Method and apparatus for determining the salinity of a sample

An apparatus for determining the salinity of an ionic sample consisting essentially of: a single current probe having a central aperture large enough to accommodate a tube that contains the ionic sample; a network analyzer electrically coupled to the current probe, wherein the network analyzer is configured to transmit power into the ionic sample when the tube containing the ionic sample is positioned within the central aperture and then further configured to measure the return loss parameter of a signal voltage waveform reflected from the ionic solution; and a reference table of reference sample properties to which the measured return loss parameter may be compared to determine the level of salinity of the ionic sample.

BACKGROUND OF THE INVENTION

Previous methods for measuring the salinity of an aqueous solution involved immersing voltage probe electrodes (i.e., two inductive coils or two ring transformers using toroids that were separated from each other by a fixed distance) in the aqueous solution. In the previous methods, an alternating current (I) is applied to the two measurement electrodes or coils to determine the voltage (V) and resulting resistance (R) using Ohm's law and the conductance is defined to be 1/R. The salinity of the aqueous solution was then determined from the conductance. A new method is needed to determine the salinity of a substance.

SUMMARY

Disclosed herein is a method and apparatus for determining the salinity of an ionic sample. The apparatus consists essentially of the following elements: a single current probe, a network analyzer, and a reference table of reference sample properties. The single current probe has a central aperture large enough to accommodate a tube that contains the ionic sample. The network analyzer is electrically coupled to the current probe and is configured to transmit power into the ionic sample when the tube containing the ionic sample is positioned within the central aperture. The network analyzer is also configured to measure a return loss parameter of a signal voltage waveform reflected from the ionic solution. The reference table is used to determine the level of salinity of the ionic sample by finding the reference sample whose properties most closely match the measured return loss parameter.

The method for determining the salinity of a sample may be described as comprising the following eight steps. The first step provides for providing a toroidal current probe having a central aperture. The second step provides for positioning a tube within the central aperture such that the tube extends through and substantially fills the central aperture. The third step provides for electrically coupling the current probe to a network analyzer with a radio frequency (RF) cable. The fourth step provides for measuring with the network analyzer the return loss of the combination of the RF cable, the current probe, and the tube when nothing but air occupies the tube. The fifth step provides for filling the tube in turn with reference samples having known salt concentrations, and measuring with the network analyzer the return loss for each reference sample. The sixth step provides for populating a reference table of return loss values with the measured return losses from the fourth and fifth steps. The seventh step provides for filling the tube with a test sample having an unknown salinity and measuring the return loss of the test sample. The eighth step provides for comparing the measured return loss of the test sample with the return loss values in the reference table to find the closest match and assigning the corresponding salinity concentration of the closest match to the test sample.

Alternatively, the method for determining the salinity of a sample may be described as comprising the following eight steps. The first step provides for providing a toroidal current probe having a central aperture. The second step provides for positioning a non-conductive tube within the central aperture such that the tube extends through and substantially fills the central aperture. The third step provides for electrically coupling the current probe to a network analyzer with a radio frequency (RF) cable. The fourth step provides for measuring with the network analyzer the return loss of the combination of the RF cable, the current probe, and the tube when nothing but air occupies the tube to calibrate the network analyzer. The fifth step provides for populating a reference table with resonance frequency, bandwidth, and amplitude properties for a plurality of reference aqueous solutions. The sixth step provides for filling the tube with an aqueous solution test sample having an unknown salinity and measuring the return loss of the test sample. The seventh step provides for obtaining resonance frequency, bandwidth, and amplitude properties of the test sample based on the measured return loss. The eighth step provides for comparing the properties of the test sample with the properties in the reference table to find the closest match aqueous solution and assigning a corresponding salinity concentration of the closest match aqueous solution to the test sample.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1is an illustration of a salinity testing apparatus10for determining the salinity of an ionic sample12. The salinity tester10comprises, consists of, or consists essentially of a single current probe14, a network analyzer16, and a reference table18. The current probe14comprises a central aperture20, which is large enough to accommodate a tube22that contains the ionic sample12. The network analyzer16is electrically coupled to the current probe14with a radio frequency (RF) cable24. The network analyzer16is configured to transmit power into the ionic sample12when the tube22is positioned within the central aperture20and to measure a return loss parameter of a signal voltage waveform reflected from the ionic solution12. The measured return loss parameter may then be compared to the reference table18to determine the level of salinity of the ionic sample12.

