Water sensor that detects tank or vessel leakage

The apparatus of the invention is a water level probe comprised of a probe body, a sleeve having openings permitting fluid flow and encasing part of a magnetostrictive sensor, and an external housing encasing electronic circuitry for processing and transmitting the electric signal generated by the magnetostrictive sensor and sealing the circuitry from fluids. The sensor comprises a float slidably mounted on a rod, the float designed to float in water but sink in a petroleum product, thereby ensuring that it will float at the interface between the water and the product. The water level probe is used in the method of the invention to measure the depth of the water in a vessel storing hydrocarbon product at predetermined time intervals. The longitudinal angle of inclination of the vessel with respect of the horizon having previously been determined, the volume of water in the vessel is calculated from the depth measurements. The rate of hydrocarbon leakage is then determined from the calculated volumes to ascertain the rate of change of water volume over time.

FIELD OF THE INVENTION 
This invention pertains generally to a method and apparatus for detecting a 
leak in a container by measuring the level of an electrically conductive 
fluid contained therein. More specifically, it relates to detecting a leak 
in an underground petrochemical storage tank by measuring the level of the 
water in the tank. 
DESCRIPTION OF THE PRIOR ART 
For many years there have been concerted efforts in the petrochemical 
industry to prevent environmental contamination resulting from leaks in 
underground storage vessels. Underground storage tanks are not only found 
in refineries and other large facilities, but also in gasoline service 
stations. Such locations may have as many as three or four such tanks and 
consequently there are a large number of these tanks dispersed over an 
incredibly large area. 
The governments of the United States and other countries have for some time 
issued and enforced regulations requiring the detection and correction of 
leaks in an effort to prevent environmental damage, particularly 
contamination of underground water supplies. A sizable industry has 
developed to provide the technology and skills needed to enable service 
station owners and others to comply with these federal regulations. 
Compliance typically involves transporting manpower and materials to the 
site of the tank and performing a series of tests designed to measure 
certain conditions inside the tank on a regular basis. These conditions 
yield indications of whether a leak exists and, preferably, the size of 
the leak. In the United States, the federal government now also regulates 
and certifies the instruments used in these tests. 
One commonly tested condition is the level of water in the tank. A leak 
that allows a chemical to seep from a tank into the environment also 
allows water to seep into the tank. Because water is immiscible with many 
chemicals and fuels, and because water is heavier than gasoline, the two 
fluids separate into two layers with the water on the bottom. It is 
therefore possible to position a probe on the bottom of the tank to 
measure the level of water in the tank. By monitoring the change in the 
level of the water over a preselected period of time, it is possible to 
determine whether a leak exists and the size of the leak. 
One critical limitation on most known methods of water detection is that, 
in the United States, federal regulations require instruments for use in 
testing gasoline storage tanks to detect leaks as small as 0.1 gal/hr. 
Further, these tanks can have extremely large dimensions. Such large 
dimensions sometimes result in a very thin layer of water in the tank, 
having a depth perhaps as small as 0.020 inches. 
Because of the thin (or shallow) layer of water and the slow rate of 
change, a depth measuring device must therefore be capable of operating 
with relatively high resolution and precision. Because of the limited 
resolution capabilities of known methods for quantifying the influx of 
water and the long time required for sufficient water to leak into the 
tank to be able to be detected by these known devices as a result of their 
limited resolution, known methods may require several hours of testing 
time per tank. In installations in which there are several tanks, the time 
required to test each tank causes testing costs to approach the 
prohibitive level. 
There have been many attempts to design a suitable probe for detecting the 
presence and/or level of water in a tank, but the disadvantage of most 
known probes is their length. These probes are generally cylindrical and, 
in addition to housing the circuitry for quantifying the influx of water 
into the tank, include the necessary components for sensing temperature 
(for correcting depth changes for changes in temperature which cause 
volume changes). One such probe commonly utilized by the assignee of the 
present invention also contains instrumentation and circuitry for 
detecting the bubbles created in the fluid by an influx of air. 
The latter function results from a leak test which is conducted by 
evacuating the ullage in the tank and detecting the bubbles formed in the 
fluid in the tank by the air which enters the tank below the surface of 
the fluid in accordance with the method described in U.S. Pat. No. 
