Detachable liquid level monitoring apparatus and method

Liquid level monitoring apparatus for monitoring the level of a liquid in a container having a wall with an exterior surface and an interior surface and formed of a material which is substantially transparent to optical energy and which has an index of refraction different from that of the liquid. The apparatus comprises an optical sensor holder and adhesive means adapted to secure the sensor holder to the exterior surface of the container at a predetermined level. A sensor head is provided. An attachment mechanism is carried by the sensor head and the sensor holder for detachably mounting the sensor head on the sensor holder. A self-contained power supply is provided which is coupled to the sensor head.

This invention relates to a detachable liquid level monitoring apparatus 
and method. 
Liquid level apparatus has heretofore been provided particularly for use 
with intravenous bags or bottles to indicate when the bag becomes nearly 
empty and requires replacement. A typical device is disclosed in U.S. Pat. 
No. 3,588,859 which discloses a level detector which uses a capacitance 
bridge. Such devices still have a number of disadvantages. They are not 
self-contained. They are not readily detachable from the bag or bottle. 
There is therefore a need for a new and improved liquid level monitoring 
apparatus and method of using the same. 
In general, it is an object of the present invention to provide a liquid 
level monitoring apparatus which is particularly suitable for rigid 
bottles and flexible bags. 
Another object of the invention is to provide an apparatus and method of 
the above character in which an opto-electronic sensor is utilized. 
Another object of the invention is to provide an apparatus and method of 
the above character in which an adhesive backed mounting pad is adapted to 
be applied to the bag or bottle. 
Another object of the invention is to provide an apparatus and method of 
the above character in which the sensor is removably attached to the 
mounting pad. 
Another object of the invention is to provide an apparatus and method of 
the above character in which the pad is discarded with the container. 
Another object of the invention is to provide an apparatus and method of 
the above character which will operate with colored liquids. 
Another object of the invention is to provide an apparatus and method of 
the above character which has a self-contained power supply. 
Another object of the invention is to provide an apparatus and method of 
the above character in which circuitry is provided to maximize battery 
life. 
Another object of the invention is to provide an apparatus and method of 
the above character which is self-contained and can be moved with the 
patient. 
Another object of the invention is to provide an apparatus and method of 
the above character which does not require the use of power cables. 
Another object of the invention is to provide an apparatus and method of 
the above character in which periodic testing of the fluid level occurs. 
Another object of the invention is to provide an apparatus and method of 
the above character which relies upon reflections which occur at the 
interface between the wall of the container and the liquid utilized. 
Another object of the invention is to provide an apparatus and method of 
the above character which relies upon reflections that change in 
magnitude. 
Another object of the invention is to provide an apparatus and method of 
the above character in which reflections are calculated using a Fresnel 
equation. 
Another object of the invention is to provide an apparatus and method of 
the above character in which the effect of stray light is minimized. 
Another object of the invention is to provide an apparatus and method of 
the above character in which a focused source and detector are utilized to 
make the sensor less immune to stray light. 
Another object of the invention is to provide an apparatus and method of 
the above character in which an additional reference sensor is utilized to 
provide immunity from stray light. 
Another object of the invention is to provide an apparatus and method of 
the above character in which pulse operation of the sensor source is 
utilized to make the apparatus more immune to stray light. 
Additional objects and features of the invention will appear from the 
following description in which the preferred embodiments are set forth in 
detail in conjunction with the accompanying drawings.

In general, the liquid level monitoring apparatus is used for monitoring 
the level of a liquid in a container formed of a material which is 
substantially transparent to infrared energy and which has an exterior and 
an interior surface. The apparatus comprises a pad. Adhesive means is 
provided for securing the pad to the exterior surface of the container. An 
opto-electronic sensor head is provided. Cooperative means is provided on 
the head and the pad for detachably mounting said head on said pad. A 
self-contained power supply is provided which is coupled to the sensor 
head. 
