Sonic tank monitoring system

The height of a liquid level from the bottom of a storage tank or the like is accurately determined by using a probe with dual isolated channels to measure the time for an acoustic signal to travel between fixed submerged points and from a transducer in one of the channels to the surface of the liquid. Elapsed time measurements are converted to liquid volume using predetermined velocity-temperature values to compensate for temperature variations in the tank.

FIELD OF THE INVENTION 
This invention relates to methods and apparatus for monitoring the volume 
of liquids in storage tanks and the like. More particularly, it relates to 
methods and apparatus employing a probe which measures the time required 
for an acoustic signal to travel from a known reference point to the 
surface of a liquid to determine the level of liquid in a tank and 
converts the liquid level to net volume at a specific temperature using 
predetermined conversion factors. 
BACKGROUND OF THE INVENTION 
Many liquids are commonly stored in tanks located underground or otherwise 
disposed so that the actual liquid surface level cannot be observed 
directly. Furthermore, environmental protection regulations require that 
tanks which contain certain products such as gasoline and the like be 
equipped with means for detecting very low leak rates. 
While estimates of liquid level in a tank can, in most cases, be made with 
a calibrated dip stick or the like, this method is neither accurate nor 
reliable. Furthermore, it provides no means to compensate for volumetric 
changes resulting from temperature changes. 
More sophisticated instruments have been developed which measure the time 
required for an acoustic signal to travel from a transducer located at a 
known location in the tank to the surface of the liquid and return to the 
transducer by reflection from the surface. As with a calibrated dip stick, 
the gross volume of liquid can be determined from the measured liquid 
level if the geometry of the tank is known. However, the velocity of an 
acoustic signal in a liquid varies with temperature of the liquid. Thus, 
to determine the actual height of the liquid level, the temperature of the 
liquid must be known. Unfortunately, the temperature of a liquid in a 
large storage tank is seldom constant throughout the liquid. Accordingly, 
in using acoustic methods to obtain accurate distance measurements in a 
liquid, the temperature of the liquid must be measured at various heights 
from the bottom of the tank and the vertical of the acoustic signal 
through each horizontal section of liquid compensated for temperature. 
This, of course, requires a plurality of temperature measuring devices at 
precisely known locations. Alternatively, a plurality of reflectors may be 
placed at fixed known locations with respect to the transducer and the 
time for an acoustic signal return from each such reflector measured. 
Thus, if the distance between reflectors is known and the temperature of 
the liquid between two of such reflectors is known, a temperature 
compensated average velocity can be calculated to determine the height of 
the liquid surface. However, this approach has inherent difficulties and 
limitations. At least one temperature measuring device must be submerged 
in the liquid and the average temperature calculated from various 
reflection time measurements. Furthermore, it becomes extremely difficult 
to distinguish between acoustic signals reflected from the liquid surface 
and acoustic signals reflected from the fixed reflectors, particularly 
where the reflector is near the liquid surface. 
Various attempts have been made to determine net liquid volume in a tank by 
measuring acoustic signal velocity through the liquid. Typical of such 
attempts is U.S. Pat. No. 4,805,453 wherein a complicated signal detection 
scheme is employed in an attempt to distinguish between reflections from 
the liquid surface and reflections from a fixed reflector. Temperature 
compensation is attempted in the typical manner of using at least one 
submerged temperature sensor and extrapolation of temperature data from 
actual measurement at a fixed location. 
It is well known that actual measurement of the temperature of a liquid is 
difficult and that temperature-sensing devices submerged in a liquid are 
expensive, subject to failure and frequently unreliable. Furthermore, the 
prior art has generally failed to recognize that the average velocity of 
an acoustic signal from a transducer to a reflective surface is not 
necessarily equal to true velocity at any point between the active surface 
of the transducer and the reflector because of errors introduced at the 
surface of the transducer. For example, cavitation may occur at the 
fluid/transducer interface. Velocity of the acoustic signal through any 
vapor or foam formed by cavitation would, of course, be different from 
velocity through the liquid. Any such effects occurring at the face of the 
transducer thus introduce errors in the measured elapsed time. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, highly accurate liquid level 
measurements are made and temperature compensated without using a 
submerged temperature sensor, and liquid level measurements are made 
without possible interference by fixed reflectors. To accomplish these 
measurements, the invention employs a probe comprising two parallel tubes 
or channels positioned vertically in the tank. A transducer is secured in 
the extreme lower end of each tube and the extreme end of the probe 
positioned at a known distance from or adjacent the bottom of the tank. 
