HIGH-ACCURACY, SELF-CALIBRATING ACOUSTIC FLOW METERS 
by R. A, LESTER, Research Engineer 
Westinghouse Research Laboratories 
Pittsburgh, Pennsylvania 
ABSTRACT 
A number of acoustic flow meters have been 
developed and tested for a variety of uses. All 
of the units transmit in opposition and compare 
the received phases; the phase difference is a 
function of the water velocity. The first unit 
was one which used an amplitude-modulated con- 
tinuous wave signal and measured speeds from 
0.15 ft/sec to 15 ft/sec to an accuracy of 1%. 
A second type compares the carrier frequency and 
measures speeds from approximately 0.005 ft/sec 
to 0.5 ft/sec with an accuracy of 1%. A third 
pulse-type system is being developed; it is 
simpler than the original unit and eliminates a 
number of the inherent inaccuracies of the above- 
mentioned units. One of the desirable features 
of all of these units is the ability to be 
calibrated or checked for zero reference in the 
field. 
INTRODUCTION 
For monitoring ocean currents, wave particle 
velocities, flows about ships and models, and 
flows in pipes, flow meters which are both 
accurate and dependable are required. Acoustic 
flow meters show great promise of fulfilling this 
need. Acoustic meters have several salient 
features: one, they are nonfouling since there 
are no moving parts; two, they are linear devices 
with good accuracy; three, they are directive; 
last, but no means least, they may be calibrated 
in the field. Several acoustic flow meters have 
been built and tested to measure flows in the 
ocean. The construction and limitations of these 
meters will be discussed. 
OPERATING PRINCIPLE 
These flow meters, like most acoustic flow 
meters, measure the difference in travel time of 
sound in the upstream and downstream directions 
of the flow. The fluid velocity is then: 
a 
At C 4 
Ve= OL it Vi << iC 
where = velocity of the fluid, 
distance the sound travels, 
speed of sound in the medium, 
difference in time delays. 
and 
200 
To get an order of magnitude 
involved, consider a flow of 
If L is 10 feet, then At = 2 
of the numbers 
1 ft/sec in water. 
Lv/C“ = 0.8 usec. 
To obtain accuracies of 
must be capable of resolving time delays of less 
than 1 wsec. This is most conveniently done by 
measuring the relative phases of two sinusoidal 
signals. Since the measurements of the upstream 
and downstream travel times are to be made 
simultaneously, there must be two sets of receiv- 
ing equipment with their associated time delays. 
Therefore, an error in the measurement of At will 
be introduced unless the two receivers have equal 
time delays. In the meters described here, 
provision is made to balance the receiving equip- 
ment and thus "zero calibrate" the meter from 
time to time to insure accuracy. In the first 
two of these meters, this "zero calibration" may 
be made in the presence of a flow. The third 
meter must be "zero calibrated" under zero flow 
conditions; thereafter, its calibration is a 
function of the stability of quartz crystal 
oscillators. 
-l ft/sec this system 
THE EQUIPMENT 
The first meter built was a CW dual 
frequency system which was designed to measure 
flow up to 15 ft/sec. For the particular appli- 
cation, it was desirable that the flow be averaged 
over a ten-foot distance. This system represents 
perhaps the most straightforward electronic 
circuitry which will make the measurement (see 
Fig. 1). Four transducers are used in this flow 
meter, two on each probe. The pairs of trans- 
ducers are separated by about an inch. In order 
to get a narrow acoustic beam width, high frequency 
carriers of 1.1 and 1.6 meps are used. These are 
modulated at 20 kc. The received signals are 
amplified, detected, and applied to the phase 
meter. The relative phase of the 20 ke signals 
is then proportional to the flow. Zero calibration 
is accomplished by reversing the direction of the 
acoustic path of one of the frequencies. With 
the signals in the same direction the water path 
time delays of both signals are identical; hence 
the electrical delays of the system may be 
adjusted to "zero". 
This system, although quite simple electronically, 
has a number of potential weaknesses. To begin 
with, it requires 2 cables to each post, 2 trans- 
ducers in each post, 2 transmitters operating at 