4,462,249, assigned to the owner of the present invention and hereby 
incorporated herein in its entirety by this specific reference thereto. As 
disclosed therein, a hydrophone is mounted in the probe body for detecting 
the resulting bubbles. Due to the length of the probe body and the fact 
that the hydrophone must be submerged to function properly, the tank must 
contain a minimum depth of fluid to insure that the hydrophone is 
submerged. 
There are many circumstances in which that minimum depth requirement causes 
problems. For instance, when a testing crew arrives on site and discovers 
that a gasoline service station storage tank does not include that minimum 
depth of product, the station operator must buy more product. Additional 
product takes time to procure and costs extra because of the unscheduled 
nature of the delivery. Another alternative which is also costly is that 
the testing crew must return at another time. 
The invention disclosed in application Ser. No. 07/575,089, filed Aug. 30, 
1990, now U.S. Pat. No. 5,156,047, overcame these disadvantages by etching 
conductive traces on a detector board protruding from the lower end of the 
probe body through a sleeve, the remainder of the electronic circuitry 
being environmentally isolated within the probe body. The traces were 
etched in a pattern where they terminate at successively higher levels so 
that a higher or lower number of traces are submerged to provide an 
indication of change in water depth. The probe therefore measures water 
level in discreet intervals which, although that invention's performance 
is far superior to other probes, sometimes requires inordinately long 
periods of time to register a change. Even then, because depth is measured 
in discrete intervals, interpolation is necessary to find the leak rate 
measured in volume per unit time. 
By way of example, if the parallel traces etched on the detector board of 
the probe disclosed in that co-pending application are spaced at, for 
instance, 0.15" intervals, and one additional trace is covered with water 
from an influx of water over the course of two hours of the test, it is 
necessary to assume that the leak rate is such that the depth of the water 
changes at the rate of 0.075" per hour. The actual rate could be anywhere 
from, for instance 0.01" to 0.29" per hour, depending upon how much of 
each trace is covered at the beginning of the test. Further, that depth 
change must be converted to volume per hour by multiplication depending 
upon the geometrical dimensions of the tank and, as will be developed 
further infra, that conversion is not always accurate and introduces an 
additional possible source of error into the measurement. 
It is, therefore, a principal feature of the present invention to provide a 
method and apparatus capable of overcoming these disadvantages and 
limitations of prior known methods and apparatus for detecting leaks in 
storage tanks. 
It is also a feature of this invention that it reduces the amount of time 
necessary to test a storage tank for leaks by detecting changes in the 
level of water therein relative to known methods and apparatus. 
It is also a feature of this invention that such an apparatus be 
sufficiently small and lightweight so that it can be easily transported to 
different varying locations. 
It is a further feature of this invention that the apparatus be compatible 
with existing technology in the industry. 
It is still a further feature of this invention that the apparatus complies 
with current federal regulations regarding operation and performance of 
these types of instruments and that, if properly used, it will enable 
gasoline storage tank operators and owners to also comply with federal 
regulations. 
SUMMARY OF THE INVENTION 
The apparatus of the invention is a water level probe comprised of a probe 
body, a sleeve having openings permitting fluid flow and encasing part of 
a magnetostrictive sensor, and an external housing encasing electronic 
circuitry for processing and transmitting the electric signal generated by 
the magnetostrictive sensor and sealing the circuitry from fluids. The 
sensor comprises a float slidably mounted on a rod, the float designed to 
float in water but sink in a petroleum product, thereby ensuring that it 
will float at the interface between the water and the product. 
The water level probe is used in the method of the invention to measure the 
depth of the water in a vessel storing hydrocarbon product at 
predetermined time intervals. The longitudinal angle of inclination of the 
vessel with respect of the horizon having previously been determined, the 
volume of water in the vessel is calculated from the depth measurements. 
The rate of hydrocarbon leakage is then determined from the calculated 
volumes to ascertain the rate of change of water volume over time.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 depicts water level probe 10 positioned on the bottom of vessel 12. 
