Ultrasonic liquid level gauge for tanks subject to movement and vibration

One or more high-frequency ultrasonic transducers are used to measure the liquid level of tanks containing any type of fluid, but fuel in particular. The invention relates specifically to tanks that are subject to movement and vibration which generally makes the use of ultrasonic echoes unreliable for obtaining accurate level measurements. A special algorithm is used to obtain the temporal center of the distribution of echo arrival times over a preset time interval. From this temporal center of an echo distribution, the liquid level is readily obtained through the acoustic velocity, time and distance relationship. An annular piezoelectric plate, independently driven at low ultrasonic frequencies (kHz range), mounted on the tank bottom surrounds the high frequency ultrasonic transducer. The function of the piezoelectric plate is to send out propagating ultrasonic waves (essentially longitudinal) to maintain the tank area in the immediate region of the high-frequency transducer free from debris and sediment deposits at the bottom of the tank thereby avoiding the uncertainty in the measurement that is introduced by debris on the tank bottom.

FIELD OF THE INVENTION 
The invention relates generally to the use of ultrasonic echoes to measure 
liquid levels in storage containers, particularly fuel levels in tanks 
subject to movement and vibration. The invention utilizes temporal 
averaging of the reflected acoustic wave to obtain accuracies not 
obtainable from ultrasonic gauges previously known. In addition, means are 
provided for maintaining a clean tank surface locally to avoid false 
readings due to the accumulation of sediment and debris precipitating over 
time from the liquid. Such debris cause both attenuation and additional 
time delays of the echo arrival time. 
BACKGROUND OF THE INVENTION 
Applications using ultrasonics to determine the thickness of solids and the 
height of liquids are well known. However, for the case of liquids stored 
in tanks subject to vibration or movement as well as hostile environments, 
it becomes difficult if not impossible to measure liquid levels with any 
degree of accuracy. In fact, for some vehicles such as diesel locomotives, 
airplanes and helicopters, reliable fuel gauges are presently not 
available. For diesel locomotives, the standard means for obtaining 
reliable fuel level measurements requires bringing the locomotive to a 
full stop and using a dip stick as a level marker. In numerous aeronautic 
vehicles, fuel tanks are generally filled before takeoff. Computer 
calculations based on an airplane's approximate load and distance 
travelled are made to determine the amount of fuel remaining in the tank 
as the flight proceeds. These calculations, however, are subject to 
considerable uncertainty which have at times led to unfortunate results 
including fatal loss of aircraft due to lack of fuel. Existing mechanical 
and electronic fuel level gauges for planes and trains are generally 
considered unreliable so that better schemes are very much in demand. 
U.S. Pat. No. 5,315,563 and 5,379,658 to Lichtenfels et al., describe a 
fuel gauge using ultrasonics, specifically addressing ways of mounting the 
ultrasonic transducer both within and outside of the tank. The purpose of 
the invention is to provide a simple mounting scheme along with means for 
disassembling the transducer and its mount for maintenance. Included also 
is the use of a stillwell for capturing some fluid in a cylindrical region 
encompassing the acoustic wave in order to minimize swashing and the 
accompanying distortion of the echo pattern. No method or application is 
described for obtaining accurate data when the tank or liquid is in motion 
or subject to vibration. 
U.S. Pat. No. 5,309,763 to Sinclair et al., describes an embodiment 
comprising a vertically mounted tube with holes to allow liquid flow into 
the tube. The tube has reflectors mounted at different heights from which 
ultrasonic echoes can be reflected. A comparison is made between the echo 
from the uppermost submerged reflector and the liquid-air interface. These 
echoes are processed to determine the actual liquid level, albeit 
approximate. The echoes from the various vertically mounted reflectors can 
also be used to determine the ultrasonic velocity of the liquid as a 
function of height. This permits an average velocity to be determined and 
accounts for stratification and velocity variation with temperature of the 
liquid. No means are provided for averaging the velocity due to vibration 
or swashing of the liquid. 
