Detector device and method for distinguishing between fluids having different dielectric properties

A device and method for distinguishing between different fluids on the basis of their dielectric properties. In an preferred embodiment, the device comprises two substantially parallel conductive surfaces which are positioned on opposite sides of a fluid conduit so as to form a substantially parallel plate capacitor. The first conductive surface is electrically connected through an inductor to a voltage source, and such first conductive surface is also electrically connected to the source electrode of a field-effect transistor. A resonator device is electrically connected between the second conductive surface and the gate electrode of the field-effect transistor. The gate electrode of the field-effect transistor is further electrically connected through a fixed resistance to ground, and the drain electrode of the field-effect transistor is electrically connected directly to ground. The inductance of the inductor is selected such that the resonant frequency of the inductor and the capacitor is substantially the same as the operating frequency of the resonator device when a desired column of fluid is positioned between the two conductive surfaces. In this state, the output voltage of the detector device will oscillate. However, when a column of fluid having significantly different dielectric properties is positioned between the two conductive surfaces, the resonant frequency of the inductor and the capacitor will change, and the output voltage of the detector device will stop oscillating.

BACKGROUND 
1. The Field of the Invention 
This invention relates to devices and methods for distinguishing between 
different kinds of fluids and, more particularly, to a novel device and 
method for distinguishing between liquids and gases on the basis of their 
differing dielectric properties. This invention is particularly, but not 
exclusively, useful as an air-in-line detector for an IV infusion device. 
2. The Background Art 
Various kinds of fluid systems are currently in wide use in industrial, 
medical and many other applications. As used herein, the term "fluid" 
refers generally to any substance which is not solid and which is capable 
of flowing through a tube or conduit and, thus, includes both gases and 
liquids. 
In fluid systems, it is often desirable to be able to distinguish between 
different fluids, such as, for example, water and air. Such detection is 
essential for the proper operation of many fluid systems. 
For example, a fluid system may be intended to convey a liquid through a 
conduit. If sufficient quantities of air enter the conduit, however, the 
system may malfunction. For instance, if the system includes a pump for 
conveying the liquid through the conduit, the pump may cease to operate 
properly in the presence of sufficient quantities of air. The intended 
function of the system will, in any event, not be served unless the air is 
detected and removed from the system. 
In fluid systems which are intended for medical applications, the early 
detection of an air bubble in a column of liquid can be vital. For 
example, a patient is often supplied with medication or other essential 
liquids through a tube which is connected to the patient through an 
intravenous (IV) catheter. If air bubbles enter the tube and are conveyed 
to the patient, the patient can be subjected to significant discomfort. 
Such air bubbles can in some cases even become life-threatening. 
In order to insure the proper operation of fluid systems, especially those 
intended for medical use, those skilled in the art have attempted to 
develop devices that will automatically detect an air bubble in a column 
of liquid. Unfortunately, however, the prior art devices have typically 
been somewhat limited in their application, such as, for example, being 
limited to use with either an opaque liquid or a substantially transparent 
liquid. Those prior art systems which have been intended for use with both 
transparent liquids and opaque liquids have generally been quite complex 
and expensive, and also somewhat unreliable. 
One type of system which is commonly used to detect the presence of an air 
bubble in a column of liquid uses a light source which is directed through 
the column. A photocell or some other light sensitive device is positioned 
adjacent the fluid column opposite the light source. Then, by detecting 
the intensity of the light transmitted through the column, the nature of 
the fluid in the column is ascertained. 
For example, if an opaque liquid is being conducted through a transparent 
conduit, the absence of light transmitted through the conduit is 
indicative of liquid being in the column. If an air bubble enters the 
conduit, however, the fluid column will suddenly become transparent. This 
can be readily detected with the photocell and communicated to appropriate 
control circuitry. 
When the liquid being conveyed through a transparent conduit is also 
substantially transparent, however, detecting an air bubble in the conduit 
becomes somewhat more challenging. In such cases, prior art detection 
devices are often based upon the different light transmission properties 
of air and liquid. For instance, liquid will generally cause a light beam 
to be refracted as it passes through the conduit, while air will not. 
