Method and apparatus for determining fluid level in oil wells

A method and apparatus for continuously monitoring fluid level in an oil well, by measuring changes in the weight of the oil well tubing which result from changes in the buoyancy of the tubing caused by the rise and fall of fluid level within the oil well. In the apparatus of the invention, a transducer is either affixed to the downhole tubing of the oil well, or alternatively, is affixed to the drive head support structure of a well having a progressive cavity well pump. The transducer continuously senses changes in the weight of the tubing as the buoyancy of the tubing changes in proportion to the rise and fall of fluid level within the well. The transducer is preferably connected to a control unit which, in turn, is connected to the pump motor and which functions to automatically control the pump motor as a function of fluid level within the well.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a method and apparatus for 
measuring fluid level in oil wells. More particularly, the invention is 
concerned with a novel sensor apparatus and the method of using the same 
for measuring the changes in weight of the tubing in an oil well which 
occur as a result of changes in buoyancy of the tubing due to the rise and 
fall of the fluid level within the well. 
2. Description of the Prior Art 
Oil field pumping units can be of several types. For example, a common type 
of unit is the so-called "sucker rod" apparatus which uses a standard 
walking beam coupled with a down hole sucker rod. Pumping units other than 
the "sucker rod" type use either a submersible pump and motor with power 
being supplied to the motor from the surface or a surface-mounted motor 
which drives a submersible rotary pump via an elongated drive shaft. 
The oil well itself typically comprises a drilled bore which is lined with 
a steel casing. Within this casing is a down hole tubing which, in other 
than the submersible pump and motor type unit, carries either the sucker 
rod or the drive shaft which drives the impeller of the submersible pump. 
In the case of the submersible pump and motor type unit, the down hole 
tubing carries the cables which provide power to the submersible motor. 
In any of the conventional prior art oil field pumping units, if the well 
runs dry or if the fluid level within the well drops significantly, the 
load on the pump motor will decrease causing the speed of the motor to 
rapidly increase. Accordingly, in wells wherein fluid levels vary during 
pumping, control means must be provided to either turn off the pump motor 
when the fluid level is low, or, alternatively, to control the speed of 
the motor to accommodate changes in load on the motor due to the rise or 
fall of fluid level within the well. Without such controls, substantial 
damage to the pump can occur. More particularly, in the case of the 
submersible pump and motor, if the motor is allowed to run in the absence 
of fluid, potentially devastating wear and tear on the mechanical parts of 
the pump can quickly occur. Retrieval of the damaged or destroyed pumping 
unit and its repair or replacement can be quite costly. Similarly, in 
surface-mounted motor units, which include a down hole rotary pump and 
impeller, operation of the pump, when the fluid level is too low can also 
result in substantial damage to the system. 
In addition to protecting the oil well pump from running in the absence of 
fluid, speed control is also necessary for the efficient operation of the 
pump and to avoid energy waste. For example, if the fluid within the well 
is not being pumped fast enough due to a rapid rise in the fluid level, 
means for adjusting the stroke speed of the pump is highly desirable to 
maintain peak production. Conversely, if the rise in fluid level within 
the well slows, slowing of the stroke speed of the pump is desirable. 
Various types of methods and apparatus for pump control in oil wells have 
been suggested in the past. The most common type of prior art control 
system involves the use of a load cell type device in conjunction with a 
pump controller which generates a well card. The well card typically 
monitors the condition of the well and is a visual depiction of oil well 
conditions. In operation of this type of system the controller 
periodically energizes the pump for a few cycles to determine if there is 
sufficient fluid within the well to warrant continued operation of the 
pump. If the system detects the absence of fluid, the pump will 
immediately be de-energized. If, on the other hand the system determines 
that ample fluid is available for pumping, the pump will be permitted to 
operate until the system senses the absence of sufficient fluid level 
within the oil well. In actual practice after a number of periodic 
energization and de-energization steps occur, a trend is established and 
the controller functions to automatically energize and de-energize the 
pump based on this established trend. However, if the well conditions 
change radically, a new trend must be established in order to maintain 
peak operating efficiency and to prevent damage to pumping system. 