The method for detecting the level of a liquid in a container having a wall 
formed of a material which is substantially transparent and having an 
exterior surface and an interior surface and an index of refraction 
different from the index of refraction of the liquid in the container is 
accomplished by directing optical energy through the wall of the container 
so that energy is reflected from the exterior and interior walls of the 
container. The method further comprises detecting the reflection of energy 
from one of the exterior and interior surfaces of the wall of the 
container and detecting when a change occurs in the reflections from the 
one wall of the container to ascertain when the liquid level falls below a 
predetermined level in the container. 
More in particular the liquid level monitoring apparatus comprising the 
present invention consists of a sensing unit 11 and a monitor unit 12 
which are interconnected by a cable 13. This liquid level monitoring 
apparatus 11 is utilized for monitoring the liquid level in a container 
16. The container 16 can be of any suitable type. However, in connection 
with the present invention it is desired that the container be formed of a 
material which is substantially transparent to optical energy, as for 
example, infrared energy. The container 16 can be formed of a flexible bag 
as shown in FIG. 1 which is typically used for dispensing medical 
solutions such as IV solutions to a patient. It also can be in the form of 
a rigid container such as shown in FIG. 1A also formed of a suitable 
material such as plastic. Also containers such as glass bottles which are 
circular in cross section can be utilized. Each of the containers 16 is 
provided with a hanger portion 17 which is provided with a hole 18 to 
permit the hanger portion 17 to be mounted on and supported by the arcuate 
extremity 19 of a generally horizontally extending support member or arm 
21 which is formed as a part of a vertical support pole 22. The support 
can be of a conventional telescoping type mounted upon a cross frame 23 
provided with casters 24, all of which form a part of an IV support stand 
26. The support stand 26 is of a conventional type and can be readily 
moved with the patient as hereinafter described. 
The sensing unit 11 consists of the sensor head 31 which is removably 
attached to a sensor holder 32 (see FIGS. 2 and 3). The sensor holder 32 
is shown in detail in FIGS. 5, 6 and 7 and consists of a plate-like member 
33 formed of a suitable material such as a rigid plastic which is 
generally rectangular in configuration having spaced apart top and bottom 
edges 34 and 36 and spaced apart parallel side edges 37 and 38 that extend 
in directions perpendicular to the edges 34 and 36. An elongate generally 
rectangular opening 39 is provided in the plate-like member 38 and extends 
through the parallel planar surfaces 41 and 42 of the plate-like member 
33. The major axis of the opening 39 extends in a direction which is 
parallel to the side edges 37 and 38 and its minor axis extends in a 
direction which is parallel to the top and bottom edges 34 and 36. The 
inside corners of the opening 39 are rounded as shown. 
The sensor head 31 is provided with a case or housing 44 which can be 
formed of a suitable material such as plastic. Cooperative mating means is 
provided on the case or housing 44 of the sensor head 31 and the 
plate-like member 33 of the sensor holder to permit attachment of the 
sensor head 31 to the sensor holder 32 or detachment of the sensor head 
from the sensor holder 32. Such cooperative mating means can take a number 
of forms. For example, as shown in the drawing, such cooperative mating 
means can consist of L-shaped members 46 and 47 which are formed integral 
with the plate-like member 33. Alternatively if desired, the retaining 
members 46 and 47 can be formed of separate parts and then secured to the 
plate-like member 33 by suitable means such as an adhesive or screws. The 
retaining members 46 and 47 are L-shaped in cross section. Thus the 
retaining member 46 is provided with a portion 46a which extends 
perpendicular to the surface 41 of the plate-like member 33 and a portion 
46b which extends in a direction which is generally parallel to the 
surface 41 and inwardly from the side edge 38. Similarly, the retaining 
member 47 is provided with an upwardly extending portion 47a and an 
inwardly extending portion 47b. Spaced apart parallel slots or recesses 48 
and 49 are provided in the two spaced parallel walls 51 and 52 of the case 
or housing 44 on opposite sides of the housing 44 which are adapted to 
receive the portions 46b and 47 b of the retaining members 46 and 47. The 
recesses 48 and 49 extend in a direction perpendicular to walls 53 and 54 
of the case or housing 44. The case or housing 56 is also provided with a 
top wall 56 which has an opening 57 therein through which the cable 13 
extends. 