Thus, if the photocell is carefully positioned and shielded so as to 
detect only a refracted light beam, the detection of light by the 
photocell is an indication that clear liquid is in the conduit. On the 
other hand, when such a properly positioned and shielded photocell does 
not detect a significant amount of light, it is likely that air or an 
opaque fluid is in the conduit. 
It will be readily appreciated that the above-described devices for 
detecting the presence of an air bubble in a column of liquid are quite 
complex. These devices require careful alignment and/or shielding of the 
light source and the photocells. Additionally, when these detection 
devices are to be used with both opaque and transparent liquids, some 
means must be provided for initially detecting whether a given liquid is 
either opaque or transparent. This again adds complexity and cost to the 
devices and may tend to make their performance somewhat unpredictable. 
In addition to photo-optical systems, various ultrasonic devices have also 
been proposed for the detection of an air-in-line condition. Ultrasonic 
devices, unlike the photo-optical devices, are not affected by fluid 
opacity. Instead, they depend on differences in the ultrasonic 
transmissive properties of the fluid in the tube to distinguish between 
whether a liquid or a gas is flowing through the tube. Ultrasonic 
air-in-line detection devices, like sophisticated photo-electric devices, 
however, are relatively more expensive to manufacture. 
The present invention recognizes that an air-in-line condition can be 
detected by means that are reliable yet less expensive than the optical or 
ultrasonic systems. In accordance with the present invention, an 
air-in-line condition can be detected by effectively incorporating the 
fluid tube as a component of an electrical circuit. Specifically, the 
present invention takes advantage of the capacitive changes between an 
air-in-line condition and a liquid-in-line condition to determine when 
there is an unwanted air-in-line condition. 
BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
In view of the foregoing, it is a primary object of the present invention 
to provide a simple electrical device which can readily distinguish 
between liquids and gases in a fluid column. 
It is also an object of the present invention to provide a device for 
distinguishing between different fluids whose operation is substantially 
independent of the optical or acoustic properties of the fluid. 
It is a further object of the present invention to provide a device for 
distinguishing between different fluids which is inexpensive to 
manufacture and reliable in operation. 
Also, it is an object of the present invention to provide a device which 
can readily be used in medical applications to rapidly detect an air 
bubble in a column of virtually any liquid. 
Consistent with the foregoing objects, the present invention is directed to 
a novel device and method for distinguishing between different fluids on 
the basis of their dielectric properties. The device comprises two 
substantially parallel conductive surfaces which are positioned on 
opposite sides of a column of fluid so as to form a substantially parallel 
plate capacitor. The first conductive surface is electrically connected 
through an inductor to a voltage source. A resonator device is 
electrically connected to the second conductive surface, and a 
transconductance amplifier means is electrically connected between the 
resonator device and the first conductive surface. 
The inductance required by the device is ascertained by first determining 
the capacitance of the two conductive surfaces when a typical desired 
liquid is in the fluid column. The inductance is then selected such that 
the resonant frequency of the inductor and the capacitor is substantially 
the same as the oscillating frequency of the resonator device. 
Thus, the output voltage of the detector device will oscillate whenever the 
fluid column is filled with the desired liquid. However, when a fluid 
having significantly different dielectric properties passes through the 
fluid column (such as, for example, air), the effective capacitance of the 
capacitor will change, thereby changing the resonant frequency of the 
inductor and the capacitor. Consequently, since the resonant frequency 
will no longer be substantially the same as the operating frequency of the 
resonator device, the output voltage of the detector device will stop 
oscillating. Hence, one can readily distinguish between different fluids 
in the fluid column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
It will be readily appreciated that the components of the present 
invention, as generally described and illustrated in the figures herein, 
could be arranged and designed in a wide variety of different 
configurations. Thus, the following more detailed description of the 
embodiment of the device and method of the present invention, as 
represented in FIGS. 1 through 3, is not intended to limit the scope of 
the invention, as claimed, but it is merely representative of one 
presently preferred embodiment of the invention. 
The presently preferred embodiment of the invention will be best understood 
by reference to the drawings, wherein like parts are designated with like 
numerals throughout. 