Disadvantageously, prior to the establishment of the new trend, the pump 
may operate for several cycles without fluid to pump, thereby exposing the 
pump to damage. In addition to being somewhat inefficient, the prior art 
load cell system cannot be used in connection with surface mounted pump 
systems. 
As will be better understood from the description which follows, the method 
and apparatus of the present invention provides continuous monitoring of 
the fluid level within the well and interfaces with controls that maintain 
the correct pumping speed at all times. Additionally, in accordance with 
the method of the present invention, should fluid not be available to the 
pump, the pump will be automatically de-energized until fluid levels are 
sufficient for the resumption of safe pumping. 
The need for continuous and precise pump control is even more important 
when surface-mounted or progressive cavity pumps are used as distinguished 
from sucker rod-type pumps. Accordingly, many of these types of prior art 
pumps embody a torque transducer which measures load level on the pump and 
when a decrease in torque occurs on the impeller shaft, the pump is 
automatically de-energized. Again a trend is established as the pump 
controller energizes and de-energizes the pump under varying conditions of 
fluid level. However, due to the possibility of immediate and extreme 
damage to the pump in the progressive cavity pump systems, it is highly 
undesirable to allow the pump to start up even momentarily when the fluid 
level within the well is too low. 
Advantageously, in accordance with the method of the present invention 
there is a continuous monitoring of fluid level within the well and 
simultaneous control of pump operation will positively prevent the pump 
from operating when the fluid level within the well is too low. 
Additionally, in accordance with the method of the present invention the 
stroke speed of the pump is continuously controlled so as to maintain 
proper pumping conditions as fluid levels rise and fall within the oil 
well. 
Another widely used prior art method for determining fluid level in an oil 
well involves the use of a small capillary tube which is coupled with a 
pressure transducer located near the top of the tubing. With this type of 
system, the pressure transducer measures the rise and fall of fluid within 
the oil well as the pressure changes in the capillary tube. This 
continuous monitoring of the well conditions satisfactorily avoids 
starting of the pumps when the fluid level is too low and also can be 
interfaced with the controller that will adjust the pumping speed to 
accommodate a rise in fluid level within the well. However, due to the 
fact that a very long length of very costly capillary tubing is required, 
this type of prior art system is quite expensive to manufacture. Further, 
the fragile capillary tubing is easily damaged during installation and 
operation. Additionally, the small capillary tubing is prone to blockage 
which results in faulty pressure measurements by the pressure transducer 
associated with the capillary tubing. Disadvantageously, when the small 
and fragile capillary tubing is either blocked or damaged, a specialized 
crew is required to take remedial action which can be both time consuming 
and quite costly. 
The novel method and apparatus of the present invention overcomes the 
drawbacks of the prior art capillary tubing systems while at the same time 
achieving superior results. Further, the apparatus of the present 
invention is considerably more economical to operate, is more reliable in 
operation, and does not require a special crew for installation and repair 
since all the measurements occur at a location proximate the surface of 
the ground. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the substantial 
drawbacks of the prior art systems by providing a method and apparatus for 
continuously monitoring fluid level in an oil well, by measuring the 
changes in the weight of the oil well tubing which result from changes in 
the buoyance of the tubing caused by the rise and fall of fluid level 
within the oil well. 
Another object of the invention is to provide a method and apparatus of the 
aforementioned character which can be used to control both sucker rod type 
pumps as well as progressive cavity type pumps. 
Another object of the invention is to provide a method for continuously 
controlling pump operation by monitoring the buoyancy of the oil well 
tubing within the oil well on a continuous basis. More particularly, it is 
an object of the invention to provide a method as described in the 
preceding paragraphs which not only protects the pump from damage, but 
also functions to achieve peak productivity by automatically increasing 
pump speed as the fluid level within the well rises. 