U.S. Pat. No. 5,053,978 to Solomon describes the use of an acoustic 
transducer mounted from the top of a stationary fuel storage tank with the 
sound directed through the air space onto the liquid to obtain the depth 
of liquid (fuel). The invention includes means for collecting additional 
tank data relating to the tank's condition using additional sensors. These 
data can be used for purposes such as automated calls to a service agency 
for needed boiler maintenance or fuel refill. 
U.S. Pat. No 4,821,569 to Soltz et al., describes a liquid level meter 
using ultrasonic echoes from an air-liquid interface and means for 
discriminating and rejecting false or parasitic echoes. Such echoes can 
arise from walls of the container, for example. A time window is provided 
which allows a comparison between the real echo and the false echo, the 
false echo generally taking more or less time in its roundtrip path. Thus, 
if the echo does not fall within the prescribed time window, means for 
rejecting it are described. If values occur consecutively for a pre-set 
number of times within the time window, they are accepted as the new true 
reading. 
U.S. Pat. No. 5,131,271 to Haynes et al., describes the use of an 
ultrasonic level detector that uses a comparator to determine whether a 
signal from a pulse echo is sufficient in amplitude to be recorded as 
valid. The ultrasonic transmitter/receiver is mounted at the top of the 
tank and measures the time of flight of the echo as it is reflected from 
the air/liquid interface. Also generated is a time window during which a 
received signal can be accepted below a certain set level. Outside of this 
time window, signals can be accepted that are higher in intensity or 
exceed a certain threshold. The system is connected to a computer and 
amplifier circuit, the latter in turn connected to a gain table to adjust 
the amplituides accepted by the computer to be counted as valid for 
determining the level of the tank. 
U.S. Pat. No. 5,319,972 to Oblak et al., describes a specific application 
that measures the liquid level in a reactor but can be used for other 
purposes. Echo signal heights are averaged over time and the various 
signals are put into time bins. The earliest arriving signal is used to 
determine the liquid level, but background pulses are analyzed to make 
sure that the the foreground signal is the true signal representing the 
correct arrival time. Means are provided for setting different levels of 
the arriving signals to ascertain their validity. There is no time 
averaging performed to determine the correct arrival time from the 
histogram that develops from the array of recorded arrival times. 
An ultrasonic method for measuring liquid height where the signal from the 
transducer is reflected from a float rather than the air-liquid interface 
is described in U.S. Pat. No. 5,319,973 to Crayton et al. Also described 
is a means for monitoring the temperature of the liquid to correct for 
ultrasonic velocity temperature dependence. This has certain advantages 
over the more conventional manner of echo reception but there is no 
indication it would work in liquid environments subject to vibration and 
splashing. 
U.S. Pat. No. 5,400,376 to Trudeau describes means specifically designed 
for measuring fuel levels in aircraft fuel tanks using a large number of 
independent sensors. Each sensor is located at a different location within 
the tank to give a multiplicity of readings, each of which is recorded and 
stored. From the multiplicity of readings given by the logic counter, a 
level can be determined. The problem of how these data are correlated to 
take into account shaking and vibration of the tank are not described nor 
does the patent describe specific solutions to that problem. 
In general, when fuels and many other liquids are kept in storage for any 
length of time, a buildup of debris will occur at the bottom of the tank. 
This buildup can greatly interfere with ultrasonic measurements in that 
the debris attenuates the ultrasonic wave. Also, the irregular surface 
tank bottom surface which is likely to result distorts the echo pattern 
both spatially and temporally. None of the above-cited art describes a 
means for accurately obtaining liquid levels for the case where the liquid 
is vibrating or in motion using a computer algorithm in combination with 
ultrasonic echo techniques. None of the cited art describes means for 
eliminating debris and sediment that occurs from liquids such as fuel oils 
in storage tanks. Thus, the techniques of the cited art are incapable of 
determining liquid levels with any degree of accuracy using the echo 
technique, especially in environments where vibration and shaking of the 
liquid are present. 
It is extremely important for essentially all moving vehicles to keep track 
of fuel levels. Aside from the dangers inherent in running out of fuel it 
is important for diesel locomotives, for example, to know at what location 
fuel should be taken on. With accurate fuel level data, a decision could 
be made by a central tracking station as to where to stop for refueling to 
obtain the best or lowest available fuel cost, consistent with the fuel 
remaining in the engines, thereby giving rise to substantial cost savings. 
