirectional sensors with five-bladed im- 

 pellers in 10.16-centimeter (4 inches) ducts. In the latter model, range 

 is to 5 knots (0 to 257 centimeters per second) with a threshold velocity 

 of 2 centimeters per second. In the range 1 to 100 pulses per second, 

 meter response is linear (Fig. 12). For calibration purposes, 16 pulses 

 per second equal to 1-knot current is used. 



b. Wave Gage. Measurement of the height and period of waves, con- 

 currently with the sensing of current speed, is obtained from a pressure 

 wave gage (Fig. 13). This instrument consists of a solid state, strain 

 gage pressure transducer with a range of to 11.34 kilograms per square 

 centimeter-absolute (0 to 25 pounds per square inch-absolute) and housed 

 in a 0.63- by 27.3-centimeter (2.0625- by 0.896-foot) (outside diameter) 

 aluminum cylinder. The gage senses the total pressure at a given depth 

 from which the dynamic pressure component is extracted and used as the 

 measure of wave height given the pressure correction factor at that loca- 

 tion. Since it is difficult to determine the pressure correction under 

 various conditions of shoaling waves, the gage is usually mounted adjacent 

 to one or more current meters which provide an independent estimate of the 

 correction factor. The wave gage is designed to switch ranges in incre- 

 ments of 3.048-meter depths with a ±0. 61-meter overshoot. This gage was 

 designed at CERC, and permits conduct of continuous surveys in variable 

 water depths to a maximum of 9.14 meters. 



c. Auxiliary Instrumentation . A comprehensive coastal oceanographic 

 experiment usually requires the measurement of several additional param- 

 eters, such as windspeed and direction, barometric pressure, salinity, and 

 water temperature. The TODAS is capable of accommodating these future 

 requirements in one of two ways. Sensors can be added to the sea sled 

 part of TODAS, thereby requiring only the installation of one voltage- 

 controlled oscillator (VCO) for each new sensor (assuming an output of 



to 5 volts (direct current)). Signals from these sensors can be mixed 

 and demodulated in the same fashion as for existing current meters and 

 the wave gage. One frequency discriminator has to be installed at the re- 

 ceiving unit for each sensor. 



Another capability of TODAS is the direct input of signals from sen- 

 sors not housed on the sled (e.g., from a shore-based anemometer). The 

 signal input, in this case, can be either analog or digital (Binary Coded 

 Decimal- BCD) format; a total of 32 sensors can be accommodated. 



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