By way of summary, these and other objectives are achieved by the method 
and apparatus of the present invention wherein a transducer, which is 
either applied to the down hole tubing of the oil well, or alternatively, 
is affixed to the drive head support structure of a progressive cavity 
well pump continuously senses changes in the weight of the tubing as the 
buoyancy of the tubing changes in proportion to the rise and fall of fluid 
level within the well. More particularly, because of the difference 
between the density of oil and air, as the fluid level within the well 
rises, the tubing becomes lighter. The strategically mounted transducer of 
the apparatus of the invention continuously senses this change in weight 
and provides real-time output signals to a cooperatively associated 
controller which, in turn, controls the operation of the pump in 
accordance with the output signals. Therefore, as the fluid level within 
the well decreases, the pump tubing becomes less buoyant and the resultant 
increase in weight is sensed by the transducer. Signals produced by the 
transducer are then transmitted to the controller which functions to 
de-energize the pump motor when the fluid level drops below the location 
of the pump. Conversely, as the fluid levels within the well increase, the 
pump tubing becomes more buoyant (lighter) and this change in weight is 
once again sensed by the transducer which signals the pump to begin 
pumping at a rate consistent with the increase in buoyancy of the tubing 
caused by the increase in fluid level within the well.

DESCRIPTION OF THE INVENTION 
Referring to the drawings and particularly to FIGS. 1 through 3, one form 
of the apparatus of the invention is there shown in use with a 
conventional type of "sucker rod" oil well. The oil well comprises a 
downhole bore 14 which is lined with a steel oil well casing 16. Extending 
downwardly of casing 16 is a sucker rod 18 which is housed within a tubing 
system comprising an elongated conduit or tube 20 which includes a lower 
segment 20a having a length "L" and a diameter "D". As depicted in FIG. 1, 
conduit 20 extends downwardly within the bore 14 and is normally partially 
surrounded by fluid "F" which rises and falls within the well. 
Sucker rod 18 is interconnected with a pump jack 22 of conventional 
construction having a driving motor 24. Sucker rod 18 extends through a 
tubing head 26 of conventional construction and is connected proximate its 
upper end 18a with pump jack 22. Connected proximate the lower end 18b of 
sucker rod 18 is a conventional sucker rod pump 28. 
Interconnecting segments 20a and 20b of length of conduit 20 and forming a 
part of the tubing system of the invention is an intermediate coupling 
portion shown here as a coupler 29, which has a length "L-1" and a 
diameter "D" (see FIG. 1). In the form of the invention shown in FIGS. 1 
through 3, the sensor means of the invention is operably interconnected 
with coupler portion, or coupler 29. The sensor means functions to sense 
physical changes in the tubing system and to generate at least one signal 
corresponding to a physical change in the tubing system, which is dictated 
by the sensor means, as the fluid level within the well rises and falls. 
As best seen by referring to FIGS. 2 and 3, the sensor means of the form of 
the invention there shown comprises transducer means which is connected to 
the coupler 29 which is interposed between lower segment 20a of tubing 20 
and upper segment 20b thereof (FIG. 2). More particularly, as best seen in 
FIG. 2, coupler 29 comprises an elongated tubular member having external 
threads 34 formed on either end thereof and an enlarged diameter central 
body portion 36. As shown in FIG. 2, the sensor means of the invention, 
which here comprises a strain gage array 40, is interconnected with body 
portion 36. As best seen by also referring to FIG. 8, strain gage array 40 
comprises four interconnected strain gages 40a, 40b, 40c, and 40d 
respectively. 
While various types of strain gages can be used in accomplishing the method 
of the invention, strain gages 40a, 40b, 40c, and 40d are preferably 
foil-type strain gages which are constructed from very thin, precision 
etched foil. The deformation of the foil is repeatable when exposed to 
various temperature ranges and as it is periodically stretched or 
compressed. Foil type strain gages of the character here employed are well 
known in the art and are readily commercially available from sources such 
as Micro Measurements, Inc. of Raleigh, N.C. 