For example, one railroad estimates that for their 3000 locomotive fleet, 
an effective 1 cent/gallon fuel cost reduction could result in a $6 
million saving per year. 
SUMMARY OF THE INVENTION 
The present invention uses a continuous or quasi-continuous signal 
averaging technique to present a distribution of echo signals as a 
function of time from which liquid levels can be accurately determined and 
monitored on a continuous or quasi-continuous basis. Several averaging 
methods are described from which the temporal center of the distribution 
can be determined, this time representing that of the undisturbed or true 
liquid level. A means for preventing debris collection in the region of 
the echo transducer is provided in order to ensure level height accuracy 
by preventing attenuation and distortion of the ultrasonic wave. 
One or more ultrasonic transducers are firmly mounted on the bottom of a 
fuel tank (tightly coupled acoustically) to transmit and receive acoustic 
pulses. At least one transducer is surrounded by a piezoelectric (piezo) 
annular plate which operates at low ultrasonic frequencies, typically on 
the order of 10-50 kHz, to maintain a local region of the bottom of the 
tank free from debris such as fuel sediment, algae growth etc. The 
received echo pulses are preferably rectified and filtered prior to 
processing by a channel analyzer or boxcar integrator. With a convenient 
pulse repetition rate on the order of 1000 pps it is possible to obtain 
ultrasonic echoes which will, however, vary in amplitude and also have a 
temporal distribution in their arrival time due to vibration and/or 
movement of the tank causing splashing of the liquid within the tank 
confines. Signal processing is used to determine the center of the echo 
arrival time from the arrival time distribution. This computed time is 
used by a computer to obtain the level height of the tank after 
temperature corrections have been made to the velocity of the acoustic 
wave in the liquid using one or more thermocouples or similar thermal 
sensors in the liquid connected to the computer. Thermal corrections are 
made to the acoustic velocity based on data stored in the computer's data 
file. A measure of the sediment level can also be obtained by comparing 
the echo signals with a transducer mounted without the annular piezo plate 
or from a transducer where the piezo plate is not activated. Signals from 
this transducer will have a delayed arrival time from which a thickness of 
sediment can be approximated knowing the acoustic velocity in the sediment 
medium.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 depicts a fuel tank 10, typically steel, onto which is mounted a 
piezoelectric crystal or piezoelectric high frequency transducer 12 
typically of a piezoelectric material such as PZT 
(lead-zirconate-titinate). The preferred frequency operating range for 
transducer 102 is about 0.5-10 MHz though it is not necessarily limited to 
that. The pulse repetition rate is preferably in the range of about 
500-2000 pulses per second although this, too, is not a strict limitation. 
The emitted ultrasonic pulse travels through tank wall 30, into liquid 16. 
The ultrasonic echo reflection of interest occurs at liquid/air interface 
17. 
A piezoelectric plate 14 preferably in the shape of an annulus, surrounds 
center mounted echo transducer 12. Circular plate 14 functions as a low 
frequency transducer to maintain the area in the vicinity of inner 
transducer 12 free from debris or sediment that can precipitate from the 
fuel or other liquid 16 stored in tank 10 over a period of time. Annular 
transducer 14 (also referred to as the transducer plate) consists 
typically though not limited to PZT material (leadzirconate-titinate) and 
is designed to resonate in the kHz range, typically 10-50 kHz though not 
limited to that range. It is connected to its own driver unit 18, which 
can be controlled independently from transducer 12, and is normally 
operated in a cw (continuous wave) mode for an interval of time that may, 
for convenience and best signal to noise purposes, be interrupted during 
the time that the echoes from high frequency transducer 12 are sampled for 
level height measurement. Driving unit 18 is not normally equipped with a 
receiving section. At sufficiently high driving voltages from driver unit 
18, a cleaning action occurs from longitudinal vibrations emanating from 
plate 14 at relatively low frequencies. Acoustic energy densities of a few 
watts/cm.sup.2 generally cause sufficient cavitation (sudden bubble 
collapse within the liquid due to ultrasonic excitation) to keep debris 
and dirt from collecting on the bottom surface of the tank in the 
immediate region of the transducer. 
