Fluid presence and qualitative measurements by transient immitivity response
An apparatus and method for obtaining a measurement of various qualities of an electrochemical cell (12). The apparatus includes first (9) and second (10) electrodes and an excitation source (8) for providing a time varying excitation voltage to the first electrode (9). The excitation voltage (8) is switched between two voltage levels with the first and second voltages alternately applied to the first electrode for predetermined times. An external capacitance (Cout) is connected between the second electrode (10) and ground. The apparatus is capable of determining the time related rates at which electrical charge is transferred from the first electrode (9) to charge the external capacitance (Cout). These rates, here termed Transient Immitivity Response (TIR), may be provided as a digital or analog output (11).
The present invention relates to a process for measuring the presence and various qualities of fluids, and materials containing fluids. More specifically, the present invention describes a process for detecting minute compositional changes in single sampling or continuous flow monitoring of fluids which offers extreme sensitivity, simplified temperature compensation, probe design, materials and control electronics.
BACKGROUND OF THE INVENTIONA myriad of fluids are used in many scientific and industrial processes, as well as in end user applications. Initial, in-process and in-use testing of these fluids can often help prevent potential problems. Many processes rely on precise mixtures of fluids, slurries, suspensions or wetted materials and require accurate feedback on the resultant mixtures. End users often depend on accurate compositions of fluids, slurries, suspensions or wetted materials for safe and efficient use. Qualitative measurement of these materials can often prevent costly mistakes, damage or injury.
Electronic analysis of fluid compositions has historically been complicated by the fact that generally any such fluid has a dielectric constant, conductance and double-layer effects, each of which produces complex electrical responses. While measurements of these qualities are commonplace, they are plagued with instrumental difficulties such as probe design, erratic temperature dependencies and complex control electronics in the effort to get accurate and sensitive results.
In-use or in-process controls often require sensors capable of properly handling varying levels of flow, pressure and temperature while accurately measuring compositional changes. Current methods of measuring the dielectric constant or conductance of a fluid require either a very small range of variance in any of these effects, or extreme and technically complex compensations for them.
The dielectric constant of fluids is a common qualitative measure associated with fluids. It is known that the dielectric constant in solids is a measure of the ability of molecules to polarize or shift their internal charges in response to external fields. In fluids, the molecules are also able to move about, rotating to orient in a field and/or migrating within the fluid. In electronic terms, the dielectric constant is the analog of a capacitor.
Many patents exist that are directed to measuring the capacitance of fluids. U.S. Pat. Nos. 4,132,944, 5,497,753 and 5,507,178 are representative of capacitance-measuring techniques.
Conductivity (the reciprocal of electrical resistance) is another common measure used to produce a qualitative indication of fluid compositions and charged species in a fluid. Charged species, or ions, present in a fluid provide a means for the passage of electrons through a fluid. The more ions present, the lower the electrical resistance of the fluid and the greater the magnitude of current that can flow through the fluid. In electronic terms this phenomenon is the analog of a resistance.
Numerous patents have been directed to fluid conductivity measurements, including U.S. Pat. Nos. 4,132,944, 4,634,982, 6,169,394 and 6,232,783, all representative of conductivity based applications.
Both of the above-described measures are greatly affected by temperature and other influences. In many cases, the precise theory behind these wide variances is not directly known or reliably predicted and varies considerably dependent on composition.
Measurements of conductivity and dielectric properties together have been performed in the past in efforts to simplify and solve many of the problems highlighted above. U.S. Pat. Nos. 4,516,077 and 6,169,394 are representative of this approach. In the latter patent, complex measurements were made of the electrical impedance of a fluid (i.e., the effect of a parallel resistance and capacitance). Unfortunately, this invention used complex electronics in generating a wide range of excitation frequencies, while variances such as temperature dependencies were not addressed.
In U.S. Pat. No. 4,516,077, a sensor is described which is useful in a limited number of solvent solutions including water, alcohols and glycols. This invention included a method of electronically charging a fluid, disconnecting the charging means, and then measuring the time necessary for the charge across the fluid to dissipate (termed the “intrinsic time constant”). This invention essentially measures the re-diffusion rate of the polarization and electrical charges as they return to equilibrium devoid of any external electrical influences and is greatly affected by temperature and fluid flow rates.