Reflectors are positioned at known distances from the transducer in one of 
the parallel tubes. The other tube contains no reflectors. 
The velocity of an acoustic signal in known liquids over a range of 
anticipated temperatures is determined experimentally and stored in memory 
in a computer. The time for an acoustic signal to reflect from the liquid 
surface to the first transducer is measured to make a gross determination 
(not compensated for temperature) of liquid height. The time for an 
acoustic signal to reflect from each of the submerged reflectors in the 
second tube is then made and compared to the velocity values stored in the 
computer to determine the average temperature of liquid between any two 
submerged reflectors. The actual height of the liquid is then calculated 
using the temperature compensated velocity values and converted to gross 
volume using the known geometry of the tank. By isolating the fixed 
reflectors in one tube and using two separate transducers to determine 
reflection times from the liquid surface and the fixed reflectors, it is 
extremely easy to distinguish between reflections from the surface and 
reflections from fixed reflectors, even when the submerged fixed reflector 
is at or near the liquid surface. Furthermore, by using experimentally 
determined velocity vs. temperature measurements stored in computer 
memory, all need for submerged temperature sensors is eliminated and 
temperature compensated velocity values can be determined for the liquid 
between any two submerged reflectors. By using a plurality of fixed 
reflectors, one of which is located relatively near the transducer, 
velocity errors due to cavitation or the presence of a second liquid (such 
as water) can be easily eliminated and the liquid level of a second liquid 
can be readily determined. These and other features and advantages of the 
invention will become more readily understood from the following detailed 
description taken in connection with the appended claims and attached 
drawing in which:

DETAILED DESCRIPTION 
The preferred arrangement of tank monitoring probe and its position within 
a tank to employ the principles of the invention are illustrated in FIG. 
1. The probe 10 comprises an elongated body defining dual linearly 
elongated parallel cylinders 11 and 12 referred to herein as channel 1 and 
channel 2, respectively. The probe body may be formed as a unitary molded 
or extruded body or may be fabricated from a pair of tubes. The probe is 
preferably formed of non-metallic materials which are resistant to the 
environment in which it is to be used. Various plastics, fiberglass and 
the like are suitable. Significant criteria in selecting the material of 
the probe are that it be inert with respect to its operating environment 
and that it not act as an acoustic wave propagation device at the 
frequencies of the transducer used. The probe must also be relatively 
rigid, straight and have a low coefficient of expansion with temperature. 
First acoustic transducer 21 and second acoustic transducer 22 are secured 
within the extreme ends of channels 1 and 2, respectively, and the probe 
10 supported vertically within a tank by suitable suspension means 18 with 
the extreme end of the probe 10 adjacent or at a known distance from the 
bottom 13 of the tank. 
Suspension means 18 may be any suitable mechanical means such as a bale, 
chain or the like which supports the probe 10 vertically within the tank 
and is normally secured within a necked opening 20 in the top 19 of the 
tank. In order to convert the collected data to volume, the exact geometry 
of the tank and the distance from the bottom of the tank to the active 
surface of the transducers 21 and 22 must, of course, be known. 
Furthermore, since distance measurements are made within the confines of 
the channels 1 and 2, means such as holes 14 in the channels must be 
provided so that the surface level 15 of the liquid in the tank is at all 
times the same as the liquid level in channels 1 and 2. 