Suitable means is provided to ensure that the case 44 can be inserted into 
the slots 48 in only one orientation of the case. In the present 
embodiment this is accomplished by having one of the slots 48 and 49 be at 
a higher elevation on the associated wall than the other slot. Similarly 
one of the L-shaped retaining members 46 and 47 extends at a higher 
elevation than the other retaining member. Thus the retaining member 46 
extends above the surface 41 at a greater distance than the retaining 
member 47, and the slot or recess 48 extends at an elevational which is 
higher than that of the slot 49 so that the case can only be inserted into 
the retaining members in one orientation. The fit between the retaining 
members 46 and 47 and the slots or recesses 48 and 49 is relatively tight 
so that the case or housing is frictionally retained within the sensor 
holder 32. 
Suitable means is provided for preventing movement of the sensor head 31 
beyond a predetermined location on the plate-like member 33 so that it 
will be properly positioned with respect to the opening 39 and consists of 
a stop member 59 formed integral with the plate-like member 33 and 
extending upwardly from the surface 41. As shown it can be generally 
rectangular so that it is engaged by the outer side of the wall 53 of the 
case 44. 
A detent lug 60 is positioned near the stop member 59 to retain the sensor 
head 31 on the plate-like member 33. The detent lug extends upwardly from 
the surface 41 of the plate-like member and engages the inner side of the 
case wall 53 when the sensor head is in the proper position on the 
plate-like member. The detent lug is relatively short and is rounded so 
that the sensor head snaps into place as the wall 53 passes over the lug. 
Like the stop member 59, the detent lug 60 is formed as an integral part 
of the plate-like member. 
Adhesive attaching means 61 (see FIGS. 6 and 7) is provided for securing 
the sensor holder 32 to the container 16. The adhesive attaching means 61 
can be in any suitable form. For example, it can be comprised of a 
pressure sensitive layer 62 of an adhesive which is mounted on the surface 
42 of the plate-like member 31. The adhesive layer 62 is covered with a 
removable flexible protective sheet 63 which can of paper or other 
suitable material which overlies the layer 62 and protects the same until 
the sensor holder 32 is ready to be used. When it is desired to utilize 
the sensor holder 32, the sheet 63 can be peeled away from the pressure 
sensitive adhesive layer 62 and the holder 32 secured to the container 16 
in the desired location. The pressure sensitive adhesive 62 is provided 
with an aperture 64 (see FIG. 4) which is in registration with the opening 
39 in the member 33. The stop member 59 can be used as a guide to 
facilitate positioning of the sensor holder 32 at the desired elevation on 
the container 16 relative to level markings 67 (see FIGS. 1 and 1A) 
provided on the container. 
Sensor modules 71 and 72 forming a part of the sensor head 31 are provided 
within the case or housing 44. The modules 71 and 72 can be formed of a 
suitable material such as plastic. Each of the modules is provided with a 
pair of bores 73 and 74 aligned at a suitable angle as, for example, 
45.degree. with respect to a front surface 76 provided on the modules 71 
and 72. The two modules 71 and 72 are positioned within the case or 
housing 44 in such a manner that the front surface 76 is generally in 
alignment with the forward extremity of the housing or case 44 and is 
immediately adjacent the sensor opening 39. Each of the modules 71 and 72 
is provided with inclined sidewalls 77 and 78 which adjoin the front 
surface 76. Each of the modules 71 and 72 is also provided with a rear 
wall 79 which is adapted to seat against a shoulder 81 formed within the 
case 44. The modules 71 and 72 can then be held in the desired position 
against the shoulder 81 in the case 44 by a suitable means such as an 
epoxy 82. 
Optical level sensing means in the form of a sensor pair is provided in 
each of the modules 71 and 72 and consists of a source of optical energy 
as, for example, an infrared source such as a light emitting diode (LED) 
86 (FIG. 4) mounted in the bore 73 and a photoelectric detector 87 for 
detecting the infrared light from the LED source 86 is provided in the 
other bore 74. Lenses 88 and 89 are provided in the bores 73 and 74 and 
are positioned so their focal points are at the same point on the inside 
surface 91 of the wall 92 of the container 16. The outside surface 93 of 
the wall 92 has the pressure sensitive adhesive layer 62 bonded to the 
same as shown in FIG. 4. Wires 96 from the cable 13 are connected to the 
light emitting diode 86 and the detector 87 by connecting the wires 96 to 
conducting posts 97 carried by the modules 71 and 72 connected to the 
light emitting diode 86 and the detector 87. 