One presently preferred embodiment of the detector device of the present 
invention, designated generally at 10, is illustrated in its entirety in 
FIG. 1. As shown, detector device 10 comprises a housing 12 which may be 
formed of plastic or some other suitable material. Importantly, housing 12 
has a channel or groove 13 formed therein for receiving a substantially 
non-conductive fluid conduit 14. 
As shown in FIG. 2, two conductive surfaces 22 and 24 are affixed to or 
embedded within housing 12 in proximity to channel 13. Conductive surfaces 
22 and 24 form the plates of a capacitor 20 (see FIG. 3). Importantly, for 
reasons which will become apparent from the discussion which follows, 
conductive surfaces 22 and 24 are positioned adjacent channel 13 so as to 
lie on substantially opposite sides of a conduit 14 which is located 
within channel 13. 
Conductive surfaces 22 and 24 are illustrated in FIG. 2 as being 
substantially planar, parallel surfaces. However, conductive surfaces 22 
and 24 may have virtually any suitable shape and still serve their 
essential function as a capacitor. Moreover, one advantage of the detector 
device 10 of the present invention is that conductive surfaces 22 and 24 
do not need to be precisely aligned, thus making it easier and less 
expensive to manufacture detector device 10. 
The electrical circuitry of detector device 10 may be conveniently 
positioned within housing 12, as shown generally in FIG. 2. Such circuitry 
can then be connected to an appropriate power source and to suitable 
monitoring devices by means of a cable 70. One presently preferred 
embodiment of the electrical circuitry of detector device 10 is 
illustrated schematically in FIG. 3. 
As shown in FIG. 3, conductive surface 22 of capacitor 20 is electrically 
connected to a voltage source 16 through an inductor 30. Conductive 
surface 22 is also electrically connected to the source electrode 52 of a 
field-effect transistor (FET) 50. 
A resonator device 40 is electrically connected between conductive surface 
24 of capacitor 20 and the gate electrode 54 of FET 50. Gate electrode 54 
of FET 50 is further electrically connected through a resistor 60 to 
ground 18, and the drain electrode 56 of FET 50 is connected directly to 
ground 18. 
Resonator 40 comprises an electrical network which is designed to oscillate 
at a specific frequency. Resonator 40 may, for example, comprise the 
resonator device which is currently available from Frequency Control 
Products as part No. FCP 35, and which is designed to oscillate at a 
frequency of 3.579 Mega Hertz (MHz). 
As will be appreciated by those skilled in the art, FET 50 and resistor 60 
form a transconductance amplifier in a feedback loop between the output of 
resonator 40 and the junction between capacitor 20 and inductor 30. The 
purpose of this transconductance amplifier in detector device 10 is to 
maintain an operative current level which will allow resonator 40 to 
oscillate at a substantially constant frequency. A suitable 
transconductance amplifier circuit could, of course, be configured a 
number of different ways. The FET amplifier circuit illustrated in FIG. 3 
is presently preferred because of its simplicity. 
FET 50 is an n-channel FET and can readily be obtained from a number of 
different sources. For example, one suitable FET 50 is currently available 
from National Semiconductor as part No. 2N3918. 
The purpose of resistor 60 is to adjust the gain of and properly bias FET 
50. A suitable resistance value for resistor 60 when using the FET 50 
described above is 10 meg Ohms. 
It is well known that capacitor 20 and inductor 30 have a resonant 
frequency at which they will oscillate. Such resonant frequency is 
directly dependent upon the capacitance of capacitor 20 and the inductance 
of inductor 30. This relationship can be expressed mathematically, as 
follows: 
##EQU1## 
where f=the resonant frequency (Hz); 
L=the inductance of inductor 30 (henries); and 
C=the capacitance of capacitor 20 (farads). 
Significantly, when the resonant frequency of capacitor 20 and inductor 30 
is substantially equal to the oscillating frequency of resonator 40, 
detector device 10 will produce an oscillating output voltage at 17. On 
the other hand, if the resonant frequency of capacitor 20 and inductor 30 
is significantly different from the oscillating frequency of resonator 40, 
the output voltage 17 of detector device 10 will be substantially 
constant. Thus, detector device 10 can be designed to produce an 
oscillating output voltage 17 under certain desired conditions by properly 
selecting the inductance of inductor 30 in relation to the capacitance of 
capacitor 20. 