A plurality of reflectors R1-R5 is positioned within channel 2 at known 
distances from transducer 22. For convenience, the reflectors may be 
spaced at equal volumetric intervals (the distance between each reflector 
representing an equal portion of the total tank volume) and/or the 
reflectors may be made progressively larger with distance from the 
transducer 22. The number and location of reflectors may, of course, be 
varied as desired. However, since the acoustic signal sensed by the 
transducer is a reflection from one of the reflectors, care should be 
taken to avoid placing any reflector at a distance from the transducer 
which is a multiple of the distance between the transducer and any other 
reflector. 
The channels are used to isolate the acoustic signals from each other and 
prevent echo returns from anything other than the surface of the liquid in 
channel 1 and the reflectors in channel 2. By isolating the acoustic 
signals from each other and from the remainder of the tank, signal 
interference from extraneous reflections is avoided. Signal degradation by 
dispersion within the tank is also avoided and the transducer is 
effectively isolated from external noise. Thus, the signal-to-noise ratio 
for reflected signals is greatly improved. 
In the preferred embodiment, the probe 10 is a one-piece molded structure 
of plastic or fiberglass having parallel cylindrical channels which are 
approximately one inch in diameter. Transducers 21 and 22 are secured at 
the extreme ends of channels 1 and 2, respectively, and arranged to 
transmit an acoustic signal toward the surface of the liquid and sense a 
reflected echo. Various conventional acoustic devices are suitable. A 
typical transducer for use in gasoline storage tanks operates at five 
hundred kilohertz. The frequency selected, however, is usually determined 
by the fluid in which it is to be used since the characteristics of the 
fluid may affect the effective propagation of sound waves at different 
frequencies. 
It should be noted that only the probe 10 which contains transducers 21, 22 
and reflectors R1-R5 is positioned within the tank. The probe contains no 
temperature sensors and no moving parts or electronic devices other than 
transducers 21, 22 and electrical cables for connecting the transducers to 
the operating system. 
The operating system, comprising a signal processor, a computer and a 
display, is located in a console at a convenient remote location and 
connected to the probe by suitable electrical cables. Obviously, the 
operating system console may be suitably interconnected with a plurality 
of probes, if desired, positioned in various storage tanks or the like 
and/or with other condition monitoring systems, alarms, etc. 
In order for the system of the invention to accurately determine liquid 
volume in a tank, the geometry of the tank must be known and the equation 
for converting liquid height to gross volume stored in the computer. 
Furthermore, since the velocity of an acoustic signal through a liquid 
varies with temperature of the liquid, the temperature vs. velocity 
characteristics of the liquid must be known. 
The present invention provides accurate volumetric measurement without 
directly determining temperature (or density) of the liquid. Instead, 
velocity vs. temperature measurements are made under precisely controlled 
laboratory conditions over a range of temperatures for each liquid in 
connection with which the monitoring system is to be used and the 
laboratory-determined velocity-temperature coefficients for each such 
liquid are stored in the computer memory. The computer is programmed to 
use these values to calculate the temperature of the liquid between the 
transducer and any reflector or between any two reflectors upon 
determination of the measured time for an acoustic signal to traverse the 
known distance in the liquid. Thus, the monitoring system only measures 
travel times of the acoustic signal between reference points positioned at 
known distances apart. The computer, using stored data, converts the time 
data to gross volume. After gross liquid volume has been determined, this 
value can be converted to net volume (volume of liquid at 60.degree. F.) 
by using the American Petroleum Institute (API) algorithm known as TAB 6B. 
The TAB 6B subroutine (or any other suitable conversion factor) is also 
stored in the computer to convert gross volume to net volume as required. 
Operation of the system only requires use of the probe to determine time 
for an acoustic signal to traverse known distances in the liquid. 
Transducer 21 is activated to emit a signal pulse and the elapsed time for 
the signal to travel to the surface 15 of the liquid and return to the 
transducer is measured. This measurement is made by conventional 
electronic means employing a clock which is started when the signal is 
transmitted and stopped when the echo is received. Time of travel is thus 
represented by clock counts during the time interval. In the preferred 
method of operation, a plurality of such measurements are performed in 
rapid succession and an average time computed to improve statistical 
accuracy. Since the temperature of the liquid is unknown, however, this 
measurement only determines signal travel time, not distance. 