As indicated in FIG. 9, the individual strain gages are preferably arranged 
in a wheatstone bridge configuration with interwiring 41 interconnecting 
the individual strain gages. Interwiring 41, which is preferably etched on 
the same foil as that upon which the individual strain gages are etched, 
is interconnected with electrical leads 41a which extend from the strain 
gage array to the exterior of the well. In a conventional manner, the 
etched foil is laminated to a substrate 43 (FIG. 8) that comprises a 
flexible material such a mylar or polyamide so that the strain imposed by 
the structure being measured on the substrate will be transferred directly 
to the foil. In the preferred form of the invention, the array of gages 
40a, 40b, 40c, and 40d and the interwiring thereof is arranged in a 
poisson configuration of the character illustrated in FIG. 9. 
As indicated in FIGS. 3 and 8, the strain gages are equally spaced around 
the circumference of central portion 36 of coupler 29 and are affixed 
thereto by any suitable means such as adhesive bonding. With the strain 
gages suitably affixed to coupler 29, any strain imposed on the coupler 
will cause the strain gages which are bonded thereto to elongate or 
compress. It is well known that metals generally change in resistance when 
this occurs. Accordingly, as a given strain gage is minutely stretched, 
the resistance increases and, vice versa, when the strain gage is 
compressed, the resistance decreases. In a wheatstone bridge construction 
of the character shown in FIG. 9, when all of the strain gages are 
identical in there omic values and, when a direct current is applied as, 
for example, across strain gages 40a and 40b, a signal output of zero 
volts will result across strain gages 40b and 40c. As is well understood 
by those skilled in the art, a change in any one leg of the wheatstone 
bridge will result in an output proportional to the change. Therefore, 
resistance changes in opposite arms of the wheatstone bridge allow current 
flow which can be used to generate and transmit signals to a controller 45 
(FIG. 9). To correct for pressure changes in tubing 20, additional strain 
gages 47 and 48 are bonded to coupler 29 and are interconnected with the 
strain gage array in the manner shown in FIGS. 8 and 9. The operation and 
methods of interconnection of the pressure correcting strain gages 47 and 
48 are well known to those skilled in the art. 
With the construction shown in FIGS. 8 and 9 of the drawings, it is 
apparent that the resistance of the strain gages 40a, 40b, 40c, and 40d 
which circumscribe coupler 29 will change as physical changes occur in the 
coupler. For example, since segment 20a of tubing 20 if threadably 
connected to coupler 29 in the manner shown in FIG. 2, changes in the 
apparent weight of segment 20a will be reflected by proportionate physical 
changes in the coupler and in the tubing at a location proximate its point 
of interconnection with the coupler. More particularly, and by way of 
example, if segment 20a of the tubing is suspended in air from coupler 29, 
measurable internal stresses on the coupler will result, which stresses 
will cause minute changes in the length and diameter of the tubing as well 
as in the length and diameter of the coupler. These changes will, of 
course, cause a change in resistance to the strain gages which are 
connected to the coupler and a concomitant change in current flow through 
leads 41a. Accordingly, with segment 20a of the tubing extending 
downwardly of casing 16, if the fluid in the well is at a level below the 
lower extremity of the segment, physical changes in the coupler will occur 
which corresponding to the actual weight of the length "L" of segment 20a. 
However, if the fluid level within the well rises, as, for example, as 
shown in FIG. 1, a portion of segment 20a will be immersed within the 
fluid and the tubing will be buoyed upwardly. This buoyancy will result in 
an apparent loss of weight in segment 20a which, in turn, will result in a 
change in the stresses imposed upon the tubing and upon the coupler to 
which it is attached. The physical changes in the tubing and coupler will 
be sensed by the sensor means or strain gage array of the invention and 
will be evidenced by a change in current flow through the device. 
As the level of fluid "F" within casing 16 continues to rise, segment 20a 
will be further buoyed upwardly, thereby further decreasing the apparent 
weight of segment 20a. Conversely, as the level of fluid within the well 
falls, the apparent weight of tubing segment 20a will increase in direct 
proportion to the buoyance exerted on the segment. These changes in the 
apparent weight of the length of tubing 20a will, of course, cause changes 
in the stresses imposed on coupler 29, which stresses will be precisely 
sensed by the sensor means or strain gage array and can be directly 
correlated to the level of fluid within the well. When the output of the 
strain gage array is connected to controller 45 and when the controller is 
appropriately interconnected with motor 24 in the manner shown in FIG. 9, 
the operation of motor 24 can be precisely controlled as a function of the 
rise and fall of the level of the fluid "F" within casing 16. 
In accordance with the method of the present invention, the resistance of 
the strain gages of the sensor means can be determined and recorded when 
the tube segment 20a of the conduit 20 is supported only in air, that is 
when the fluid level in the well is lower than the bottom extremity of 
segment 20a. Similarly, the resistance of the strain gage can be 
determined and recorded when segment 20a of the coupler is totally or 
partially submerged. 
This done the resistance of the strain gages at intermediate fluid levels 
within the well can be determined either empirically or through actual 
measurement. Using these readings, those skilled in the art will have no 
difficulty in correlating the resistance of the strain gages with fluid 
level with the bore of the well. In a manner well understood by those 
skilled in the art, the sensor means can be operably interconnected with 
the controller 45 so that the speed of motor 24 can automatically be 
regulated and so that the motor can be stopped and started as a function 
of fluid levels within the well. For example, if readings on the sensor 
means correspond to the initial set of readings taken when the full weight 
of segment 20a was supported by the well, the controller will be 
programmed to automatically de-energize the motor since this reading means 
that the level of fluid in the well is below the pump. 
Referring next to FIGS. 4 and 5, the apparatus of the invention is shown 
being used in connection with a downhole rotary pump type of pumping 
system. In this system, the rotary pump 51 is driven by a surface mounted 
motor 52 via a drive shaft or rod 54. The oil well itself is similar in 
character to that shown in FIG. 1 and comprises a downhole bore 14 which 
is lined with a steel oil well casing 16. As best seen in FIG. 4, drive 
shaft 54 is housed within a tubing system comprising an elongated tube 20 
similar to that previously described which includes a lower segment 20a 
having a length "L" and a diameter "D". As depicted in FIG. 4, tube 
segment 20a extends downwardly within the bore 14 and normally is 
partially surrounded by fluid "F" which rises and falls within the well. 
Because of the similarity between the oil well and the tubing system shown 
in FIG. 4 and that shown in FIG. 1, like numbers are used in FIGS. 4 and 5 
to identify like components. Drive rod 54 is of conventional construction 
and is supported by a support structure 56 which here also forms a part of 
the tubing system and which functions to support driving motor 52. The 
drive rod or shaft 54 extends through a tubing head 60 of conventional 
construction and is drivably connected proximate its upper end 54a with 
drive motor 52. The lower end 54b of the drive shaft is, of course, 
connected to the downhole rotary pump 51. 
Disposed intermediate segments 20a and 20b of tubing 20 is a coupler 29 to 
which the sensor means of the invention is interconnected. Coupler 29 as 
well as the sensor means, are identical in construction and operation to 
those previously described in connection with FIGS. 1 through 3. As 
before, the sensor means functions to sense physical changes in the tubing 
system of the invention and is designed to generate at least one signal 
corresponding to a physical change in the tubing system as the fluid level 
within the well rises and falls. As before, the sensor means of the 
invention comprises transducer means which is connected to coupler means 
or coupler 29 in the manner previously described. Coupler 29 is interposed 
between lower segment 20a of tubing 20 and upper segment 20b thereof and 
comprises an elongated tubular member of the identical character 
previously described. Reference should be made to FIGS. 8 and 9 which 
illustrate the manner of interconnection of the strain gage array 40 of 
the sensor means with body portion 36 of the coupler 29. 
