u .u 



TM Wo. 377 



(2) current direction relative to magnetic north j (3) instrument tilt 

 angle; and (k) tilt direction. Each of these data is photometrically- 

 averaged over a preset time intervale This photometric averaging process 

 is accomplished in the following manner . The Savonius rotor (speed sensor) 

 located at the base of the meter is magnetically coupled to a gear train 

 within the cylinder housing,, During the sampling interval, this gear 

 train, which has a reduction ratio of 7200 to 1, rotates a radioactive, 

 phosphorescent light source through an angle proportional to the number of 

 revolutions. This light source is photographed with the l6 ma camera as it 

 traverses an arc. The estimate of speed is obtained from the film as an 

 arc of varying intensity, with the highest exposure density at the slowest 

 speeds. 



Current direction is obtained by referencing the direction of the current 

 meter cylinder housing with respect to a precision fluid-damped compass 

 mounted in the meter. The cylinder housing is connected to a fiberglass 

 vane about 1 square meter in area (see figure XV-5). The cylinder case 

 and vane systems tend to align themselves with the direction of mean flow 

 by pivoting about the support shaft. The vane direction is recorded on 

 film by use of two phosphorescent light sources, one of which remains at 

 magnetic north while the other rotates with the instrument. As with the 

 speed record, the position on the arc of the highest optical density 

 indicates the most prominent direction of flow, whereas the extremes of the 

 arc delineate the azimuthal excursion over the sampling interval. 



The instrument incorporates a tilt sensor in the form of a viscous~ 

 damped plumb bob containing at its lower extremity a phosphorescent light 

 source. A disk indicator, which is mounted between the plumb bob light 

 source and the film, permits simultaneous recording of the tilt angle and 

 direction, referenced again to the compass. The tilt record allows one to 

 compare the tilt angle and azimuth with the velocity vector, and also to 

 correct the speed record for extreme tilt. For both records the tilt was 

 not over 5° so that no correction was needed. 



A cam system driven by an electric clock controls the periodic advance 

 of the film record (i.e., the sampling interval). The period of sampling 

 can be adjusted to the desired interval (from 5 minutes to many hours) by 

 varying the cam geometry. The photographic film data from the current meter 

 system were processed and decoded by the Braincon Corp. The punch-paper 

 tape listings were then taken to the KIUW3 computer laboratory and placed 

 on digital magnetic tape, giving mean current speed and direction at equal 

 time intervals. Wind speed and direction were recorded at four-hour inter- 

 vals during the periods of current observation and are Issted in appendix C. 



Throughout both series of current measurements, the G&GS pressure-recording 

 tide gage (bubbler gage) monitored the free surface elevation. This device 

 registers the variation. of the sea surface pressure head, as transmitted hydro- 

 statically from a submerged sensor fastened to the southeast leg of the BBELS. 

 The bourdon type pressure gage is coupled to and moves the ink pen of a single- 



78 



