The transducer bridge circuit sensitivity for each channel was adjusted so that 

 a full-scale pressure of approximately +15 psig (103 kPa) , with respect to atmos- 

 pheric pressure, produces a +10 mV output signal. The VCO accepts a +10 mV full- 

 scale input signal and produces +2 kHz (constant bandwidth) deviation in center 

 frequency. The discriminator gain for each channel was adjusted to convert a +2 kHz 

 center frequency deviation into a full-scale output signal of +5 V. Therefore, the 

 overall sensitivity of each channel was designed so that an applied full-scale pres- 

 sure of +15 psig (103 kPa) produced approximately +5 V at the output of a discrimi- 

 nator unit. Thus, the overall system sensitivity was approximately 0.33 V/psi 

 (48V/MPa) . 



A Baldwin optical shaft-angle encoder, geared to the propeller shaft, was used 

 to determine shaft angular position. Two types of pulses were generated; a single- 

 pulse per revolution and a multipulse per revolution (60 equally spaced pulses). 

 The single pulse is referred to a known angular position relative to the propeller. 

 The pulses generated by this encoder are accurate to within +0.01 deg. 



An ultrasonic wave-height probe was used to measure changes in the water level 

 above the propeller. This measurement permitted a correction to the initial pressure 

 zeros taken at the beginning of the run due to any variation in the average water 

 level during the run. The wave probe was zeroed out at the start of a day's running, 

 when the basin's water surface was undisturbed. The instrument zero was checked 

 between runs by inserting a flat plate a known distance below the probe, but above 

 the disturbed water level. 



CALIBRATION 



An extensive calibration program was conducted to arrive at accurate calibra- 

 tions for test purposes, and to investigate possible systematic errors in pressure 

 measurement instrumentation. 



This program was based on a computer-automated calibration system. A cylindri- 

 cal aluminum pressure tank was constructed to house the entire propeller and hub 

 assembly. Pressure inside the tank was monitored with a precision pressure standard 

 and used to calibrate the 40 pressure gages simultaneously. The tank pressure was 

 stepped automatically by computer through a range from -5 to 10 psig (-34 to 69 kPa) . 



