to represent the signals from the buoy. Care 

 must be taken in distinguishing the meter signals 

 from outside interference. 



Where facilities do not permit the installation 

 of a shore station, the receiver is placed aboard 

 a survey vessel with much the same equipment 

 and observational procedure. 



9-61 Operating the Roberts Current Meter 

 Directly From Aboard Ship. — When current 

 studies are made directly from a survey ship 

 with a Roberts meter, there is no need for the 

 transmitter and receiver as the meter may be 

 connected directly to the relay box. A dia- 

 grammatic sketch for shipboard meter use is 

 shown at bottom of figure 9-7. 



The ship must be anchored to obtain current 

 readings. The meter is lowered to the desired 

 depths by its cable, which is then connected to 

 the relay box; signals are recorded on the 

 chronograph. 



9-62 Maintenance and Repair of Roberts 

 Radio Current Meter Equipment. — Extensive 

 repairs to the interior mechanism of the meter 

 are practically impossible in the field because 

 of the meter's delicate construction. The outer 

 shell, however, where damage commonly occurs, 

 usually can be repaired where ordinary machine 

 shop facilities are available. In most cases, 

 ordinary repairs or replacements of accessory 

 equipment can be carried out easily. 



9-63 OBSERVING AND SCALING PRO- 

 CEDURE.—!. Method oj Recording.— The sig- 

 nals from a radio current meter station, having 

 been identified according to the wavelength 

 broadcast by the buoy, are carefully tuned to 

 produce the maximum signal as nearly as pos- 

 sible clear of interference or noise and then 

 switched to the chronograph. The latter, 

 running and marking seconds of time, records 

 the signals on a trace superimposed on or 

 closely parallel to the seconds record. This 

 is possible only if the buoy signal strength 

 is clearly above the noise or interference level. 



A tape record is obtained, free of inter- 

 ference, or with only slight interference which 

 the observer identifies as such and marks out 

 on the tape as it occurs. This need not be long; 

 ordinarily ji to ji minute of unmistakable sig- 

 nals is sufficient. The tape, shown in figure 

 9-8, is scrutinized to find a series of valid 

 marks at regular intervals, which are labeled 

 "V" (velocity). Between each second pair 

 of such marks should be found another mark, 

 which is labeled "D" (direction). 



If the radio signals are below the noise level, 

 and cannot be recorded clearly, though dis- 

 tinguishable by ear over the earphone or 

 loudspeaker, the chronograph may be run, 



100 



-tv-'iA — V-t^A- 



SHOWING METHOD OF COMPUTING VELOCITY 

 = CONTACT INTERVAL "^^ 



INTERVAL 

 1 01-099 



VELOCITY 

 KNOTS 



SHOWING BISECTION TO INDICATE POSITI 

 OBSCURED VMARK WHEN SIGNALS OVERLAP 



SHOWING METHOD OF SCALING 

 DIRECTION WITH CELLULOID 

 SCALE 



EEI3E 



Figure 9-8. Tape scaling. 



automatically marking seconds while the ob- 

 server produces hand-controlled marks (by 

 key or by tripping the stylus) to represent the 

 buoy signals. This procedure is less accurate 

 than that inherent in fully automatic recording. 



A special case occurs when the direction of 

 current flow is N (magnetic), causing the 

 velocity and direction signals to merge or over- 

 lap. If it is an unmistakable overlap, produc- 

 ing alternate long and short marks, the short 

 ones may be taken to be V-signals, and the 

 (obscured) intervening V-marks plotted by 

 bisecting the space between two of the short 

 marks, as shown in figure 9-8. If the co- 

 incidence is so perfect as to indicate no system- 

 atic difference in signal length, then the current 

 direction may be regarded as N (magnetic) 

 and the marks labeled alternately V and VD. 



2. Determination of Velocity. — A definite 

 number of V-signals is counted from the tape. 

 The corresponding time interval is determined, 

 using a chronograph scale if necessary (though 

 experience will demonstrate that a substantial 

 portion of the observations can be made by 

 estimation of the second fractions). A simple 

 division affords the time-per-contact interval 

 with which to enter the rating table for velocity. 

 At high current velocities where the intervals 



H. O. 607 