The ionic sample12may be any liquid, soil, or sediment. Suitable examples of the ionic sample12include, but are not limited to, sea water, urine, brine, and ocean sediment. The sample12is held in the tube22. The tube22may be any container capable of holding the sample12and fitting within the central aperture20of the current probe14. One example of a suitable embodiment of the tube22is a sealed polyethylene container where the salinity tester10may be used to determine the salinity of the sample12within the tube22without breaking the seal or penetrating the sealed tube in any fashion. Another example of a suitable example of the tube22is a polyethylene pipe where the sample12is a fluid that flows through the pipe. In the pipe embodiment, the salinity tester10may be used to determine the salinity of the sample12within the tube22without any part of the salinity tester10coming into physical contact with the sample12. The tube22may be made of metal provided the tube22is open on one end. In another embodiment, the tube22may be a test tube.

The current probe14may be any toroidal current transformer having a single coil and having any desired size and shape. A suitable example of the current probe14is the current injection device disclosed in U.S. Pat. No. 6,492,956 to Fischer et al., which is incorporated herein by reference. The current probe14may have a solid, ferromagnetic, toroidal core or the core may be split into two or more sections to allow it to be clamped around the tube22without cutting into or penetrating the tube22. The network analyzer16may be any vector network analyzer or performance network analyzer. For example, a suitable embodiment of the network analyzer16is a model 8753ES, 2-port network analyzer made by Agilent Technologies®.

The reference table18may be any look-up table comprising properties of reference samples. For example, the reference table18may be a database stored on a computer. The reference table18may store measured return loss values from various known reference samples. The reference table18may also comprise resonance frequency, bandwidth, and amplitude information pertaining to each measurement of a reference sample. For example, the network analyzer16may measure the return loss of each reference sample from 2 MHz to 1600 MHz. The reference table18may also comprise reference sample properties from other sources such as industry literature. For example, a reference table18of salt concentrations was developed using the following reference samples at multiple known salt concentrations: reverse osmosis water, tap water, drinking water. Each reference sample was measured at each of the following salt concentrations: 0.62, 1.23, 2.46, 9.86, 14.79, 19.72, 24.64, and 29.57 milliliters (0.125, 0.25, 0.5, 2.0, 3.0, 4.0, 5.0, and 6.0 teaspoons) of salt per liter of reference sample.

FIG. 2is a flowchart which depicts one example method26of how the salinity tester10may be used to determine the salinity of the sample12. The first step26ainvolves providing the current probe12having the central aperture20. The next step26bprovides for positioning the tube22within the central aperture20such that the tube22extends through and substantially fills the central aperture20. The next step26cprovides for electrically coupling the current probe14to the network analyzer16with the RF cable24. The next step26dprovides for measuring with the network analyzer16the return loss of the combination of the RF cable24, the current probe14, and the tube22when nothing but air occupies the tube22. The next step26eprovides for filling the tube22in turn with reference samples having known salt concentrations, and measuring with the network analyzer16the return loss for each reference sample. The next step26fprovides for populating the reference table18with the measured return losses from steps26dand26e. The next step26gprovides for filling the tube22with the test sample12having an unknown salinity and measuring the return loss of the test sample12. The next step26hprovides for comparing the measured return loss of the test sample12with the return loss values in the reference table18to find the closest match and assigning the corresponding salinity concentration of the closest match to the test sample12. The salinity tester10may be used to determine the salt content in the sample12without immersing the current probe14in the sample12and without the current probe14contacting the sample12.