The capacitance of capacitor 20 is dependent upon the surface area of 
conductive surfaces 22 and 24, the distance between conductive surfaces 22 
and 24, and the dielectric properties of the material between conductive 
surfaces 22 and 24. For example, the capacitance of an ideal parallel 
plate capacitor is approximated by the following equation: 
##EQU2## 
where C=capacitance (farads); 
K=dielectric constant of material between capacitor plates; 
.epsilon.=permittivity constant (8.85.times.10.sup.-12 farad/meter); 
A=surface area of capacitor plates (meter.sup.2); and 
d=distance between capacitor plates (meters). 
Due to fringing and other effects, the actual capacitance of capacitor 20 
will be different from the value given by equation (2). Equation (2) does, 
however, illustrate in a general way how the capacitance of capacitor 20 
will depend upon the size and position of conductive surfaces 22 and 24, 
as well as upon the dielectric nature of the specific material which 
separates conductive surfaces 22 and 24. 
In using detector device 10, the inductance of inductor 30 is selected so 
that the resonant frequency of capacitor 20 and inductor 30 is 
substantially the same as the oscillating frequency of resonator 40 when a 
desired column of fluid is positioned between conductive surfaces 22 and 
24 of capacitor 20. This can be done by measuring the capacitance of 
capacitor 20 with the desired column of fluid in place and then selecting 
an inductor 30 having a fixed inductance of the required value. 
Alternatively, inductor 30 may be a variable inductance device which can 
be adjusted so as to provide the necessary inductance for each desired 
application. 
With the inductance of inductor 30 properly selected, the output voltage 17 
of detector device 10 will oscillate as long as the desired column of 
fluid remains between conductive surfaces 22 and 24. However, in the event 
the column of fluid between conductive surfaces 22 and 24 is interrupted 
by a column of fluid having different dielectric properties, the 
capacitance of capacitor 20 (and consequently the resonant frequency of 
capacitor 20 and inductor 30) will be changed. In such case, the resonant 
frequency of capacitor 20 and inductor 30 will no longer be substantially 
the same as the oscillating frequency of resonator 40, and the output 
voltage 17 of detector device 10 will cease oscillating. 
For example, the dielectric constant "K" of air is approximately 1.00054, 
while the dielectric constant of water is approximately 78. Thus, in 
accordance with equations (1) and (2), if a column of water between 
conductive surfaces 22 and 24 is replaced by a column of air, the 
capacitance of capacitor 20 will decrease by a factor of approximately 78 
and the resonant frequency of capacitor 20 and inductor 30 will increase 
by a factor of approximately 8.8. Hence, if detector device 10 has been 
designed to produce an oscillating output voltage 17 when a column of 
water is positioned between conductive surfaces 22 and 24, the output 
voltage 17 of detector device 10 will cease to oscillate if the column of 
water is replaced by a column of air. 
Those skilled in the art will recognize that the sensitivity of detector 
device 10 can be selected by varying the dimensions of conductive surfaces 
22 and 24 and the `Q` of the LC circuit. As a general rule, the space 
between conductive surfaces 22 and 24 should be approximately just large 
enough to receive the smallest volume of fluid one desires to detect. For 
example, the embodiment of detector device 10 illustrated in FIG. 2 could 
readily detect the presence of air bubble 15, since air bubble 15 occupies 
virtually the entire space between conductive surfaces 22 and 24. Detector 
device 10 might not, however, be able to detect air bubbles which are 
significantly smaller than air bubble 15 because such air bubbles might 
not be large enough to significantly affect the capacitance of capacitor 
20. 
Those skilled in the art will also appreciate that conductive surfaces 22 
and 24 need not necessarily be in direct contact with the column of fluid. 
In fact, as shown in FIGS. 1 and 2, the column of fluid will generally 
reside within some type of conduit. Further, conductive surfaces 22 and 24 
may be embedded within the walls of housing 12. The volume and dielectric 
properties of the other materials positioned between conductive surfaces 
22 and 24 will, however, affect both the capacitance of capacitor 20 and 
the overall sensitivity of detector device 10. For this reason, both 
housing 12 and fluid conduit 14 should be substantially non-conductive so 
that capacitor 20 can function properly. 