Alternatively, the sensing head can be formed as a unitary structure with 
the optical sensors being molded into the case rather than being mounted 
separately as to the present embodiment. This would reduce the cost of the 
device by reducing the number of parts and eliminating the steps required 
to mount the sensors, and it would also provide more uniform alignment of 
the sensors. 
The cable 13 extends to the monitor unit 12 which consists of a box-like 
case 101 formed of a suitable material such as plastic and which has 
mounted therein a printed circuit board (not shown) which carries the 
electronic circuitry which is shown in FIG. 8. An on-off switch 106 is 
mounted in the case 101 for turning the electronic circuitry on and off. 
The case 101 also contains self-contained power supply means in the form 
of a 9-volt battery B1 which under the control of switch S1 as shown in 
FIG. 8 controls the application of the 9 volts to the electronic 
circuitry. 
The light emitting diode 86 and the detector 87 in each of the modules 71, 
72 form a sensor pair. The sensor pairs are disposed horizontally with the 
upper module 71 serving as the signal sensor pair and the lower module 72 
serving as the reference sensor pair. Alternatively, the two sensor 
modules can be positioned side-by-side, rather than being separated 
vertically. This would position both sensor pairs at the same height and 
would eliminate the need to install the sensor head on the mounting plate 
in the same direction every time. 
The electronic circuitry as shown in FIG. 8 includes four operational 
amplifiers OA1, OA2, OA3 and OA4 which are connected to various electronic 
components shown in FIG. 8 to operate in the manner hereinafter described. 
It will be noted that operating power is applied to the light emitting 
diode 86 in the module 72 and that the sensor in this module operates in a 
passive mode to monitor ambient light. 
Operation and use of the liquid level monitoring apparatus may now be 
briefly described as follows in performing the method of the present 
invention. Let it be assumed that it is desired to utilize the liquid 
level monitoring apparatus in a critical care application, as for example, 
a situation in which it is desired to monitor the liquid level in an IV 
bag or container 16 having a liquid 111 therein having a liquid level 112 
so that an alarm is initiated when the liquid level 112 approaches a level 
mandating replacement of the bag or container 16. The monitoring unit 12 
can be placed on the IV support stand 26 by utilizing the hanger 102 to 
mount the same on the horizontal member 21 as shown in FIGS. 1 and 1A. A 
sensor holder 32 can then be secured to the bag in the appropriate 
location by first removing the protective sheet 63 to expose the adhesive 
layer 61. The sensor holder 32 is then secured to the bag in the 
appropriate location by utilizing the stop member 59 in FIG. 2 as a 
reference mark to locate the elevation at which it is desired to initiate 
an alarm. After the sensor holder 32 has been positioned, the sensor head 
31 can be slid into position in the holder 32 until it snaps into position 
between the stop 59 and the detent lug 60. The outlet tube 114 attached to 
the container 26 can then be positioned in the patient's vessel which is 
to receive the IV liquid contained within the container 16. After the 
liquid in the container 16 has been drained from the container to a level 
causing operation of the liquid level monitoring apparatus as hereinafter 
described and an alarm initiated, the nurse can remove the sensor head 31 
from the sensor holder 32 by sliding it out of the sensor holder away from 
the stop 59. The container 16 can then be removed from the support member 
21 and discarded along with the sensor holder 32 which previously had been 
secured thereto. A new sensor holder 32 is then taken and secured to the 
new bag 16 in the manner hereinbefore described and the new bag 16 is hung 
onto the support member 21. Thereafter, the sensor head 31 can again be 
slid into the sensor holder 32 in the manner hereinbefore described so 
that the liquid level can be monitored. 