As previously discussed, with segment 20a of the tubing extending 
downwardly of casing 16, if the fluid in the well is at a level below the 
lower extremity of the segment, physical changes in the coupler will occur 
which corresponding to the actual weight of the length "L" of segment 20a. 
However, if the fluid level within the well rises, as, for example, as 
shown in FIG. 4, a portion of segment 20a will be immersed within the 
fluid and the tubing will be buoyed upwardly. This buoyancy will result in 
an apparent loss of weight in segment 20a which, in turn, will result in a 
change in the stresses imposed upon the tubing and upon the coupler to 
which it is attached. The physical changes in the tubing and coupler will 
be sensed by the sensor means or strain gage array of this form of the 
invention and will be evidenced by a change in current flow through the 
device. 
As the level of fluid "F" within casing 16 continues to rise, segment 20a 
will be further buoyed upwardly, thereby further decreasing the apparent 
weight of segment 20a. Conversely, as the level of fluid within the well 
falls, the apparent weight of tubing segment 20a will increase in direct 
proportion to the buoyance exerted on the segment. These changes in the 
apparent weight of the length of tubing 20a will, of course, cause changes 
in the stresses imposed on coupler 29, which stresses will be precisely 
sensed by the sensor means or strain gage array and can be directly 
correlated to the level of fluid within the well. When, as before, the 
output of the strain gage array is connected to a suitable controller such 
as a controller 45 and when the controller is appropriately interconnected 
with motor 52, the operation of motor can be precisely controlled as a 
function of the rise and fall of fluid within the well. 
Turning now to FIGS. 6 and 7, the apparatus of the invention is shown being 
used in connection with an oil well system embodying a submergible rotary 
pump with motor attached. In this well system, the rotary pump and motor 
subassembly 60 is mounted downhole of a well which is similar in character 
to that shown in FIGS. 1 and 4. More particularly, as before, the well 
comprises a downhole bore 14 which is lined with a steel oil well casing 
16. As best seen in FIG. 6, the electric power cables 62 which drive motor 
subassembly 60 extend from a surface mounted power source 64 downwardly 
within a tubing system comprises a conduit 20 similar to that previously 
described which includes a lower segment 20a having a length "L" and a 
diameter "D". As depicted in FIG. 6, tube segment 20a of conduit 20 
extends downwardly within the bore 14 and normally is partially surrounded 
by fluid "F" which rises and falls within the well. Because of the 
similarity between the oil well and the tubing system shown in FIG. 6 and 
that shown in FIGS. 1 and 4, like numbers are used in FIGS. 6 and 7 to 
identify like components. The power cables 62 extend through a tubing head 
66 of conventional construction and are connected proximate their upper 
ends 62a with power source 64, which may be a conventional type of high 
voltage supply unit. The lower end 62b of the cables are, of course, 
connected to the motor of the downhole rotary pump and motor assembly 60. 
Disposed intermediate segments 20a and 20b of tubing 20 is a coupler 29 to 
which the sensor means of the invention is interconnected. Coupler 29 as 
well as the sensor means, are identical in construction and operation to 
those previously described in connection with FIGS. 1 through 5. As 
before, the sensor means functions to sense physical changes in the tubing 
system in the exact manner as previously described. Accordingly, the 
details of the construction and operation of the sensor means will not be 
repeated here. 