By way of further illustration, detector device 10 of the present invention 
can be conveniently used by medical personnel to detect the presence of an 
air bubble in the flexible tube which carries liquid from a reservoir to a 
patient. This tube is commonly referred to as an "IV" tube, and it is 
connected to a patient by means of an intravenous (IV) catheter. 
For such use, housing 12 of detector device 10 may advantageously be 
configured such that it can be readily secured to a conventional hospital 
IV stand or directly attached to an IV infusion device. In either case, 
the IV tube 14 can be inserted into channel 13 of detector device 10 and 
will be releasably held within channel 13 by friction. 
With tube 14 thus in place, tube 14 may be filled with the liquid to be 
administered to the patient. The capacitance of capacitor 20 may then be 
measured, and the inductance of inductor 30 is selected such that the 
resonant frequency of capacitor 20 and inductor 30 is substantially the 
same as the oscillating frequency of resonator 40. 
For example, resonator 40 may have an operating frequency of 1.0 MHz. 
Further, the capacitance of capacitor 20 might be approximately 5.0 
picofarads when tube 14 is filled with the desired liquid. Using equation 
(1) above, one finds that the inductance of inductor 30 must be 
approximately 5.1 millihenries in order for the resonant frequency of 
capacitor 20 and inductor 30 to be substantially the same as the 
oscillating frequency of resonator 40, (i.e., 1.0 MHz). 
If the inductance of inductor 30 is so selected, detector device 10 will 
produce an output voltage 17 which oscillates at a frequency of 
approximately 1.0 MHz as long as tube 14 is filled with the desired 
liquid. At some point, however, an air bubble 15 (see FIG. 2) may enter 
tube 14 and flow along tube 14 until it is positioned between conductive 
surfaces 22 and 24 of capacitor 20. When this happens, the capacitance of 
capacitor 20 changes, along with the resonant frequency of capacitor 20 
and inductor 30. 
For example, when an air bubble is positioned in tube 14 between conductive 
surfaces 22 and 24 of capacitor 20, the capacitance of capacitor 20 may 
drop to approximately 1.25 picofarads. Using equation (1), one calculates 
that the resonant frequency of capacitor 20 and inductor 30 is now equal 
to approximately 2.0 MHz. Since this resonant frequency is now 
significantly different from the oscillating frequency of resonator 40 
(1.0 MHz), detector device 10 will no longer produce an output voltage 17 
which oscillates. Thus, by appropriately monitoring output voltage 17, the 
presence of an air bubble 15 between conductive surfaces 22 and 24 of 
capacitor 20 may be readily detected. 
Advantageously, the output voltage 17 of detector device 10 may be 
connected to conventional electrical circuitry which will activate some 
type of alarm whenever output voltage 17 stops oscillating. Such circuitry 
is, for example, commonly used in medical devices for monitoring a 
patient's heart. The alarm will thus promptly notify the medical attendant 
that an air bubble or some other undesirable fluid is now located in tube 
14 so that the patient can get the needed attention. 
From the above discussion, it will be appreciated that the present 
invention provides a simple device which can readily distinguish between 
liquids and gases in a fluid column. The detector device of the present 
invention operates on electrical principles, and its operation is 
independent of the light transmission properties of the fluid in the 
column. Moreover, the detector device of the present invention uses 
readily available electrical components which can be quickly and easily 
assembled into a lightweight, compact unit which is very reliable. The 
present invention thus provides a device for distinguishing between 
different fluids which is inexpensive to manufacture and reliable in 
operation. Further, as discussed above, the detector device of the present 
invention can readily be used for medical applications to rapidly detect 
the presence of air bubbles in a column of virtually any liquid. 
The invention may be embodied in other specific forms without departing 
from its spirit or essential characteristics. The described embodiments 
are to be considered in all respects only as illustrative and not 
restrictive. The scope of the invention is, therefore, indicated by the 
appended claims, rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope.