The Portable Conductivity Meter

The Portable Conductivity Meter December 11, 1998 Group T2 Kathryn Guterl Lytal Kaufman Maryam Malik Andrea Sultenfuss Portable Conductivity Meter Group T2 ABSTRACT A portable, inexpensive conductivity meter was constructed using a signal generating circuit, a wheatstone bridge, and a voltage comparator The conductivity meter was powered by two 9-volt batteries and conductivity cells were constructed from a mixture of tin and copper wire (bench wire) as well as from pure copper wire Testing numerous cells, the average optimal conductivity cell measurements that caused the constructed circuit to work effectively were determined The cell constants of these optimal cells were determined to be 3.0 + 0.16 cm-1 and 1.7 + 0.21 cm-1 for the bench and copper wire cells, respectively Using these cells, the resistance readings of NaCl solutions (0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 M) were determined It was found that the measured resistance from the constructed bench wire and copper wire conductivity cells deviated from the expected resistance readings by 165% and 51%, respectively; the average voltage error across the wheatstone bridge was 143 mV and 98 mV; the percent deviation from the expected cell constant was 84.7% and 87.3% Four urine samples with specific gravity measurements ranging from 1.0036 g/mL to 1.0257 g/mL were tested using the constructed optimal conductivity cells in conjunction with the circuit thus forming the conductivity meter The constructed bench wire cells were found to be more effective in determining the difference in concentration of the urine samples, illuminating the LED as expected in 66.7% of the trials From a regression equation for each individual cell relating the resistance reading to the concentration of the sample tested, it was found that highly concentrated urine samples deviated significantly from estimated concentration values determined from the specific gravity measurements Portable Conductivity Meter Group T2 TABLE OF CONTENTS Background Urine Analysis Conductivity Meter .4 NaCl Solutions .6 Materials and Apparatus Procedure Cell construction Testing of Cells in NaCl Solution Circuit construction .9 Testing of circuits 10 Urine Specimen Collection 10 Results 12 Testing of Cells .12 Tests of Cells and Circuit with NaCl Solutions 14 Tests of Cells and Circuit with Urine 14 Discussion 16 Testing of the Circuit 16 Optimal Cell Constant 16 Bench Wire vs Copper Wire 18 Urine Analysis 19 Error Analysis 19 Effectiveness as a Conductivity Meter 21 Appendix A 23 Appendix B 26 References 27 Portable Conductivity Meter Group T2 BACKGROUND Urine Analysis Sodium concentration in the urine is an important indicator of high sodium intake as well as dehydration People suffering from high blood pressure must monitor sodium intake throughout the day As more NaCl is consumed, the osmotic pressure of the blood increases, thus causing water to flow from the body tissues into the capillaries This increase in blood volume increases blood pressure In order to counteract this effect, the kidneys remove much of this added NaCl, which is shown by an increase of NaCl concentration in the urine Similarly, when a person is dehydrated, the kidneys reabsorb as much water as possible, increasing the concentration of NaCl in the urine (4) The volume and solute composition of urine can vary greatly depending on an individual’s diet and health Because of these variables, it is difficult to establish the specific concentration for each component of the urine In examining the composition of urine, sodium and chloride ions make up the greatest concentrations of ions in the solution Under normal conditions, there is a 0.1 M concentration of NaCl in the urine (3) However, when the urine is concentrated due to a high level of salt intake or dehydration, as described above, the concentration of NaCl rises to 0.26 M NaCl or greater Table 1: Major components of urine (3) Component Water (1.2 L) Sodium Chloride Potassium Initial ultrafiltrate (mmol) 9,500,000.00 32,500.00 37,000.00 986.00 Final urine (mmol) 67,000.00 130.00 185.00 70.00 % reabsorbed 99.3 99.6 99.5 92.9 Analysis of urine is used in the laboratory to test for various abnormalities including diabetes, dehydration, thyroid disorders, kidney malfunction, and central nervous system damage Urine is readily available and easily collected An inexpensive, portable device to test the concentration of the urine would prove to be an asset to urinalysis (6) Conductivity Meter A conductivity meter serves as an effective tool in measuring the concentration of ions in solution In an electrolyte solution, the motion of the charged particles constitutes an electric current The conductivity of an electrolyte solution is a function of the Portable Conductivity Meter