Also shown is driver/receiver unit 20, also known as a transceiver, 
connected to the inner mounted high frequency echo transducer 12. The 
transceiver 20 consists in part of a voltage pulse unit which applies a 
periodic voltage pulse to transducer 12 for generating a propagating 
ultrasonic pulse in a manner well known in the art. In a typical 
embodiment, the ultrasonic pulse is emitted and the echo sensed by the 
same transducer, i.e., transducer 12. The pulser unit 20 both sends out 
the acoustic pulse and receives and amplifies the returning echo. This 
unit is sometimes referred to in the art as a pitch/catch transceiver 
whose pulse repetition rate can be varied, typically from 10-1,000 pps and 
whose variable gain can produce electrical excitation pulses as high as 
250 V or greater. To produce the ultrasonic excitation pulse for the high 
frequency transducer, one typically uses electrical excitation pulse 
widths on the order of 0.5 microseconds or less. Unit 20 may contain an 
electronic switch which permits a received echo pattern or return pulse to 
be sensed by transducer 12 which is amplified by amplifier unit 22. The 
output of amplifier 22 is connected to a signal averaging unit such as a 
channel analyzer or a boxcar integrator 24. The output of signal averager 
24 serves as the input signal to computer processing unit 26 which 
computes the received echo amplitude distribution with time. 
In FIG. 2, the input to averager 24 can, for example, be the output of 
transducer 12 or an amplifier 22 which has amplified the output of 
transducer 12. A channel analyzer or boxcar integrator are two examples of 
signal averagers 24 that can be used to obtain the required temporal 
(time) distribution 50 from which the level height can be determined. The 
echo signals from the transceiver output is averaged by the signal 
averager over a predetermined fixed interval of time. For example, echo 
signals might be averaged over a period of time of from about 5 seconds to 
5 minutes using a pulse repetition rate of 1000 pps. Operating with these 
parameters gives rise to a representative distribution of echoes and echo 
arrival times (e.g., time to transit a path from the transducer and back 
from the liquid air interface) when the tank is experiencing motion, 
vibration or a combination thereof. 
A computer algorithm stored in computer 26 processes the output of signal 
averager 24 in the following way. The center of the temporal distribution 
is subtracted from the time the ultrasonic pulse was emitted, to, thus 
corresponding to the time required for the pulse to traverse a roundtrip 
path in the undisturbed liquid. A quasi-continuous value for the fuel 
level can be obtained by echo sampling in a quasi-continuous manner as the 
fuel level decreases with time. More specifically, the mean distribution 
can be obtained from the following: Consider a set of echoes A.sub.i 
(t.sub.i). Here A.sub.i is the amplitude of the echo arriving at the 
receiver at time t.sub.i. Over a specified sampling time, the average 52 
of the distribution of arrival times, t.sub.av, is determined from the set 
of arrival times, irrespective of the amplitude. Thus, the averaged 
arrival time from which the liquid level can be determined is given by 
EQU t.sub.av =t.sub.1 +(t.sub.f -t.sub.l)/2! (1) 
where t.sub.1 is the first echo to be counted, and t.sub.f is the last echo 
to be counted in the temporal averaging scheme. 