The following is a description of an example embodiment of the method described above where the test sample12is a urine sample. In this embodiment, the network analyzer16was set to perform return loss measurements and the salinity tester10was calibrated with a mechanical cal-kit from 2 MHz-1600 MHz by sequentially measuring calibrated open, short, and load terminal modules. Next, the RF cable24was connected to the current probe14. Then the return loss measurement was taken with the network analyzer16while the tube22was in the central aperture20and while the tube22was filled with an aqueous solution of 4.93 milliliters (1 teaspoon) of salt dissolved in one liter of drinking water. This last step was repeated for different calibrated plain salt concentrations such as 0.62, 1.23, 2.46, 9.86, 14.79, 19.72 . . . etc. milliliters (0.125, 0.25, 0.5, 2, 3, 4 . . . etc. teaspoons) to develop the reference table18. Next the urine sample was placed in the tube22and the return loss measurement was performed. Then a correlation analysis of the urine sample was performed with the reference table18to determine the salt concentration of the urine sample.

Urine comprises dissolved ions of sodium Na+ and chloride Cl−. The Na+ and Cl− ions in a urine sample solution are disassociated and charged respectively. In the urine sample embodiment described above, the tube22was a sealed polyethylene container, which was inserted into the central aperture20of the current probe14and subjected to radio frequency transmission from the network analyzer16for return loss measurement. Positively charged ions will respond differently to an alternating electric field than negatively charged ions. The positively charged sodium ions move toward the negatively charged side of the electric field during the negative cycle. Negatively charged chloride ions move toward the positive side of the electric field during the positive cycle. Because these ions are charged and moving in the aqueous solution with the polarity of the alternating electric field set up by the current probe and the network analyzer, they constitute a specific impedance. This specific impedance of the sodium in the urine solution can be derived from the network analyzer scattering parameter measurement.

Reflections of a transmit signal can occur at an impedance mismatch. The ratio of the amplitude of the reflected wave Vrto the amplitude of the incident wave Viis known as the reflection coefficient Γ. The reflection coefficient Γ is shown below in Equation 1.

Γ=VrViEq.⁢1
When the source ZSand load ZLimpedances are known values, the reflection coefficient Γ may be given by the following equation:

Γ=ZL-ZSZL+ZSEq.⁢2
In Equation 2, ZSis the impedance toward the source and ZLis the impedance toward the load. Return loss is the negative of the magnitude of the reflection coefficient Γ in decibels dB. Since power is proportional to the square of the voltage, return loss may be given by the following equation:
RL(dB)=−20 log10|Γ|  Eq. 3
According to Equation 3, a good impedance match between the source and the load may be accomplished if the reflected power is small in comparison to the incident power—such as would be indicated by a large, positive return loss value.

When the network analyzer16is set to perform return loss measurements, the network analyzer16transmits power to the sample12via the current probe14and measures the reflected power from the sample12. Return loss measurements may be taken by the network analyzer16of many different reference samples, each sample having a unique known concentration of sodium and chloride ions. The reference table18may be populated with the resulting return loss measurements. The incident power to the reflected power return loss measurements of the reference samples may be used to obtain corresponding resonance frequency, bandwidth, and amplitude values for each reference sample, which values may also be stored in the reference table18. The referenced return loss measurement is a good salinity indicator of the total number of sodium ions in the sample12. Statistical correlation techniques may be used to compare the return loss measurement of the sample12with the values in the reference table18to find a closest match reference sample. The properties and/or identity of the closest matched reference sample may then be ascribed to the sample12. This method of using the salinity tester10is not limited to sodium chloride in urine analysis, but can also be used for any ionic sample conductivity measurement. For example, potassium K+ in electrolyte solution has different resonance and amplitude than sodium Na+. Accordingly, the salinity tester10may also be used to determine the level of potassium in a given sample12.

FIG. 3is a plot of measured return loss values from 2 to 1600 MHz for several conditions of the salinity tester10. A line trace for each condition is shown separately inFIGS. 4A through 12B. For all the conditions measured and depicted inFIGS. 3-12B, the following experimental setup was used. The tube22was a polyethylene tube having a length of 14.2875 cm (5.625 inches) and an inner diameter of 2.8575 cm (1.125 inches). The current probe14was a UHF2 420-450 MHz current probe manufactured by Fischer Custom Communications, Inc. having an inner diameter of 3.175 cm (1.25 inches), an outer diameter of 9.2075 cm (3.625 inches), and a height of 7.62 (3.0 inches). The network analyzer was a Site Master S312D Cable and Antenna Analyzer from Anritsu®. The network analyzer16was connected to the current probe14with a 91.44 cm (3 feet) RF cable from Cable X-perts, Inc.

Line trace28, as shown inFIGS. 3 and 4A, represents the measured return loss of the salinity tester10with nothing but air in the central aperture20of the current probe14. Line trace30, as shown inFIGS. 3 and 4B, represents the measured return loss of the salinity tester10with an empty tube22positioned in the central aperture20. Line trace32, as shown inFIGS. 3 and 5A, represents the measured return loss of the salinity tester10with the tube22—positioned in the central aperture20—filled with reverse osmosis (RO) water. RO water is more saturated with salts and oxygen than distilled water. Line trace34, as shown inFIGS. 3 and 5B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with tap water. Line trace36, as shown inFIGS. 3 and 6A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with filtered, drinking water from a drinking fountain. Line trace38, as shown inFIGS. 3 and 6B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 0.62 milliliters (0.125 teaspoons) of salt. Line trace40, as shown inFIGS. 3 and 7A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 1.23 milliliters (0.25 teaspoons) of salt. Line trace42, as shown inFIGS. 3 and 7B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 2.46 milliliters (0.5 teaspoons) of salt. Line trace44, as shown inFIGS. 3 and 8A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 4.93 milliliters (1.0 teaspoons) of salt. Line trace46, as shown inFIGS. 3 and 8B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 9.86 milliliters (2.0 teaspoons) of salt. Line trace48, as shown inFIGS. 3 and 9A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 14.79 milliliters (3.0 teaspoons) of salt. Line trace50, as shown inFIGS. 3 and 9B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 19.72 milliliters (4.0 teaspoons) of salt. Line trace52, as shown inFIGS. 3 and 10A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 24.64 milliliters (5.0 teaspoons) of salt. Line trace54, as shown inFIGS. 3 and 10B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with the reference solution of drinking water and 29.57 milliliters (6.0 teaspoons) of salt. Line trace56, as shown inFIGS. 3 and 11A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with a test sample12of brine. Line trace58, as shown inFIGS. 3 and 11B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with a test sample12of ocean water from Imperial Beach in San Diego, Calif. Line trace60, as shown inFIGS. 3 and 12A, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with a test sample12of urine collected on a first day. Line trace62, as shown inFIGS. 3 and 12B, represents the measured return loss of the salinity tester10with the tube22in the central aperture20filled with a test sample12of urine collected on a second day.

FIG. 13is a table showing the results of a correlation analysis. A correlation analysis may be performed on the return loss measurement results to determine the sodium level of a given sample12. For example, Microsoft® Excel's® correlation function was used to analyze the two urine samples from different days and ocean water from San Diego Imperial Beach. The results of that analysis are given inFIG. 5. Several instances of high correlation are circled. The ocean water sample12correlated with the reference solutions containing 24.64 to 29.57 milliliters (5-6 teaspoons), which is consistent with known salt concentrations of ocean water. The urine sample12from the first day correlated with the reference solution with 9.86 milliliters (2.0 teaspoons) of salt. The urine sample12from the second day correlated with the reference solution with 4.93 milliliters (1.0 teaspoons) of salt. The urine sample12from the second day also correlated with the urine sample12from the first day.

From the above description of the method for using the salinity tester10, it is manifest that various techniques may be used for implementing the concepts of the method without departing from the scope of the claims. The described embodiments are to be considered in all respects as illustrative and not restrictive. The method/apparatus disclosed herein may be practiced in the absence of any element that is not specifically claimed and/or disclosed herein. It should also be understood that the salinity tester10is not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.