The liquid level sensing apparatus operates on the reflections which occur 
between the inner surface 91 of the wall 92 of the bag 16 and the 
interface with the liquid 111 within the bag and which has a liquid level 
112. The sensor holder 32 when attached to the bag serves to hold the 
inner surface 91 of the bag 16 very flat adjacent the sensor head 31 to 
improve the accuracy of the liquid level monitoring apparatus. This is 
particularly important where the container 16 is in the form of a flexible 
plastic bag because the sensor holder prevents deformation of the 
interface to which the light beam from the sensor head 31 is transmitted 
and reflected. 
It is well known to those skilled in the art, the difference in the indices 
of refraction of the two different materials, as for example, plastic and 
air or liquid that the reflection at each interface can be calculated by 
using the Fresnel equation: 
##EQU1## 
where n.sub.2 and n.sub.1, are the indices of refraction of the two 
materials. 
When the liquid level sensing apparatus is considered in connection with 
the present invention, the ray diagrams which would be encountered are 
shown in FIGS. 9A and 9B. When light beam L1 strikes the plastic container 
wall 92 above the liquid level 112 reflections occur, one at each of the 
two surfaces 91 and 93 of the container wall 92 which are identified as 
points I and II. The magnitude of each reflection is given by the Fresnel 
equation. Since air is disposed on both sides of the wall 92, the 
reflected rays R.sub.1A and R.sub.1B are given by the equations 2 and 3 
below wherein the index of refraction of the plastic wall 92 n.sub.2 is 
approximately 1.4 and that of air in n.sub.1 is 1.0. 
##EQU2## 
When the light beam L2 strikes the plastic container wall 92 below the 
liquid level as shown in FIG. 9, then two reflections still occur at 
points III and IV. However, the magnitude of the reflections are different 
and can be calculated as follows as shown by equations 3 and 4 below: 
##EQU3## 
The difference in reflection of R.sub.1B of 2.8% and that of R.sub.2B of 
0.001% can be utilized to determine when a liquid is present at the 
position of being sensed. In the present invention, the apparatus senses 
only the reflections that change in magnitude since it is impractical and 
unnecessary to discriminate between the reflections R.sub.1A and R.sub.2B 
and R.sub.1B and R.sub.2B so that the total reflection is measured. The 
change in reflection between the presence of liquid and the absence of 
liquid adjacent the plastic wall 92 can then be readily calculated as set 
forth below in Equation 6. 
##EQU4## 
This equation shows the changes in reflection can be easily detected 
provided that stray light from other sources can be ignored or minimized. 
The detection of stray light is minimized by utilizing three different 
techniques. The first technique or method is to utilize the lenses 88 and 
89 hereinbefore described for the light source and for the detector so 
that the source of light is focused at the interface between the plastic 
wall 92 and the liquid with the detector being similarly focused. This 
reduces the effect of room light by placing a sensor source in focus with 
the detector. 
The effect of stray room light is also reduced by pulsing the source of 
light to provide a second technique or method. This makes it possible to 
provide very high intensive light pulses to reduce the effect of stray 
room light. This has the additional advantage of decreasing power 
utilization and therefore increasing battery life. 
The third technique or method for reducing or minimizing the effect of 
stray room light is obtained by comparing the signal detected by the first 
detector in one sensed pair at one location which is compared with the 
output from a second detector used as a reference in the other sensor pair 
placed adjacent to the first detector but at a different location. In such 
a situation, both detectors see the same amount of stray room light but 
only the first detector sees the additional light from the source. By 
taking the difference between the two signals, the stray room light signal 
is subtracted leaving only the source signal. Thus it can be seen that in 
operation of the sensor, Fresnel reflections are used for sensing the 
presence of a liquid in the container. The focused source and detector 
make the sensor relatively immune to stray room light. The use of a second 
sensor pair as a reference makes the first sensor pair immune to stray 
room light. The pulsed operation of the sensor source makes the sensor 
also relatively immune to stray room light. 
Utilization of these principles in connection with the circuitry shown in 
FIG. 8 may now be briefly described as follows. The operational amplifier 
OA1 with the associated circuitry including the capacitor C1 provides a 
square wave having a period of approximately two seconds. The capacitor C2 
and the operational amplifier 0A2 convert the square wave signal into a 
short pulse which drives the light emitting diodes 86 in the sensor pairs 
of the modules 71 and 72 to provide short pulses of light. The detectors 
87 which are provided in the sensor pairs in the modules 71 and 72 are 
connected as common collector photo transistors and generate a voltage 
which is proportional to the intensity of the light that strikes them. The 
outputs from the two detectors 87 of the modules 71 and 72 are subtracted 
from each other by the operational amplifier OA3 and in addition, a fixed 
reference voltage is generated by the adjustable resistor R15 and the 
resistor R14 which is subtracted from the voltage from the signal source. 
