2 widely separated frequencies and, because of 
the physical separation necessary due to trans- 
ducer size, two acoustic paths are required. 
Errors may occur because of electrical leak- 
through or due to acoustic path separation in a 
highly inhomogeneous medium; it is also possible 
that errors of a secondary nature may occur due 
to the large frequency separation of the trans- 
mitted signals. 
The following system, shown in Fig. 2, was 
constructed to circumvent these two weaknesses. 
In this flow meter, the high frequency carrier 
with a 20 ke modulation is again used. Two 
transducers are driven from the same pulsed 
1.1 mcps transmitter. The transmitter is turned 
off when the space between the transducers is 
filled with acoustical energy. The signals are 
detected and again the phase difference of the 
20 ke signal is proportional to the flow. A 
sampling circuit after the phase meter remembers 
this phase difference until the phase is altered 
by the next received signal. In this way, 250 
samples of the fluid velocity are taken each 
second. The resistors shown are included to 
insure that the transducers will see the same 
impedance when transmitting as when receiving, 
thus insuring their reciprocity. This system 
uses a single cable to each post, one transducer 
in each post, a single acoustic path and a single 
frequency. Crosstalk problems and acoustic path 
problems are minimized and equipment problems 
due to water exposure are halved. 
The third system, shown in Fig. 3, isa 
pulse system which uses free running crystal 
oscillators to store the phase information. It 
was built to measure flows of less than two knots 
maximum velocity. In this meter the transducers 
are spaced at a two foot interval. A pulsed 
1 meps signal is applied to the two transducers 
for 400 usec. This signal is received and used 
to lock two slave oscillators. These slave 
oscillators are then heterodyned with a local 
oscillator and the resulting audio signals are 
applied to the phase detector. The output of 
the phase detector then indicates the flow. 
During the time no energy is received, the slave 
oscillators free-run, thus providing continuous 
information to the phase meter. The range of the 
velocities to be measured or the post spacing 
may be varied by changing the carrier frequency. 
SOURCES OF ERROR 
The error of these systems due to the 
electrical circuits is quite small. The phase 
meter has been the limiting component and its 
error is approximately 0.34%. 
In homogeneous water the accuracy of the 
instantaneous readings is limited by the vortices 
created by the leading transducer when the 
acoustic beam is in the wake of this probe. 
This vortex shedding causes a fluctuation in the 
201 
output which increases in magnitude and frequency 
with increasing speed. For the 2-3/8" probes 
used, the measured velocity fluctuated about 0.2 
knots at a speed of 8 knots, and the fluctuation 
had a frequency of about 17 cps. If the flow 
meter is run at an angle to the flow, such that 
the acoustic beam is not in the wake of the lead- 
ing transducer, there is no error due to vortex 
shedding. 
In inhomogeneous water, the errors introduced 
are a function of the velocity of the fluid, the 
magnitude of the velocity anomalies, the patch 
size and the distance between transducers. These 
inhomogeneities can introduce errors in several 
ways. 
One mechanism which can cause errors is the 
phenomenon of multiple path transmission which 
results in signal amplitude fluctuations. This 
occurs when parts of the acoustic beam are re- 
fracted in a nonuniform manner so that they arrive 
at the receiving element out of phase. This 
effect varies as the distance between the trans- 
ducer elements; however, the exact manner in which 
it varies is, at present, not completely 
understood. The effect of this secondary 
path is to add to the direct signal another sig- 
nal of slightly different phase. The sum of 
these two signals results in a fluctuating 
received amplitude. Tests were conducted at sea, 
at Scripps Institute, and in the Laboratory to 
determine the magnitude of this effect. It was 
determined that the signal level fluctuated 
approximately 12 db under fairly severe thermal 
conditions. Using this as a figure for the kind 
of fluctuation one might expect at sea, the 
velocity error due to the multiple path trans- 
mission may be calculated. If it is assumed that 
the second path signal is equal in magnitude to 
the direct signal, a delay of 166 electrical 
degrees of the carrier frequency would be required 
in order for the resulting amplitude to change to 
one-fourth of its original value. The resultant 
phase of the received signal would then be 83 
different from the phase of the direct path. 
This condition would lead to an apparent time 
delay of about 0.25 usec or an instantaneous 
error of about 0.3 ft/sec in the measured 
velocity. This error does indeed occur occasion- 
ally under severe thermal gradients in the order 
of 1 to 2 F per foot. A secondary effect of 
the multiple path is the effect of the amplitude 
change on the time delay of the receivers. The 
receivers may be designed so that error due to 
amplitude fluctuations are negligible. 
Inhomogeneities can also cause errors due to 
the fact that the acoustic signals traveling in 
opposite directions are not in the same water at 
the same time. If the water velocity is large 
enough and the thermal patches sufficiently 
small, the sound traveling in one direction will 
travel through different water than the sound 
traveling in the opposite direction. This effect 
is quite difficult to separate from the effect of 