Vessel 12 is partially filled with layers of a fluid such as gasoline, or 
other hydrocarbon product, 14 and a fluid such as water 16. Although 
vessel 12 is only shown partially filled, the practice of the invention is 
equally effective in tanks that are completely full. Gasoline 14 and water 
16 are immiscible and therefore layer, the heavier fluid (water 16) 
layering beneath the lighter. Vessel 12 is located beneath ground surface 
18 and is typically oriented at a small angle .theta. longitudinally with 
respect to the horizon, as a result of the installation of the vessel 12. 
A properly installed 10,000 gallon underground storage tank, for instance, 
has a tilt amounting to a difference in the height of one end of the tank 
of about four inches compared to the other end. 
Probe 10 is connected to a truck 20 via cable 22 and suspended in tank or 
vessel 12 by that cable. Truck 20 stores and processes electronic data 
collected and transmitted by probe 10 to truck 20 via cable 22. The 
methods and electronics necessary for transmitting and processing the data 
can be any of several well known to those ordinarily skilled in the art. 
FIG. 2 depicts a lower portion of probe 10 in greater detail and in a side, 
quarter-sectional view. Probe 10 generally comprises probe body 24 to 
which external housing 26 and protective sleeve 28 are threadably engaged 
at either end of probe body 24. Probe body 24, in the preferred 
embodiment, is typically constructed from a solid, cylindrical slug of 
aluminum or stainless steel in which threads are milled on both ends. 
Now referring to FIGS. 2 and 4, the preferred embodiment of probe body 24 
includes hydrophone mounting means 30 on the side thereof oriented 
perpendicularly to the length of probe body 24. Probe body 24 also 
includes, as shown in FIG. 4, a temperature sensor mounting means 37 and 
pressure transducer 34 located on the top 36 of probe body 24. Connector 
35 mates with a connector (not shown) on the inside of external housing 26 
to connect probe body 24; detector board 40, to logic board 42 and to 
cable 22. 
Referring to both FIGS. 2 and 4, slot 38 is milled throughout the length of 
probe body 24 through which rod 50 of magnetostrictive sensor 52 passes. 
Magnetostrictive sensor 52 in the preferred embodiment is the MTS LEVEL 
PLUS industrial tank gauge manufactured by MTS Systems Corporation, 
Sensors Division, Research Triangle Park N.C. and is commercially 
available. Float 54 incorporating a ring magnet (not shown) is slidably 
mounted on rod 50 below probe body 24 and in sleeve 28, float 54 sliding 
in response to changes in fluid level. The construction of sleeve 28 is 
discussed below in connection with FIG. 3. 
External housing 26 encases detector board 40 and logic board 42. Detector 
board 40 is electrically connected to magnetostrictive sensor 52 by an 
electrical connector (not shown) in any of several ways commonly known to 
those in the art. To some degree, as in the preferred embodiment, the 
manner of connection is dictated by the chosen embodiment for 
magnetostrictive sensor 52. Logic board 42 is similarly connected to 
detector board 40 via an electrical connector (not shown) in a manner well 
known to those in the art. Together, detector board 40 and logic board 42 
receive the data from magnetostrictive sensor 52, process and condition 
the data, and then transmit the data to truck 20 (shown in FIG. 1) via 
cable 22. 
FIG. 3 depicts probe 10 from the bottom end and, hence, the bottom end of 
sleeve 28. Protective sleeve 28 is shown mounted to probe body 24 on 
threads 25 in FIG. 2 and encases float 54 as it slides on rod 50 of 
magnetostrictive sensor 52. As shown in FIG. 4 in addition to FIG. 3, 
sleeve 28 also has openings 56a-d and 58 milled or cut therein to permit 
the fluid flow into the interior of sleeve 28. 
Threaded connection 23 is a fluid seal which prevents fluid flow into probe 
10 above probe body 24. Furthermore, there are two O-rings 39, one in slot 
38 through which rod 50 passes and one at the interface between detector 
board 40 and magnetostrictive sensor 52, which also provide fluid seals. 
O-rings 39, in conjunction with threaded connection 23 (shown in FIG. 2), 
environmentally isolate those portions of probe 10 above probe body 24 
from the fluid in which probe 10 operates. Thus, magnetostrictive sensor 
52 operates entirely within the fluid while the electronic circuitry on 
detector board 40 and logic board 42 are environmentally isolated 
therefrom. 
In operation, power is transmitted from truck 20 via cable 22 to probe 10 
once probe 10 is lowered to the bottom of vessel 12 and the test starts. 
Referring now to FIG. 2, float 54 is especially designed to float on water 
16 but submerse in gasoline 14. In addition to being immiscible, water and 
gasoline have different specific gravities, the specific gravity of 
gasoline being approximately 0.74 and of water being 1.0. Thus, float 54 
must be constructed of a material having a higher specific gravity than 
gasoline but lower than water. 
As time passes during the test, float 54 will rise and fall with the change 
in water level in vessel 12 if there be any. In accord with the general 
principle of operation for magnetostrictive sensors, magnetostrictive 
sensor 52 will generate an electrical signal proportional to the position 
of float 54 on rod 50 and, hence, the level of water 16 in vessel 12. The 
data generated from the electrical signal is then processed on boards 40 
and 42 and transmitted to truck 20 for further processing. Such processing 
is accomplished in accordance with methods known in the art to produce, 
for instance, a printed record of the rate of change in the level of the 
electrically conductive fluid in vessel 12. Test time is approximately 
less than an hour. 
As noted above, properly installed underground storage tanks are slightly 
inclined from the horizon at some predetermined angle .theta.. 
Consequently, the possibility exists that at very low depth levels of 
water in the tank, the full length of the tank will not be wetted with 
water. In that instance, the difference between the volume of water which 
would be expected in the tank from a measurement of depth and the actual 
volume of water present would be larger than that accounted for by 
standard calculations of volume as a function of the depth of the water 
measured by the magnetostrictive sensor 52. An additional calculation is 
therefore utilized in detecting a charge in the depth of the water 16 at 
very low depths in the tank. So far as is known, the measurement of the 
tilt of the tank can be obtained only with the device described in 
co-pending application Ser. No. 07/833,306, also assigned to the owner of 
the present invention and hereby incorporated in its entirety by this 
specific reference thereto. 
Probe 10 is employed as described above to measure water depth. However, 
accurate determination of the volume of water in an underground tank is 
critical to conformance with federal regulations limiting leakage, and 
similarly influx, to 0.1 gal/min. The rate of influx is determined by 
calculating the change in the volume of water in the tank over time, which 
requires that the volume of water be determinable instantaneously. 
Basic trigonometric relationships show that the depth of water may be 
defined as a function of its length from the lower end 13 of the tank 12, 
along the tank bottom, as follows: 
EQU X=X.sub.o +l tan .theta. 
where as illustrated in FIG. 1, X.sub.o is the vertical distance from the 
tank centerline to the water level at the lower end 13 of the tank 12; 1 
is the length of the water along the bottom of the tank; .theta. is the 
angle at which the tank 12 is inclined with respect to the horizon; r is 
the radius of the tank (assumed to be generally round in cross-section); 
and X is the vertical distance from the center-line of the tank to the 
surface of the water 16 (e.g., the interface) at any point along the 
length of the water. 
Having defined this relationship, it may be seen that the volume of water 
at any time in the tank 12 may be determined by: 
##EQU1## 
where 1 is the length of the water along the bottom of the tank, measured 
form the lower end 13 of the tank. A solution of this integral will result 
in the volume of water in the tank 12 at any given time. The rate of 
influx may then be determined by calculating the rate of change in the 
volume of water over time, the output of magnetostrictive sensor 52 being 
interrogated at selected time intervals under the control of a computer 
(not shown) aboard truck 20. In this manner, an accurate, real-time 
measurement of any change in the depth of the water 16 in the tank, or 
vessel 16 is obtained in a much shorter period of time than with 
previously known methods for detecting the influx of water. 
Those skilled in the art who have the benefit of this disclosure will 
recognize that the processing of the acquired data could be accomplished 
entirely by suitable electronic equipment located in truck 20. Other 
changes and modifications to the presently preferred embodiment 
illustrated and described above will be apparent to such persons, and all 
such changes are included within the spirit and scope of the invention as 
set out in the following claims.