The measurement of any fluid quality is complicated by the electrode-fluid interface. Each such interface includes its own resistance and capacitance, which are known to often be larger than those of the fluid itself. Electrochemical reactions caused by the introduction of an electrical current into a fluid can cause electrode corrosion and contamination. Sensed voltages or currents often need amplification and signal conditioning to provide suitable readings. These, and other problems, have seldom been addressed in previous inventions.
Therefore, it is desirable to develop an invention that uses the electrical qualities of the fluid to provide the primary measure while avoiding the above-described complications.
BRIEF SUMMARY OF THE INVENTIONThe present invention relates to a process for measuring the presence and various qualities of fluids and materials containing fluids. It offers improved performance over previous methods in its range and sensitivity, as well as relative insensitivity to temperature and fluid flow. In addition, this process offers simplified design and measurement.
The present invention includes a process and an apparatus for controlling and measuring various electrochemical effects of simplified electrochemical cells. However, the underlying effects measured are complex in nature. The present invention controls some of the individual influences of those effects to derive a measurement that has advantages over previous techniques and is termed here Transient Immitivity Response (TIR).
The primary feature of the invention is the use of a capacitance external to the cell to accumulate, control and limit the electrical currents passing through the cell. Transient immitivity response refers to the interactions between this capacitance and the current transfer mechanisms within the electrochemical cell. These interactions create a complex rate of electrical charging and discharging of this external capacitance that can be measured in many different ways. This capacitance, the cell configuration and other external components may be adjusted to enhance or reduce the effect of various charge transfer mechanisms and to fit the invention to virtually any fluid. The transient immitivity response is the time related complex rate at which charge is passed through the cell and accumulated on the external capacitance.
One embodiment according to the invention includes two electrodes spaced apart from each other and both in contact with a fluid being tested. This embodiment includes an excitation source for providing a time-varying excitation voltage to a first one of the electrodes. The excitation voltage is switched between a first defined voltage level and a distinct second defined voltage level. The first and second voltage levels are alternatively applied to the first electrode for specific time periods. This source has a low source resistance such that it is able to supply sufficient electrical current to change the first electrode's electrical potential in a minimal time and thereby rapidly charge the first electrode's capacitance.
According to the invention, a defined capacitance is located between the second electrode and an electrical or circuit ground. The ground has a defined voltage. This embodiment also includes a voltage detector for detecting a sensed voltage induced on the defined capacitance. The sensed voltage is proportional to electrical charges conducted through the fluid from the first electrode to the second electrode as a consequence of the excitation voltage applied to the first electrode. This voltage detector has a very high resistance to electrical ground such that there is no substantial current flow through it from the cell. Examples of suitable voltage detectors include current generation FET transistors, op amps and CMOS logic circuits having input resistances greater then 1011 ohms.
In this embodiment, the voltage level at the excitation source is held constant at least until the cell is at equilibrium when a fluid is present. If there is no fluid present, no voltage will be detected at the sensing or detecting means. If a fluid is present at equilibrium, all portions of the electrochemical cell of the embodiment will be at essentially the same voltage as the excitation voltage and the voltage sensed at the second electrode will be essentially equal to the voltage at the first electrode. The excitation voltage of the first means is then switched to a second voltage level. The cell will now work to come to equilibrium at this second voltage level.
The embodiment further includes a means for determining one or more time intervals between the switch in first and second defined voltage levels and when a sensed voltage at the capacitance attains one or more selected voltage levels. These time intervals represent the transient immitivity response of the fluid. Alternately, this means may measure the voltage attained at the capacitance at one or more predetermined time intervals after the switch in first and second defined voltage levels. Once again, providing a measure of the Δvoltage/Δtime nature of the transient immitivity response. The voltage level attained at the second electrode is a time-related function of all of the resistances and capacitances of the electrode interface and the fluid, and the change in voltage of the first excitation source. This embodiment is further capable of providing the transient immitivity response as a digital or analog output. A lack of a changing sensed voltage may indicate a lack of fluid between the electrodes. While this single time, or rate, measurement embodies the basis for the present invention, two or more measurements of the time-related response of this electrochemical cell system may be used to elucidate more subtle information.