Group T2 concentration of ions in the solution Because of this, each solution has a resistance that can be measured with conductivity cells Conducting sensors are constructed of an insulating material imbedded with metallic pieces The metal pieces, serving as sensing elements, are placed at a fixed distance apart and make contact with the solution The cell constant, K, for a particular conductivity cell is determined by the geometry of the cell The value of K can be estimated by the following equation: K= d A Equation where d is the distance between the sensing elements and A is the surface area of one sensing element exposed to the solution By varying K or c, the resistance varies according to the equation: R= K Λcq Equation where R is the resistance, K is the cell constant, Λ is the equivalent conductance, c is the molar concentration of the electrolyte, and q is the ionic charge We can see from this equation that as the concentration goes down, the resistance will go up rapidly (2) Equation gives an approximation for the cell constant of the conductivity cell However, for effective calculation of the resistance of a solution, the actual cell constant must be determined through a KCl standardization The conductance of KCl is known over a wide range of concentrations and thus, using these values of equivalent conductance, and the measured resistance of the solution with a particular conductivity cell, the actual cell constants can be determined These cell constants, along with the measured resistance, can then be used in the calculation of the equivalent conductance of other solutions (2) When measuring the resistance of a solution it is impractical to use an ohmmeter, since passing a current through the solution will lead to errors, such as heating and polarization Using AC potential eliminates the latter problem, but an AC wheatstone bridge minimizes both of the above problems In Figure 1, Rf1 and Rf2 are fixed resistors of known resistance; Rb is a resistance box in which the resistance can be changed very precisely over a wide range; and Rm is the resistance to be measured, in this case the conductivity cell with the solution of interest Portable Conductivity Meter Group T2 Figure 1: Schematic of wheatstone bridge (2) The bridge works correctly when V2 = V4 and therefore Rm = Rb(Rf1/Rf2) If the two fixed resistors are chosen to be identical, then Rm is equal to the reading of the variable resistance box Rb (2) NaCl Solutions The expected values of equivalent conductance and resistance, assuming a cell constant of cm-1, are shown in Table The values of equivalent conductance were obtained from an extrapolation of the values given in the CRC (5) The expected resistance was determined using Equation and a cell constant of cm-1 Table 2: Expected resistance for a range of NaCl solutions with concentration values similar to that of urine (5) Concentration (M) Equivalent Conductance Exp Resistance (Ω) ( cm S/mol) 0.05 111.010 180.164 0.1 106.690 93.729 0.125 103.557 77.252 0.15 101.493 65.686 0.2 97.829 51.110 0.25 94.601 42.283 0.3 91.682 36.358 Portable Conductivity Meter Group T2 MATERIALS AND APPARATUS Wire (at bench) Breadboard and kΩ potentiometer Green light emitting diode Rectifier (diode) Resistors: 33,000 Ω (x1) 10,000 Ω (x4) & 1,000 Ω (x1) 0.01 μF capacitor LM741 operational amplifier (x2) Electrical tape Laboratory wheatstone bridge Deionized water volt battery (x2) Ruler 0.01 M KCl solutions 0.05 – 0.3 M NaCl solutions Deionized water Beakers Pipettes Scale LabView Voltmeter (x2) Variable Resistance Box Model RSU-280 Alligator clips Copper wires Mettler Toledo AB54 Balance Specific gravity bottle Caliper Portable Conductivity Meter Group T2 PROCEDURE Cell Construction Conductivity cells were constructed using wire made from a mixture of copper and tin (wire from the bioengineering laboratory bench) as well as pure copper wire The wires were attached to a firm support as shown in Figure An approximation of the cell constant was determined from the following equation: where d is the distance between the wires, r is the radius of the wire, and L is the length Equation d Cell constant (K) ≈ (2πrL + πr ) of each wire exposed to the solution Ten bench wire cells and five copper wire cells were constructed with expected cell constants ranging from 0.77 cm-1 to 83.67 cm-1 The dimensions for these conductivity cells are listed in Appendix A, Table 11 and Table 13 When using the bench wires, the plastic coating was stripped indicating the length of wire to be submerged in the solutions Electrical tape was used to cover the copper wire and indicate the length of the wire to be submerged in the solutions Figure 2: Diagram of Cell prototype Testing of Cells (in NaCl solutions) After the cells were constructed, the actual cell constants were determined in a 