After the signal travel time to the surface of the liquid 15 is determined 
in channel 1, transducer 22 is pulsed to determine the signal travel time 
from transducer 22 to each of the reflectors in channel 2 which are 
submerged. Note that a reflection will also be received from the liquid 
surface 15 in channel 2. However, if the surface level of liquid 15 is 
near a reflector, it will be difficult to determine whether the last 
reflection is from a fixed reflector or the surface of the liquid. Thus, 
if the elapsed time is close to the time measured in channel 1, the last 
reflected signal will be disregarded and the next lower reflector 
recognized as the highest submerged reflector. Since the distance from 
transducer 22 to each of the reflectors is known, the temperature of the 
liquid between any two submerged reflectors can be readily calculated by 
the computer using the measured time and the velocity vs. the computer 
using the measured time and the velocity vs. temperature data stored in 
the computer. If more than one reflector is submerged, it is preferable to 
disregard the measured elapsed time from the transducer to the first 
reflector R1 since this value may be distorted by interface effects such 
as cavitation at the transducer surface and/or the presence of a second 
liquid such as water of sufficient volume to cover the transducer, thus 
forming a liquid/liquid interface between the transducer and the reflector 
R1. Since the actual velocity of the acoustic signal between the 
transducer 22 and each of the reflectors can be calculated from the 
measured times, and since the distances are known, the average temperature 
of the liquid between each of the reflectors can be determined and these 
temperatures used to calculate an average temperature of the liquid to the 
height of the highest submerged reflector. This average temperature of the 
entire column of liquid in channel 1 as well as channel 2. Thus, a 
temperature correction factor for the column of liquid can be determined. 
By applying the temperature correction factor as determined by the 
measurements in channel 2, the elapsed time measured in channel 1 can be 
used to accurately determine the precise level of liquid in channel 1. 
Since the system measures elapsed time for a signal to travel from the 
transducer to the surface and return to the transducer, actual level of 
the surface of the liquid is determined by 
##EQU1## 
where d=distance from transducer to liquid surface 
v=signal velocity in the liquid 
c=clock cycles 
f=clock frequency 
Once the liquid level is determined, the gross volume of liquid is 
determined by reference to a tank table or by applying a known tank 
geometry factor. Net volume may then be determined by applying the TAB 6B 
subroutine or other suitable conversion factor. 
It will be recognized that under certain conditions two immiscible liquids 
may be contained in the same tank. In this case, the less dense liquid 
will float on the more dense liquid forming a liquid/liquid interface. The 
velocity of an acoustic signal ill, in most cases, be different in each of 
the liquids. 
A two liquid situation frequently occurs in storage tanks for petroleum 
products such as gasoline by condensation of water or introduction of 
other contamination. Determination of the presence and volume of such a 
second liquid is thus critical for making accurate net volume 
determinations and to making accurate leak detection determinations. 
As noted above, the probe 10 is suspended in the tank with the transducers 
21, 22 at or near the bottom of the tank. Since the acoustic signal is 
directed upwardly from the top surfaces of the transducers, any liquid 
level below the top surfaces of the transducers will not be detected. With 
the transducers as close as possible to the bottom of the tank, any liquid 
below the top surface of the transducer can ordinarily be disregarded as 
negligible. However, if the level of the second liquid is above the active 
surfaces of the transducers, its volume can be determined by various 
techniques. For example, if the volume of the lower liquid is sufficiently 
large, its volume can be determined as described above by treating the 
liquid/liquid interface as the surface of the liquid. 
In ordinary use for monitoring fuel storage tanks, the gradual accumulation 
of water, if it occurs, will be observed before a large volume is present. 
Since the accumulation of water is anticipated, temperature vs. velocity 
information for water is also stored in the computer memory. 
As noted above, if the water level is above the first fixed reflector, the 
level of water can be readily determined by using the reflection from the 
water/fuel interface as the liquid surface. However, if the water level is 
below the first reflector, different methods may be used to determine 
water level. 
