TM No. 377 



Mecca Cable and Service, Inc., Houston, Texas). The output from the modulator 

 circuit is fed through a shielded sea water cable to a zero balance adjuster 

 and, from this, is recorded on a two-channel 720 Sanborn recorder (Sanborn 

 Co., Waltham, Mass.). 



The repetition rate of the pulses of the modulated output voltage is 

 inversely proportional to the angular velocity of the impeller. The shape 

 or signature of the pulse provided by the emf- genera ting circuit indicates 

 in which sense the impeller is spinning. The result is a kind of algebraic 

 frequency analog of the angular velocity and sense of impeller rotation as 

 a function of time. 



The pulse trace was not completely satisfactory, since it was often 

 difficult to distinguish positive from negative rotation signatures. Also, 

 the Marine Advisors circuitry was too sophisticated for the job for which 

 it was intended. Still, the elaborate amplifier and circuitry, involving 

 transistors and diodes, allowed the small perturbations due to the iron 

 slugs passing the coils to be well detected. This facilitated the use of 

 very light iron slugs, which added little to the moment of inertia (hence, 

 to the response time) of the impellers. However, since the entire circuitry 

 of each ducted meter system was potted with an epoxy resin within the 

 pillbox mounted on the cylinder, failure of any electrical components meant 

 complete destruction and replacement of the entire circuitry. Within three 

 months, after two complete circuit replacements, the system was discarded 

 in favor of a simpler system consisting solely of a miniature pickup coil 

 and small alnico magnets. 



Dynamic Characteristics and Calibration — The ducted meters were 

 calibrated for steady and accelerated flow by the manufacturer, and calibra- 

 tion data in the form of a curve were supplied with the meters. These data 

 were obtained by towing the ducted meters individually in a Lockheed 

 Corporation test tank, but only in the direction parallel to the axis of 

 the individual cylinder. 



The calibration curve for steady flow provided by the Marine Advisors 

 for the ducted meters is shown in figure II-5. The abscissa is towing speed 

 Vj (cm sec" 1 ), and the ordinate is the repetition rate or output frequency 

 of the voltage pulses fy (cps). Since the impeller has five blades, fy 

 represents five times the impeller frequency. This curve was taken from 

 data plotted on semi- logarithmic paper, and originally contained 38 points 

 ranging from 2.1 to 361 cm sec" 1 . The linearity of the calibration plot was 

 preserved extremely well for even these extreme high speeds. The relation 

 of fy to the towing speed is linear with a proportionality constant Kf of 

 4.25 cm. It appears that the threshold of velocity measurable is near 2.0 - 

 3.0 cm sec" 1 . The slope of the line Kf" 1 is about 0.233 cm" 1 . 



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