0.01M KCl solution using the variable resistance box as well as the pre-constructed wheatstone bridge An AC potential was used to measure the resistance of NaCl solutions The accuracy of the cells was analyzed by comparing the resistance readings from the variable resistance box to the expected readings for solutions of 0.05, 0.1, 0.15, Portable Conductivity Meter Group T2 0.2, 0.25, and 0.3 M NaCl The expected resistance readings for the NaCl solutions were determined using Equation 2, the measured cell constants, and the values of equivalent conductance (5) Circuit Construction In order to maintain the portability of the conductivity meter, an oscillator circuit that generates a kHz square wave signal from a DC signal was constructed as shown in Figure (1) An AC wheatstone bridge was used to minimize the effects of heating and polarization caused by a current This bridge was constructed as shown in Figure In order to compare the voltages across the bridge circuit, a voltage comparator circuit was constructed as shown in Figure Appendix B shows the conductivity meter circuit with all three circuits connected Figure 3: Schematic of AC signal generating circuit (1) Figure 4: Voltage comparator circuit V2 and V4 are the input voltages from the wheatstone bridge (1) Portable Conductivity Meter Group T2 Testing of Circuits Each circuit was tested individually with LabView to ensure that it was working as expected The three circuits were tested in conjunction forming the circuit of the conductivity meter For reasons explained in the discussion, it was determined to use only a green diode with a rectifier to indicate deviation from normal urine concentration The circuit was tested using a resistor in place of the solution resistance (Rm) to ensure the correct orientation of the diode and rectifier The bridge was then balanced at a resistance of kΩ using the potentiometer of kΩ in the Rb position Then various resistors were placed in the position of Rm to ensure that the diode would illuminate when a solution was tested with a lower resistance than the resistance at which the wheatstone bridge was balanced Resistors of 470 Ω, kΩ, and 3.3 kΩ were tested in the position of Rm Next, the circuit was tested with the 0.05 - 0.3 M NaCl solutions using the constructed cells However, it was discovered that the resistance readings from these solutions were too low to bring about a large enough voltage difference across the wheatstone bridge to cause the diode to illuminate as expected Therefore, a kΩ resistor was placed in series with Rm in order to increase the resistance to a detectable level enabling the wheatstone bridge to cause the voltage comparator to illuminate the diode In addition, a change from a kΩ potentiometer to a kΩ potentiometer was used in the wheatstone bridge Solutions were then tested with the constructed cells in the position of Rm to determine the optimal cell measurements required to create a cell constant large enough to cause the change in resistance measured by the wheatstone bridge to be detectable Three copper wire cells and three bench wire cells with optimal measurements were used to detect the resistance of the NaCl solutions (0.05 M to 0.3 M) From this data, the reproducibility of the results when using the constructed cells in conjunction with the conductivity meter circuit was determined Urine specimen collection During the third week of experimentation, urine samples were obtained from four random subjects A resistance reading with six constructed cells (three bench wire cells and three copper wire cells) having the determined optimal measurements were then taken for each sample This was done using the variable resistance box and the pre- 10 Portable Conductivity Meter Group T2 Table 4: Comparison of data for cells with distance between the two wires of the cell less than cm and greater than cm Distance ( cells of Avg Error across Avg Deviation from Avg Deviation from Expected both materials) Wheatstone Bridge (mV) Expected Resistance (Ω) Cell Constant (cm-1) Distance = 4cm 144 172 33.00 Table 5: Comparison of data for cells with length of exposed wire to solution less than and greater than cm Length (cells of Avg Error across Avg Deviation from Avg Deviation from Expected both materials) Wheatstone Bridge (mV) Expected Resistance (Ω) Cell Constant (cm-1) Length < 1cm 148 186 31.77 Length > 1cm 70 51 0.68 In order to effectively use the cells with the circuit, the cell constant of each cell needed to be high enough to ensure that changes in the resistance caused by the various concentrations (0.05-0.3M) of NaCl solutions were measurable The optimal cell dimensions were determined for both the bench wire and the copper wire cells These dimensions are shown in Table Table 6: Optimal cell dimensions for bench wire and copper wire cells Cell Type Radius (cm) Length (cm) Distance (cm) Bench Wire 0.035 0.30 4.75 Copper Wire 0.105 0.55 4.00 Using these dimensions, three separate bench wire and copper wire cells were assembled to test the precision of the constructed cells The cell constants were expected to be 68.06 cm-1 for the bench wire cells and 10.07 cm-1 for the copper wire cells The cell constants were determined experimentally with the use of 0.01M KCl solution, and the cells were used to determine the resistance readings of each NaCl solution using the variable resistance box Table shows the results for each cell The average actual cell constants were determined to be 3.0 + 0.16 cm-1 and 1.7 + 0.21 cm-1 for the bench wire cells and the copper wire cells, respectively Table 7: Data for trials of three different bench wire cells and three different 13 Portable Conductivity Meter Bench Bench Bench Average Copper Copper Copper Average Group T2 copper wire cells Cell Constant Avg Error across Avg Deviation from Deviation from Expected -1 (cm ) Wheatstone Bridge (mV) Expected Resistance (Ω) Cell Constant (cm-1) 2.97 249 222 65.09 2.97 224 504 65.09 3.25 170 165 64.81 3.0 + 0.16 214 297 65.00 1.42 83 36 8.65 1.83 145 86 8.24 1.69 98 58 8.38 1.7 + 0.21 109 60 8.42 Tests of Cells and Circuit with NaCl Solutions The cells with the measurements listed in Table were also used in conjunction with the circuit and NaCl solutions to determine the reproducibility of the illumination of the diode with the expected solution The maximum normal concentration of NaCl in the urine is approximately 0.25 M Therefore, using these cells, the bridge of the circuit was balanced at this concentration It was expected that placing the cells in solutions with concentrations greater than 0.25M would not cause the diode to illuminate while placing the cells in solutions with concentrations less than 0.25M would cause the diode to illuminate It was hoped that for separate cells of equal cell dimensions and therefore similar cell constant, the balanced voltage would be insignificantly different resulting in the expected illumination of the diode The above-expected results occurred for two of the three bench wire cells and for zero of the three copper wire cells Tests of Cells and Circuit with Urine Finally, the circuit and cells were tested in four urine samples of various concentrations The specific gravity of each urine sample was measured to determine if any of the samples had specific gravity values above the value for normal urine specific gravity (and therefore urine concentration) Table shows the specific gravity of the four urine samples 14 Portable Conductivity Meter Group T2 Table 8: Specific gravity of four urine samples tested with the constructed circuit and six cells with the optimal dimensions The circuit was balanced using sample #3 Sample # Specific Gravity (g/mL) (+ 0.0001) 1.0036 1.0037 1.0204 1.0257 The resistance of each urine sample was measured with each of the six cells and the use of the variable resistance box Using Excel, a regression of concentration versus resistance from the resistance measurements obtained for each individual cell when the 0.05M-0.3M NaCl solutions were tested was carried out to obtain a relationship for each individual cell The regression equations were used to determine the concentration of each urine sample from the resistance readings of each sample obtained from each individual cell Table shows the average values of concentration obtained for each urine sample The concentrations determined for samples and as well as for and are not significantly different from one another The constructed wheatstone bridge was balanced at the resistance caused by a solution with a specific gravity of 1.02 g/mL (sample 3) For sample the diode was not illuminated, while the diode was illuminated for samples and This illumination of the diode was as expected Thus, the constructed cells were able to detect a difference in the resistance cause by the various concentrations of the four urine samples Table 9: Average concentration values for four urine samples determined from each individual cell’s regression equations relating concentration to resistance reading Sample # Avg Concentration (M) Standard Deviation (M) 0.09 0.04 0.11 0.05 0.38 0.19 0.42 0.18 15 Portable Conductivity Meter Group T2 DISCUSSION Testing of the Circuit In constructing the conductivity meter circuit, many adjustments were made First, due to the high concentration of the NaCl solutions used in conjunction with the constructed circuit, the resistance readings of the solutions were not in the middle range of the potentiometer In order to place these resistance readings within range, a 1000 Ω resistor was placed in series with the conductivity cell (Rm) Adjustments were made to create a DC current from the AC current of the signal generating circuit It was previously determined as explained in the background that the AC current was optimal to power the bridge and measure the resistance; however, the AC current adversely affected the illumination of the diode To resolve this effect, a rectifier was placed in parallel before the diode to eliminate most of the negative voltage and allow positive voltage to pass This ensured that the diode illuminated when positive voltage was passed Optimal Cell Constant Cells were constructed with dimensions to create an optimal cell constant that would ensure that the circuit detected changes in the resistance caused by the various concentration range used resulting in the effective illumination of the diode However, the accuracy and precision of the cells were compromised when the optimal cells were used In order for the cells to sense the change in resistance of the solutions, the greatest cell constant possible was required Limited by the dimensions of the beakers used and the diameter of the wire, the most appropriate dimensions were given in Table 6, above However, as shown in Tables and 5, the cells with the greater cell constant (distance >= cm and length < cm) performed less accurately Assuming the distance between the two wires divided by the area of wire exposed to solutions approximates the cell constant (Equation 3), an increase in the distance would increase the cell constant When comparing the distance between the wires, as shown in Table 4, the wheatstone bridge could not be balanced as accurately and the millivolt error across the wheatstone bridge increased by 51% with the increase in the distance between the wires In addition, the average deviation from the expected solution resistance increased by 54% and the average deviation from the expected cell constant increased by 1575% 16 Portable Conductivity Meter Group T2 One can see from Equation that as the length of wire exposed to the solution decreases, the cell constant is expected to increase Indeed this did occur, but with this increase in cell constant, the accuracy and precision of the cell decreased (Table 5) In comparing the results of a decrease in the length of wire exposed to solution, the millivolt error across the wheatstone bridge read from the voltmeter increased by 112%, while the average deviation from the expected resistances increased by 265% In addition, the deviation of the actual cell constant from the expected cell constant increased by 4272% with the length decrease The greatest possible cell constant was needed to effectively measure the change in resistance in a 0.05 M change in concentration A large range of resistance was desired to ensure that the circuit would detect changes in resistance caused by various concentrations To acquire a large change in resistance for the same values of concentration, one can see from Equation that a high cell constant will raise the resistance readings Therefore, increasing the cell constant will also increase the range of resistance values read for a given set of concentrated solutions Using cells with the distance between the wires greater than cm and a length of wire exposed to solution less than cm, the resistance readings of NaCl solutions from 0.05 M – 0.3 M ranged from approximately 330 Ω to 60 Ω for the copper wire cells and from 500 Ω to 100 Ω for the bench wire cells Conversely, using cells with the distance between the wires less than cm and the length of wire exposed to solution greater than cm, the resistance readings ranged from 150 Ω to 30 Ω for the copper wire cells and from 100 Ω to 20 Ω for the bench wire cells Therefore, by increasing the distance between the wires and decreasing the length of wire exposed to the solution, the resistance range increased by 235% The most effective cell that worked in conjunction with the circuit was the bench wire cell with dimensions specified in Table This cell had a distance greater than cm and a length less than cm Furthermore, in using the cells in conjunction with the conductivity meter circuit, the error in the accuracy of the cell can be taken into account when balancing the wheatstone bridge at the resistance indicating the cutoff concentration of normal urine Therefore, although the bench wire cells with measurements listed in Table had a 17 Portable Conductivity Meter Group T2 greater error in measuring the actual resistance of each solution compared to the expected resistance, the cells were more effective in determining smaller changes in concentration Bench Wire vs Copper Wire The copper wire cells were less successful in correctly illuminating the diode than the bench wire cells when testing the cells in the NaCl solutions Although the copper wire cells were 4.9 times more accurate in determining the resistance of the solutions, they were not as effective in determining the differences in the resistance caused by the various concentration of NaCl solutions The copper wire cell constant is 1.8 times less than that of the bench wire cells, and thus, the resistance range to be detected by the circuit was less than that for the bench wire cells Because of this, the copper wire cells were unable to effectively sense this smaller change in resistance Attempting to increase the cell constant of these copper wire cells was difficult due to the limitation of the larger radius of 0.105 cm compared to the radius of the bench wire, 0.035 cm In addition, the two wires of each cell could not be placed at a large enough distance apart to raise the cell constant to that of the bench wire cells Another disadvantage to the copper wire cells was that the wire needed to be stripped using a blade, rather than wire strippers In using the bench wire to construct the cells, the wire was easily and accurately stripped indicating the length that was to be exposed to the solution This was not possible in stripping the copper wire Therefore, electrical tape was placed over the wire indicating the correct length of wire to be exposed to the solution However, this taping method was not as effective in insulating the wire With more effective tools, insulated wires could be obtained and stripped in a more effective manner creating more accurate and precise cells The size of the urine sample to be tested as well as the size of the beaker holding the sample limits the dimensions of the cells that could be constructed Ideally, a small urine sample would be sufficient to be tested, since large urine samples are more difficult to obtain A solution sample could be diluted causing a decrease in concentration and thus an increase in the resistance of the resulting solution Thus, a smaller sample container, such as a test tube, could be used because the distance between the wires could be reduced without a decrease in the resulting cell constant Many variations in the dimensions of the cell to be constructed as well as the dilution factor of the sample are 18 Portable Conductivity Meter Group T2 possible Further experimentation would be needed to determine the dimensions of a conductivity cell that would result in accurate and precise results of a diluted urine sample Urine Analysis The urine samples were greater in concentration than the solutions of NaCl This is as expected because urine consists of other ions, mainly potassium, which would affect the concentration and therefore the resistance of the solution For the trials using the urine samples, the bridge was balanced at the resistance corresponding to a urine sample at the normal specific gravity of approximately 1.02 g/mL Taking into account the presence of potassium in the urine, approximate values for the concentration of the solution can be determined from the specific gravity measurements Using the normal concentration of ions in the urine 24.3% Na+, 53.5% Cl- and 22.2% K+ and assuming that the urine concentration changes are due only to the addition or reabsorption of water, the concentrations of the urine samples were estimated Table 10 below gives the estimated urine concentration of each sample as well as the average experimental urine concentration of each sample determined from the regression equation for each cell and the resistance reading for each sample using this cell The estimated concentrations for samples and are not significantly different from the experimental concentrations found from the regression However, the estimated concentrations for samples and are significantly different from the average experimental concentration values as they fall out of the range of error in the average experimental concentrations A possible explanation attributes this to the low accuracy of the cells used The resistance of the highly concentrated urine samples (samples and 4) are lower than the resistance of the lower concentration samples (samples and 2) The error is greater when measuring low resistances as the range of resistance that can be measured is less Thus, for changes in concentration of the highly concentrated NaCl solutions, only small changes in the resistance were detected Thus, the regression equation relating concentration to resistance at higher concentrations was less accurate in predicting actual concentrations for the highly concentrated samples In addition, as stated before, the error of the cells in detecting the concentration can be taken into account when balancing the wheatstone bridge 19 Portable Conductivity Meter Group T2 Table 10: Average urine concentrations determined experimentally from the regression equations for each cell and the resistance reading of the sample as well as the estimated urine concentration found from the specific gravity