The present invention is also a method of using an apparatus to obtain a transient immitivity response of a fluid. Initially, first and second electrodes are selected and the electrodes, spaced apart from each other, are brought into contact with a fluid. Time varying excitation voltage is then applied to the first electrode. The excitation voltage is subsequently switched between a first defined voltage level and a distinct second defined voltage level so that the first and second defined levels are alternately applied to the first electrode for specific time periods. The excitation source is further characterized by having a low resistance in order for a minimal switch time to exist when the excitation voltage is switched between the first and second defined voltage levels.
The method further includes providing a defined capacitance between the second electrode and an electrical or circuit ground. The ground has a defined voltage. A sense voltage is then detected as having been induced on the capacitance, the sense voltage being proportional to electrical charges conducted through the fluid from the first electrode to the second electrode as a consequence of the excitation voltage applied to the first electrode. The detector used is preferably characterized by having a high input resistance to minimize external current flows.
Following detection of a sense voltage induced on the capacitances, one or more time intervals are determined between the switch between first and second defined voltage levels and when the sense voltage at the second electrode attains one or more specific voltage levels. Alternately, one or more voltage levels attained at predetermined time intervals from the time the excitation voltage is switched between a first and second defined voltage level may be measured. These time intervals and voltage levels represent the transient immitivity response of the fluid and may be subsequently provided as digital or analog output.
It is known that the above-described resistances and capacitances are themselves functions of the fluid under test, fluid flow, temperature, electric potentials and other effects. The particular combinations of these effects as measured by the present invention can produce reduced dependence on flow and potential as well as reducing the variance in temperature dependencies caused by fluid composition.
It is therefore an object of the present invention to provide a fluid sensor which overcomes many of the limitations of the prior art.
It is another object of the present invention to provide a sensor for the presence of a variety of fluids and fluid bearing materials.
It is a further object of the present invention to provide a sensor which can qualitatively measure the difference between various solvents and fluid compositions.
Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGThe previously stated features and advantages of the present invention will be apparent from the following detailed description as illustrated in the accompanying drawings wherein like reference numerals throughout the various figures denote like structural elements, and in which:
FIGS. 8A-D are graphs showing the results of a comparison study of the temperature characteristics of various known methods and the present invention on distilled water.
FIGS. 9A-D are graphs showing the results of a comparison study of the temperature characteristics of various known methods and the present invention on tap water.
FIGS. 10A-D are graphs showing the results in
Still referring to
In FIGS. 2A-C, steady state, or direct current, voltage is blocked by the capacitor C. A rapid change in the excitation voltage, however, causes an indirect current. This is the result of the electrostatic fields within the capacitor C—when a charge is placed on one side, the charges on the other side reorganize to produce a charge equal to the excitation charge, but opposite in polarity. This results in a momentary, immediate current flow on the sensing side as the charges of the same polarity as the excitation voltage rush out and are replaced by charges of the opposite polarity.
where: C=capacitance, t=time, Vin=change in excitation voltage.
As this equation shows, the pulse height is dependent on how quickly the excitation voltage (Vin) changes. Very fast rising voltages will produce a pulse height that is equal to the excitation voltage change, but never more. The amount of charge contained in a capacitor is related to the voltage present across it and its capacitance as:
q=CV
where: q=charge, C=capacitance, V=voltage across the capacitor.
The amount of current present in the resultant pulse on the sense side of the capacitor C is equal to the change in the charge of the capacitor C caused by the change in the excitation voltage. Since the present invention uses an input amplifier with a large, yet finite, input resistance, the charge will be drained through that resistance. If there were no path for current to flow from the sensing side, the voltage would remain equal to the excitation voltage as the capacitor C has reached an electrostatic equilibrium. If a lower resistance to ground is placed on the sense side, the pulse will shorten in width, as this charge is given a lower resistance path to ground and the charge is drained more quickly. For a very fast rising excitation voltage (time for excitation voltage change<Δt), this pulse shape will be equal to:
where: R=resistance to ground sensing side, C=capacitance, =Euler's number (the base of a natural logarithm), and t=time increment.