In a fuel storage tank, any reflection received from a level below the 
first reflector is first assumed to be a reflection from a water/fuel 
interface. The water level is readily calculated by multiplying the 
measured time by the velocity of sound in water. The velocity of sound in 
water is 
EQU Vw=A+(B)(T) 
where 
A=54716.34 inches/second 
B=54.26 inches/second .degree.F. 
T=temperature in .degree.F. 
In using this method, the latest measured or calculated average temperature 
of the liquid in the tank may be taken as the current temperature of the 
water if current temperature cannot be determined directly. 
Alternatively, a counts variation method may be used to determine the level 
of the second liquid where the velocity of sound is different in the two 
liquids. This method requires that at least two reflectors be submerged in 
the total liquid volume and that the level of the lower liquid is below 
the lowest reflector. The distances from the transducer to each of the 
submerged reflectors, of course, are also known. Assuming that the lower 
liquid is water and the upper liquid is gasoline, the counts per inch in 
the gasoline can be expressed as 
##EQU2## 
where CR1=count value for echo from lowest reflector 
CR2=count value for echo from second lowest reflector 
DR1=distance from transducer to lowest reflector 
DR2=distance from transducer to second lowest reflector 
The counts per inch in the region between the transducer and the lowest 
reflector can be expressed as 
##EQU3## 
Using the known values of the velocity of sound in water and the 
temperature of the water, and since the echo count represents two-way 
travel time of the acoustic signal, the value representing the number of 
counts attributable to the two-way travel time of the acoustic signal 
through one inch of water at the current temperature may be calculated and 
expressed as Cw. The level of the water/fuel interface may then be 
expressed as 
##EQU4## 
It will be readily appreciated that the counts variation method described 
above does not rely on detecting a reflection from a liquid/liquid 
interface. Thus, it may be used when the level of the second liquid is 
very near the surface of the transducer. However, the velocity of sound in 
each liquid must be different at the temperatures under consideration. 
From the foregoing, it will be appreciated that the methods and apparatus 
of the invention may be used to accurately determine the volume of liquid 
in a container and determine net volume compensated for temperature 
without actually measuring the temperature of the liquid in the container. 
Furthermore, the actual volume of each of two immiscible liquids can also 
be determined. The apparatus may thus be used to measure very low leak 
rates by measuring net volume changes with time. 
Since the acoustic signal from the transducer used for locating the surface 
of the liquid is confined to a channel which has no reflectors, detection 
of a signal reflected from the surface is readily obtained. Isolation of 
the signal in a vertical channel essentially eliminates extraneous echoes 
and random noise. Similarly, by confining the signal from the second 
transducer 22 to an isolated channel, reflections from each reflector can 
be readily identified. Furthermore, since the distance to each reflector 
is known and the approximate velocity of the signal through the liquid is 
known, the signal sensing circuitry can be designed to be activated at 
predetermined time windows corresponding with estimated time required for 
a signal return from each reflector, thus further improving signal 
detection selectivity. By appropriate automatic recycling of the pulsing 
sequence, statistical averaging can be employed to produce extremely 
accurate distance measurements. 
It will be appreciated that the system described can be programmed to 
accomplish a wide variety of tasks based on volumetric determinations. For 
example, leak tests can be performed by making precise net volume 
measurements at various intervals during a period when additional liquid 
is neither added nor withdrawn from the storage tank. Similarly, the 
volume monitoring system described can be used for automatic inventory 
control, automatic ordering, etc., by proper programming of the computer. 
While the invention has been described with reference to monitoring liquid 
volumes in a storage tank, it will be appreciated that the principles 
thereof may be applied to similar monitoring applications by appropriate 
selection of operating components and conditions. It is to be understood, 
therefore, that although the invention has been described with particular 
reference to specific embodiments thereof, the forms of the invention 
shown and described in detail are to be taken as preferred embodiments of 
the same, and that various changes and modifications may be resorted to 
without departing from the spirit and scope of the invention as defined by 
the appended claims.