measurements Sample # Avg Experimental Urine Estimated Concentration Concentration (M) from Specific Gravity (M) 0.09 + 0.04 0.1106 0.11 + 0.05 0.1137 0.38 + 0.19 0.6267 0.42 + 0.18 0.79 Error Analysis The determination of the expected cell constant and the actual cell constant contained error From the equipment available to construct the cells, it was determined that the expected cell constant contained a 40.3% error There was also error in the actual cell constant determination The cell was not fixed securely at the exact length of wire to be exposed to the solution, although attempts were made to insulate all wire not to be exposed to the solution In addition, the experimenter aimed to hold the cell the appropriate length in each solution Thus, the actual cell constant contained error due to this variation in length Furthermore, the large difference in the expected cell constants as compared to the actual cell constants can be attributed to the approximation used to calculate the expected cell constants as well as the error in making the cells described above In professionally manufactured conductivity cells used in the lab, the area used in the calculation of the cell constant is determined by the area of the metal plates facing one another However, the approximation used to determine the expected cell constant of the conductivity cells used in this experiment, Equation 3, takes into account the entire surface area of wire exposed to the solution, and not simply the surface area between the two wires A better approximation of the expected cell constant would allow for a more accurate determination of the dimensions to build an effective conductivity cell The concentration of 0.01 M KCl solution only contained an error of 0.013% affecting the value found for the actual cell constant minimally An error of 0.006% was involved in the NaCl solutions, which is negligible as well Temperature was assumed to be 25°C, and thus slight deviations in temperature should be taken into account as error in the resistance readings 20 Portable Conductivity Meter Group T2 Effectiveness as a Conductivity Meter In order for the conductivity meter to be an effective test of the concentration of urine, it is necessary that the wheatstone bridge be balanced successfully and used with many different cells of the same cell constant To test this effectiveness, the bridge was balanced at the value of resistance corresponding to a normal urine concentration using one cell of either the bench or copper wire The other two cells of bench or copper wire were tested to determine if the bridge remained balanced and the diode illuminated as expected It was found that the bridge worked consistently, illuminating the diode at the correct concentration in 66.7% of the trials for the bench wire cells Copper wire cells were less effective in determining resistance changes due to various concentrations Often, the diode remained illuminated for higher concentrations than expected A larger range in resistance for the same set of concentrations would be necessary to cause the appropriate change in voltage to cause the diode to not be illuminated when using these copper wire cells To improve the conductivity meter, several steps must be taken The cells should be constructed with more accurate measurements to minimize the error in the expected cell constant and create uniformity among actual cell constants in cells with the same dimensions In addition, more tests of actual urine samples should be conducted to determine the effectiveness of the meter’s ability to measure differences in concentration From the data collected in the three weeks of experimentation, it was determined that the setup and construction of the cells is a feasible design and has potential for an effective conductivity meter The apparatus is portable, as well as inexpensive However, more time is needed to further evaluate the circuit, cell construction, and sample testing to improve the apparatus The constructed conductivity meter is relatively inexpensive with the estimated cost of construction as $15-$20 The disposable cells are constructed of an inexpensive conducting metal and thus are not expensive The affordability of this meter and its cells is important to provide a simple and inexpensive conductivity meter to monitor the concentration of urine The constructed conductivity meter has been proposed to measure the increase in the concentration of NaCl in urine This is advantageous for patients with high blood 21 Portable Conductivity Meter Group T2 pressure or those susceptible to dehydration, but not limited to these With slight alterations, other concentrations of ions in bodily fluids could be analyzed For example, by measuring the concentration of nitrites in the urine of patients with such a conductivity meter, the onset