This equation is the same as for discharging a capacitance, with good reason. The amount of charge ‘stored’ by a capacitor is the same as that absorbed from the excitation source and the same as that released in this indirect current. The capacitor does not actually store any net charge—it maintains a separation of charges. The current required to ‘charge’ the capacitor is actually transferred to its other side. In the process, a separation of charges is built up and maintained within the capacitor until the charges are allowed to recombine when the capacitor is discharged.
The indirect current passed through the parallel capacitor Cp once again causes an immediate rise to the full excitation voltage. The current through the parallel resistance Rp will maintain that voltage while also discharging the capacitor Cp, and the sensed waveform 5 will be as seen in
By comparison,
While the output voltage is less than the input voltage, the two capacitors Cp and Cout will continue to draw current through the admittance resistance Rp in order to charge the grounded capacitance Cout and discharge the admittance capacitance Cp to the input voltage level. These are complementary processes and are described by:
In practice, after the considerations for the initial, immediate, indirect current pass-through of the admittance capacitance Cp, the two capacitances Cp and Cout can be considered as two parallel charging capacitances. Parallel capacitances can be summed together in order to find their combined influence, thus:
Combining the above equation with that for the current sharing of the indirect current through the admittance capacitor Cp, and putting it in terms of the grounded capacitor Cout we get:
This equation gives an accurate description of the output voltage from the circuit represented in FIGS. 4A-C in the special case of a rapid and discrete change in Vin (time for excitation voltage change<Δt) from 0 Volts to Vin. The output waveform shown in
One notable feature of this circuit is that the effect of the capacitance of Cp is more prominent in the term for the indirect current than for the charging current. In other words, Cp affects the output voltage rise time more by shortening it through the indirect current passed through than by the lengthening of the rise time through discharging through the parallel resistance Rp. The reason is that the indirect current is passed through immediately where the charge/discharge current is time-related. This reverses the expected effect of the admittance capacitance of Cp—a larger value actually shortens the overall rise time of the circuit, governed by:
The admittance circuit shown in
Rp=Ree+Rf+Rse
and:
When used in the equations above, this gives a good first approximation and simplified description of the actions of this circuit. A further interesting point can be seen from the equations above—that the smaller capacitance of the three admittance capacitors will have the greatest effect on the circuit. This is important because the fluid capacitance Cf will almost always be much smaller than the capacitance of the electrode interfaces Cee and Cse. This stems quite simply from the geometry of the probe used, where the capacitance can be calculated from the general equation for a simple flat plate capacitor:
where: ε=overall permitivity, A=area of each plate, and D=distance between plates.
From the above, the greater the distance between the plates, the smaller the capacitance. At the electrode interfaces A and C of
If there is little or no path to ground on the sensing side, i.e. when the sensing amplifier impedance is very high, the only currents that pass through the admittance are those required to discharge the admittance capacitance Cp and charge the output capacitance Cout. This limits the amount of current drawn through the fluid thereby reducing the possibilities of chemical changes on the surfaces of the electrodes and in the fluid. These effects can be further reduced by using a bipolar excitation voltage and/or having the excitation voltage connected only when a measurement is made.
Also, when using a high input resistance input amplifier, the sensed voltage will be near or equal to the input voltage when the cell is at equilibrium, so the sensed voltage will be as large as that input. This means that little or no signal conditioning or amplification will be needed. Adding a smaller resistance to ground at the input amplifier will decrease the sensed voltage and cause additional currents to constantly flow through the cell.
The present invention measures the time it takes for the sensed voltage to reach a particular voltage or the voltage reached at a particular time. While any voltage level could be used for the former, using a level that is 0 volts for a bipolar excitation, or half the excitation voltage for an excitation that runs to and from ground, can make the design simpler and help reduce the effects of electrical noise. This time interval, or voltage, gives a single measurement of the complex effects described herein. Such measurements are well known and well suited for digital circuitry or conversion to analog signals.