The ultrasonic velocity, v in the liquid can be corrected for temperature, 
T, using one or more thermocouples 28 in liquid 16 at different level 
heights along the tank. Thermocouples 28 are connected to computer 26 to 
provide correction data using the computer's data file of ultrasonic wave 
velocity as a function of temperature. When several thermocouples 28 are 
used, the velocity corrections can be made in a step-wise fashion in which 
case v is v (T). An additional correction is made to the echo transit 
time, t.sub.av to account for the round trip travel time of the ultrasonic 
pulse through tank wall 30. This correction equals 2D/v.sub.D or .DELTA.t 
where D is the thickness of tank wall 30 and v.sub.D is the velocity of 
the ultrasound in tank wall 30. Ultrasonic echoes that occur from multiple 
reflections from tank wall 30 and other spurious echoes of small amplitude 
can be rejected by filtering or setting the analyzing receiver (boxcar or 
channel analyzer 24) to levels such that those echoes below a 
predetermined nominal amplitude value, Am, or those echoes arriving before 
a preset time t.sub.p are not accepted by signal averager 24. Then the 
true height of the liquid, H, in tank 10 is given by 
##EQU1## 
The use of the center of the temporal distribution of echo arrival time 
results in a means for determining the level height based on the 
statistics of random motion. Generally, vibration and movement of a liquid 
container cause motion of the liquid surface whose time average is 
equivalent to that of a stationary surface. For an ultrasonic echo, the 
liquid top reflecting surface or plane has equal probability of being 
above or below the true level of the liquid. Under these conditions, the 
acoustic path lengths are both longer and shorter than that for the the 
undisturbed liquid. Thus, we find that the true level height lies in 
between the two extreme paths, that is the shortest and the longest 
acoustic path length (proportional to the echo arrival times). For 
completely random motion, the center of the echo arrival time distribution 
is directly proportional to the actual liquid level height. 
Shown in FIG. 3 is transducer 34 which can be utilized in addition to the 
one mounted on the bottom or can be used as a stand-alone. Transducer 34 
is mounted on the top of tank 10. Also shown is circular plate transducer 
44, driven by sonic driver 46, which functions in a manner similar to 
transducer 14, but here is effective in keeping the area surrounding 
transducer 34 free of condensate. In this embodiment, the ultrasonic pulse 
travels through upper tank wall 36 and then through a region 38 consisting 
of air and liquid vapor. Acoustic reflection occurs at air/liquid 
interface 40, resulting in a return echo which is received by top mounted 
transducer 34. Transducer 34 must necessarily have a peak frequency 
considerably lower than bottom mounted echo transducer 12 due to the 
strong frequency dependent attenuation of sound waves in air above 
.about.150 kHz. For tank top mounted transducer 34, a preferred frequency 
range would be about 25-100 kHz. The output of transceiver 42 for 
transducer 34 is processed in a manner similar to the one just described 
for transducer 12. These data can serve as an additional check on the 
accuracy of the data from the lower echo transducer or as an independent 
measure when used as a stand-alone. Top mounted transducer 34 gives a 
measure of the amount of fuel missing from the full tank, a fixed quantity 
which translates to a known distance. The difference between the full tank 
echo arrival time and the echo arrival time after fuel has been used, 
gives a direct measure of the fuel level from the expression, 
##EQU2## 
where v.sub.air is the velocity of the acoustic wave in air, t.sub.av is 
calculated in accordance with Equation 1 and .DELTA.t is calculated as 
described above. The tank level, h, is then given by the expression, 
EQU h=h.sub.0 -.DELTA.h (4) 
where h.sub.0 represents the full tank height. 
FIG. 4 shows an embodiment consisting of an array of two (more can be used) 
transducers 12' and 12" and 13' and 13", both types each operating in the 
same manner already described for tank bottom-mounted single high 
frequency transducer 12 and annular plate 13. The arrival times of the 
echo signals from high frequency array 12', 12" are averaged from each 
transducer element separately or as a single combined average to provide a 
more statistically extensive and therefore a more accurate temporal signal 
average than can be expected from a single unit. Noise will also be 
reduced using this multiplicity of transducers. The output is used to 
determine the center of the echo arrival time as previously described. 
When any one acoustic transducer 12', 12" of the array is arranged to 
operate for an extended period of time without activation of the 
surrounding annular piezo plate 13', 13" the ultrasonic wave from that 
transducer traverses debris 48 that may have collected. The signals from 
two transducers 12', 12", one with and one without the ultrasonic action 
of the annular plate may then be used to determine the thickness of debris 
layer 48 by simple subtraction, that is, 
EQU 2d=(v)(t.sub.d)-(v)(t.sub.nd) (5) 
where t.sub.d is the temporally averaged transit time for the echo that has 
traversed debris layer 48 and t.sub.nd is the echo time for the wave 
traversing a clean tank surface without debris. Here, the assumption is 
that the ultrasonic (acoustic) velocity in debris 48 is close to that of 
the velocity in the liquid which, to first order for small layers of 
debris (small compared to the liquid level) is a reasonable approximation. 