of sepsis, or septic shock, can be determined early, and preventive treatments could be administered The possibilities of an effective and inexpensive conductivity meter have vast possible medical application 22 Portable Conductivity Meter Group T2 APPENDIX A Below is data for each conductivity cell constructed and tested Tables 11 and 12 include the data for the trials of bench wire cells while Tables 13 and 14 include data for the copper wire cells The cell constants are given as well as the average millivolt error across the wheatstone bridge read from the voltmeter, the average deviation from the expected resistance, and the deviation from the expected cell constant Table 11: Dimensions for trials of bench wire conductivity cells as well as the expected and actual cell constants Cell # Radius Length Distance Expected Cell Actual Cell (cm) (cm) (cm) Constant (cm-1) Constant (cm-1) 0.035 0.44 1.00 9.94 2.83 0.035 2.26 0.75 1.50 0.57 0.035 0.20 4.00 83.67 4.24 0.035 0.40 4.00 43.59 2.82 0.035 2.10 1.00 2.15 0.84 0.035 2.95 0.50 0.77 0.42 0.035 0.40 4.00 43.59 4.24 0.035 0.30 4.75 68.06 2.97 0.035 0.30 4.75 68.06 2.97 10 0.035 0.30 4.75 68.06 3.25 Table 12: Data for trials of bench wire conductivity cells including average millivolt error across the wheatstone bridge, average deviation from the expected resistance, and the deviation from the expected cell constant Cell # Avg Error across Avg Deviation from Deviation from Expected Cell Wheatstone Bridge (mV) Expected Resistance (Ω) Constant (cm-1) 196 355 7.11 48 28 0.93 116 56 79.43 137 256 40.77 108 79 1.31 78 91 0.35 227 532 39.35 249 222 65.09 224 504 65.09 10 170 165 64.81 23 Portable Conductivity Meter Group T2 Table 13: Dimensions for trials of copper wire conductivity cells as well as the expected and actual cell constants Cell # Radius Length Distance (cm) Expected Cell Actual Cell (cm) (cm) Constant (cm-1) Constant (cm-1) 1c 0.105 1.46 1.00 1.00 0.86 2c 0.105 0.55 4.00 10.07 1.83 3c 0.105 0.55 4.00 10.07 1.42 4c 0.105 0.55 4.00 10.07 1.83 5c 0.105 0.55 4.00 10.07 1.69 Table 14: Data for trials of copper wire conductivity cells including average millivolt error across the wheatstone bridge, average deviation from the expected resistance, and the deviation from the expected cell constant Cell # Avg Error across Avg Deviation from Deviation from Expected Cell Wheatstone Bridge (mV) Expected Resistance (Ω) Constant (cm-1) 1c 45 0.14 2c 117 72 8.24 3c 83 36 8.65 4c 145 86 8.24 5c 98 58 8.38 Table 15 and Figure show a sample set of data as well as the regression curve for one copper wire conductivity cell Table 15: A sample chart of data taken for cell #3c of the copper wire cells Expected equivalent conductances are from the CRC (5) Concentration Expected Equivalent Expected Measured Error across Wheatstone (M) Conductance (cm S/mol) Resistance (Ω) Resistance (Ω) Bridge (mV) 0.050 111.0 255.8 300 94 0.100 106.7 133.1 200 83 0.125 103.6 109.7 160 85 0.150 101.5 93.3 140 82 0.200 97.8 72.6 130 87 0.250 94.6 60.0 110 76 0.300 91.7 51.6 100 77 24 Portable Conductivity Meter Group T2 Figure 5: Example regression curve for cell #3c of the copper wire cells Co ncent rat io n (M) Concentration vs Resistance for Cell #3c Concentration = 468.87(Resistance)-1.6062 R2 = 0.9878 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 100 200 300 Measured Resistance (ohm s) 25 400 Portable Conductivity Meter Group T2 APPENDIX B Figure 6, below, is the conductivity meter circuit schematic Each individual circuit is described in the background or procedure Here, the individual circuits are shown as they were connected Figure 6: Circuit apparatus with the signal generating circuit, the wheatstone bridge, and the voltage comparator circuit 26 Portable Conductivity Meter Group T2 REFERENCES Bioengineering Lab Manual, BE 209 Fall 1998 Bioengineering Lab Manual, BE 309 Fall 1998 Brunzel, Nancy A Fundamentals of Urine and Body Fluid Analysis Saunders: Philadelphia, 1994 Campbell, Neil A Biology 4th edition Benjamin/Cummings Publishing Co.: Menlo Park, 1996 Lide, David R., ed CRC Handbook of Chemistry and Physics 76th edition CRC Press: Boca Raton, 1995 Strasinger, Susan King Urinalysis and Body Fluids, edition F.A Davis: Philadelphia, 1994 27 ... Furthermore, in using the cells in conjunction with the conductivity meter circuit, the error in the accuracy of the cell can be taken into account when balancing the wheatstone bridge at the. .. 20 Portable Conductivity Meter Group T2 Effectiveness as a Conductivity Meter In order for the conductivity meter to be an effective test of the concentration of urine, it is necessary that the. .. cm In increasing the distance between the two wires and decreasing the length of each wire exposed to the solution, the accuracy of the cells decreased 12 Portable Conductivity Meter Group T2 Table