Since the gain of the operational amplifier OA3 is large any small 
difference between the reference voltage and the signal voltage causes a 
large change in the output of the amplifier OA3. 
The output from the operational amplifier OA3 is then utilized to charge 
capacitor C3 through the diode D2. The voltage on the capacitor C3 rises 
rapidly when no liquid is present in the container 16 at the level at 
which the sensing unit is positioned and decays slowly. This serves to 
lengthen the pulse output and is used to generate a signal to the 
operational amplifier OA4 which is utilized to provide a signal to operate 
the buzzer 116 which can be disposed within the case 101 of the monitor 
unit 12. In addition, the same signal can be supplied to external contacts 
117 which can be utilized for powering an external device (not shown) such 
as an additional remote alarm. 
The slow decay of the voltage on the capacitor C3 makes it possible to 
provide a signal to the operational amplifier OA4 so that it generates a 
relatively square wave pulse of relatively long duration to actuate the 
buzzer 116 or an external device connected to the terminals 117. The 
voltage on the capacitor C3 has a rate of decay which is determined by the 
resistance connected between the capacitor C3 and ground. This resistance 
is controlled by the transistor Q1 which forms the low battery indication 
circuit. When the voltage on battery B1 is greater than approximately 6 
volts, the transistor Q1 is turned on and connects the resistor R16 to 
ground. When the battery voltage of battery B1 drops below approximately 6 
volts, the signal formed by the resistor divider R20 and R21 is 
insufficient to keep the transistor Q1 turned on so it turns off. The 
resistance of the R16 resistor to ground becomes infinite and the 
capacitor C3 can never discharge. In fact, the capacitor C3 is discharged 
to the battery voltage through resistor R19. This causes a continuous 
buzzing which serves as a low battery indication. 
When liquid is present in the container 16 no pulse is generated by the 
operational amplifier OA3 so that the capacitor C3 remains discharged and 
the buzzer 116 remains inactivated. However, even if liquid is present in 
the container, a low battery voltage will still produce a continuous 
buzzing. Therefore, it is impossible to place into service a liquid level 
monitoring apparatus which has a low battery voltage without causing a 
continuous buzzing. 
From the foregoing it can be seen that there has been provided a liquid 
level monitoring apparatus which is particularly suitable for monitoring 
liquid levels in plastic containers, glass bottles and the like. Since 
only a single sensor head need be utilized, only one sensor head needs to 
be positioned by the operator merely by positioning the sensor holder 33, 
sliding the sensor head 31 into the sensor holder 32. Each use of a 
container 16 merely requires the disposal of the container 16 itself with 
a single adhesive backed sensor holder 32. All the remainder of the liquid 
level sensing apparatus can be saved and reused. Since the sensor holder 
32 is relatively simple, it can be formed of a single member of plastic 
provided with an adhesive backing. Thus it can be very low in cost to 
minimize the expense in the use of the liquid level monitoring apparatus 
of the present invention. Also only a single sensor housing is required 
for each unit. The alignment of the detector and the light emitting diode 
is fixed in the module eliminating the necessity for adjustment. 
The pulse method of sampling and initiating alarms significantly increases 
the battery life. Even with almost continuous use, the battery should now 
provide an operating time of approximately one month. A low battery 
indication is also provided. The liquid level monitoring apparatus can be 
utilized with colored liquids since transmission of light through the 
liquid is not required. The liquid level monitoring apparatus is self 
contained and can be readily moved with the patient without necessity for 
disconnecting power cords and the like. The sensor holder can be readily 
modified to accommodate containers which are circular in cross section, as 
for example, bottles, merely by providing the sensor holder with a curved 
surface which has a general conformation which corresponds to the 
curvature of the exterior surface of the bottle.