Most fluids have a predictable reaction to increased temperatures, both the fluid resistance and capacitance decreasing at differing, and often non-linear, rates. Other methods can require complex compensations to account for these changes, particularly when the fluid composition is subject to change. In the present invention, the various design elements, probe configuration and input capacitance, can be adjusted so that the system ‘self-compensates’ for many of the temperature changes. If the capacitance of the fluid goes down with increased temperature, it can increase the transient immitivity response, whereas a decrease of fluid resistance will work to decrease the transient immitivity response.
Specifically,
More importantly,
FIGS. 8A-D, 9A-D and 10A-D show comparisons of the present invention, assembled as described for
Tap water was chosen as a common, complex electrolyte. In comparing the results shown in FIGS. 8A-D and FIGS. 9A-D, it is clear that each method, excluding that of the present invention, has a response to temperature variations that depends in varying degree to the fluid composition, and thus a different characteristic curve for each. In FIGS. 8A-D and 9A-D the present invention has a characteristic temperature response that is very similar.
In FIGS. 10A-D, these differences are further brought out by graphing the response to distilled water against that for tap water for each method.
For accurate use of any qualitative fluid sensing system, temperature compensation is required. These graphs show that the three known methods, conductivity, capacitance and intrinsic time constant, would also require some knowledge of the fluid composition in order to effect an accurate temperature compensation. However, for the present invention, a single compensation, requiring knowledge of the temperature alone, would be useable over a wide range of temperatures and fluid compositions.
Vout=Vin−(IR
While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations, and omissions may be made without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only and should not limit the scope of the invention set forth in the following claims.
Claims
1-26. (canceled)
27. An apparatus for obtaining a transient immitivity response for a fluid, comprising:
- a first electrode and a second electrode, the first and second electrodes being spaced apart from each other and both in contact with the fluid;
- an excitation source for providing a time varying excitation voltage to the first electrode, the excitation voltage being capable of being switched between a first defined voltage level and a distinct second defined voltage level, the first and second voltage levels being alternately applied to the first electrode for specific time periods;
- a capacitance located between the second electrode and an electrical or circuit ground, the ground having a defined ground voltage;
- a voltage detector for detecting a sensed voltage at a predetermined location with respect to the apparatus, the sensed voltage being proportional to electrical charges conducted through the fluid from the first electrode to the second electrode as a consequence of the excitation voltage applied to the first electrode, and the voltage detector capable of determining the sensed voltage at one or more time intervals; and
- a timing device for determining one or more time intervals between the switch in first and second defined voltage levels wherein the one or more time intervals can be correlated with the sensed voltage at each interval and each sensed voltage per time interval measures a rate representing the transient immitivity response.
28. A method for obtaining a transient immitivity response, comprising the steps of:
- a) contacting a fluid with a first electrode and a second electrode, the first and second electrodes spaced apart from each other;
- b) applying an excitation voltage to the first electrode and switching the excitation voltage between a first defined voltage level and a distinct second defined voltage level, the first and second defined voltage levels being alternately applied to the first electrode for specific time periods;
- c) providing a capacitance between the second electrode and an electrical or circuit ground, the ground having a defined ground voltage;
- d) detecting a sensed voltage resulting from the excitation voltage applied to the first electrode by means of a series resistance wherein the sensed voltage is proportional to the electrical charges conducted through the fluid from the first electrode to the second electrode as a consequence of the excitation voltage applied to the first electrode;
- e) determining one or more rates of change in the sensed voltage per time interval, such rate or rates representing the transient immitivity response of the fluid; and
- f) providing the measured rate or rates representing the transient immitivity response as a digital or analog output.
Type: Application
Filed: Nov 18, 2002
Publication Date: Jul 7, 2005
Inventors: Allen Sampson (St. Charles, IL), Robert Davis (Hinsdale, IL)
Application Number: 10/506,113