The output of signal averager 24 serves as the input to computer 26 which 
determines the midpoint of the echo arrival time temporal distribution. 
The computer calculates the height of the fluid from the echo time and 
velocity of the signal in the fluid medium and any tank wall through which 
the ultrasonic wave must travel. The time required for averaging is in 
part determined by the gate width selected on the boxcar or channel 
analyzer. An offset time can be pre-set so that only a very narrow portion 
of the time of flight echoes are processed by the integrator, thereby 
eliminating much unnecessary integration time, that is those times where 
no echo signal is expected. Also, the operation can be such that the 
entire system is only operated at selected times since a continuous 
readout is not required for reasonable updated information. 
The output of computer 26 can be connected to any of a number of readout 
displays 54 such as a dial gauge, a digitized readout, or a chart 
recorder. The data may also be kept in the computer memory as a permanent 
or semi-permanent record. Data taken when the tank is quiescent may be 
stored in the computer and used as a reference level for comparison with 
data taken at later times. 
Ultrasonic echo experiments were performed with water to verify the 
concepts of the invention. Echoes were measured traversing water in a flat 
bottomed container 95 mm in height, 45 mm in diameter, 2 mm wall thickness 
with the the water level 32 mm in height. Data were taken with the water 
both in a static (quiescent) and highly agitated state. Agitation was 
provided by motion of a flat paddle using a back and forth movement of 2-3 
cycles per second. A PZT Panametrics transducer (Model VIP 0.75" diameter) 
centered at 1 MHz was swaged to the bottom of the container to transmit 
ultrasonic waves into and back out of the liquid. A Panametrics 
pulser-receiver unit (Model 5050 PR) was used for pulsing and receiving 
the echoes. The echoes in turn were monitored on a Tektronix 7704A 
oscilloscope using a Tektronix Model 7A22 plug-in unit. The echoes were 
recorded on film from the oscilloscope using a camera with time exposure 
of approximately 7 seconds of agitation. The center of the distribution 
was determined by measuring the time of the first significant amplitude of 
the echo and last arrival of such echoes. Data are shown for both the 
static and agitated states of the liquid, FIG. 5A and B respectively. 
In FIG. 5A, signals are recorded for multiple echoes arriving at times 
equal to the round trip of the ultrasonic signal through both the 
container wall and water. Since there is no agitation here, the signals 
arrive at the same time for each pulse so that there is a superposition of 
essentially all the pulses arriving for seven seconds at 1000 pps, with an 
arrival time of approximately 43 microseconds. Because the liquid is 
quiescent, there is only one arrival time rather than a distribution of 
arrival times. The observed time corresponds to the roundtrip time of the 
ultrasonic pulse through the container wall and the liquid. In addition, 
the second echo with an arrival time of approximately 84 microseconds for 
two roundtrip paths in the acoustic media are also shown. 
The liquid level height is determined from the velocity in the two media 
and the known thickness of the container wall as follows. The time to 
travel in the wall (0.3 cm thick) is 0.3 cm/5.64.times.10.sup.5 cm/sec 
where 5.65.times.10.sup.5 cm/sec is the speed of the pulse in the wall. 
This is equal to 0.5 .mu.s. The round trip time is twice this and equals 
At which is 1 .mu.s. Thus, for the first echo, with ultrasonic velocity in 
the liquid equal to 1.53.times.10.sup.5 cm/sec, 
##EQU3## 
For the second echo, from acoustic impedance arguments, it can be shown 
that the reflection for the second echo takes place at the liquid-wall 
interface. Therefore, the ultrasonic wave has only traversed the wall 
thickness once, i.e. 1 roundtrip, so that .DELTA.t is still only 1 .mu.s 
as before and 
EQU H=(1.53.times.10.sup.5 cm/sec)(84 .mu.s-1.mu.)!/4=3.17 tm 
Upon continuous random agitation of the liquid in the container, a 
distribution of signals is observed, FIG. 5B. The first echo arrival 
interval ranges from about 39 to 48 microseconds and the second arrival 
interval from about 78 to 90 microseconds. The first echo signal 
distribution is centered at about 43.5 microseconds, a time corresponding 
to the first peak signal of FIG. 5A, that is the quiescent liquid. The 
second echo signal distribution is centered at about 84 microseconds. 