Turning lastly to FIGS. 10 and 11, an alternate form of the invention is 
there shown being used in connection with a downhole rotary pump type of 
pumping system of the character shown in FIGS. 3, 4 and 5. As previously 
described in this system, the rotary pump 51 is driven by a surface 
mounted motor 52 via a drive shaft or rod 54. The oil well itself is 
similar in character to that shown in FIGS. 4 and 6, and comprises a 
downhole bore 14 which is lined with a steel oil well casing 16. As 
before, drive shaft 54 is housed within a tubing system comprising an 
elongated continuous conduit or tube 70. Continuous conduit, or tube 70, 
extends downwardly within the bore 14 and normally is partially surrounded 
by fluid "F" which rises and falls within the well. Because of the 
similarity between the oil well and certain aspects of the tubing system 
shown in FIG. 4, like numbers are used in FIGS. 10 and 11 to identify like 
components. Drive rod 54 is of conventional construction and is supported 
by a support structure 72 which forms a part of the tubing system of the 
invention and which functions to support driving motor 52. The drive rod 
or shaft 54 extends through a tubing head 60 of conventional construction 
and is drivably connected proximate its upper end 54a with drive motor 52. 
The lower end 54b of the drive shaft is, of course, connected to the 
downhole rotary pump 51 (not shown in FIG. 10, but shown in FIG. 4). 
Unlike the forms of the invention earlier described herein, the sensor 
means of the invention, rather than being connected to a coupler member 
such as coupler 29, is connected to one of the vertical supporting columns 
of the support structure 72, such as column 72a. As before, the sensor 
means functions to sense physical changes in the tubing system of the 
invention and, more particularly, in the structural support portion of the 
tubing system, and is designed to generate at least one signal 
corresponding to a physical change in the system as the fluid level within 
the well rises and falls. Once again, the sensor means of the invention 
comprises transducer means which is here connected to the central portion 
72b of support column 72a in the manner best seen in FIG. 11. Reference 
should also be made to FIG. 9, which illustrates the manner of 
interconnection of the strain gages which comprise the strain gage array 
40 of the sensor means. 
In the construction illustrated in FIGS. 4 and 10 of the drawings, the 
support structure 72 functions to continuously hold the drive shaft 54 in 
tension. Accordingly, the vertical columns of the structure, including 
column 72a, are continuously in compression with the tubing head 60 of the 
structure supporting the downwardly extending conduit or tube 70 which 
surrounds the drive shaft. With tube 70 extending downwardly of casing 16, 
if the fluid in the well is at a level below the lower extremity of the 
tube, physical changes in the support structure, and particularly in 
column 72a, will occur which corresponding to the actual weight of the 
length of tube 70. However, if the fluid level within the well rises, a 
portion of the tube 70 will be immersed within the fluid and will be 
buoyed upwardly. This buoyancy will result in an apparent loss of weight 
in tube 70 which, in turn, will result in a change in the compression 
stresses imposed on column 72a by the downwardly extending tube. The 
physical changes in column 72a will, of course, be sensed by the sensor 
means or stain gage array of this form of the invention and will be 
evidenced by a change in current flow through the device. For example, 
where column 72a has a length L-2 and a diameter D-1, an increase in 
compression on the column will cause a slight increase in diameter and a 
slight decrease in length. Conversely, a decrease in the compressive 
forces exerted on the column will cause a corresponding decrease in 
diameter and an increase in length. 
As the level of fluid "F" within casing 16 continues to rise, tube 70 will 
be further buoyed upwardly, thereby further decreasing the apparent weight 
of the tube. Conversely as the level of fluid within the well falls, the 
apparent weight of tube 70 will increase in direct proportion to the 
buoyance exerted on the tube. These changes in the apparent weight of tube 
70 will, of course, cause changes in the stresses imposed on column 72a, 
which stresses will be precisely sensed by the sensor means or strain gage 
array and can be directly correlated to the level of fluid within the 
well. When, as before, the output of the strain gage array is connected to 
a suitable controller such as a controller 45, and when the controller is 
appropriately interconnected with motor 52, the operation of the motor can 
be precisely controlled in the manner previously described herein, as a 
function of the rise and fall of fluid within the well. 
Having now described the invention in detail in accordance with the 
requirements of the patent statutes, those skilled in this art will have 
no difficulty in making changes and modifications in the individual parts 
or their relative assembly in order to meet specific requirements or 
conditions. Such changes and modifications may be made without departing 
from the scope and spirit of the invention, as set forth in the following 
claims.