For the agitated liquid, the height H of the liquid is 
##EQU4## 
for the first temporal echo distribution and 
##EQU5## 
for the second temporal echo distribution 
Thus, the innovative method of the present invention of temporal averaging 
of the echo signal for an agitated liquid can be used to determine the 
level of the quiescent state. 
In yet another experiment, with a water height of only .about.1.5 cm, a 
test was made to observe the effect of a layer of sediment on the bottom 
of the tank, simulated by a highly uneven, rough layer of rubber cement 
with a maximum thickness of 0.7 mm. Echo patterns for both still and 
agitated water were again found to be quite discernible with approximately 
a factor of two diminution in amplitude of the 1st and 2nd echo signal 
compared to that for a smooth inner surface bottom. Again, the temporal 
distribution of echo signals for the agitated liquid case was found to be 
centered about the time corresponding to the signal for the quiescent 
liquid. 
In general, echoes which result only from the tank wall and from 
reflections from any solidified or highly viscous material at the bottom 
of the tank will be constant with time. The signal averager can be set to 
record only those signals that have time and/or amplitude variations on 
the scale of many microseconds to milliseconds (depending on the liquid 
level height) thereby ignoring multiple wall reflections which occur on a 
sub-microsecond time scale for steel tank wall thicknesses on the order of 
1 to 2 cm. 
In another embodiment the channel analyzer and/or the box car integrator 
can be exchanged for an oscilloscope with a buffer memory. A single echo 
transducer whose output has been filtered and amplified is connected to 
the input of the memory scope to record a continuous stream of echoes for 
a given pre-set time duration duration. A cursor is set automatically by 
computer control or manually to the center of the temporal echo 
distribution from which the liquid level height is readily determined by 
techniques already described. After a preset sampling time, the buffer 
scope memory is cleared for the next set of echoes, either manually or by 
computer control. A feedback loop is provided to give instructions to the 
signal averager as to when and over what time interval signal processing 
is to take place. 
The flow chart in FIG. 6 illustrates the steps required to find the time 
average for the most accurate value of the liquid level of the tank. 
Transceiver 120 excites transducer(s) 12. The echo signal is directed back 
to transceiver 120, filtered by filter 56, then amplified by amplifier 22. 
The output of amplifier 22 is connected to a limiter 58 which determines 
what amplitude and what temporal signals to include in signal averager 24. 
The instructions to limiter 58 are given by computer 26 via feedback loop 
80. The echo signals are gated by gate 60 to reject those signals arriving 
in the microsecond time regime since these signals result from multiple 
tank wall echoes and are not meaningful in determining the center of the 
distribution of echo arrival times. Computer 26 provides the algorithm for 
determining the temporal distribution of the echoes and the mean arrival 
time based on the temporal average. The level is displayed on display 54. 
A further feature of the algorithm is that it calculates the liquid level 
height based on the arrival time and the velocity data stored in the 
computer. The velocity is corrected with the aid of the thermocouple 
reading(s) from thermocouples 28 to take into account the velocity 
dependence on temperature. 
Annular piezo-plate 13 surrounding echo transducer 12 is caused to 
oscillate in a cw or quasi-cw mode at a low frequency (kHz range) from a 
separate driver unit 130 to provide and maintain a clean tank surface, 
free of debris in the region of at least one pulse echo transducer. The 
on/off operation of driver unit 130 is controlled by computer 26. Computer 
26 also controls the repetition rate and pulse amplitude of transceiver 
120. 
While the invention has been described with respect to preferred 
embodiments thereof, it will be appreciated by those having skill in the 
art that variations may be made in the invention without departing from 
the spirit and scope of the invention.