AU2010241482A1

AU2010241482A1 - Apparatus for Measuring Water Concentration in Multi-Component Solutions

Info

Publication number: AU2010241482A1
Application number: AU2010241482A
Authority: AU
Inventors: John Murray Charlesworth
Original assignee: COMMW OF AUSTRALIA C O MINISTER FOR DEFENCE; Commonwealth of Australia Department of Defence
Current assignee: Commonwealth of Australia Department of Defence
Priority date: 2003-09-24
Filing date: 2010-11-17
Publication date: 2010-12-09
Anticipated expiration: 2023-09-24
Also published as: AU2010241482B2; AU2003248295A1

Description

Apparatus for Measuring Water Concentration in Multi Component Solutions Description of the invention. Many attempts have been made to produce devices that provide convenient and economical means of continuously measuring the concentration of water in aqueous solutions of multiple components. The vapor pressure of water, which is strongly dependent on the concentration of non-volatile solute, can be used as a measure of water concentration in liquids. As is generally known, the partial pressure of any component i in a solution can be expected to be a function of temperature T, the mole fraction x and the mole fractions of all other species in solution. Thus, component i, of a total of N components, has a partial pressure that is described by the following expression: P; x f(T,x 1 ,x 2 ,x 3 ,......XN-1) (equation 1) Furthermore the total pressure Pt in equilibrium with any liquid mixture of N components is the sum of the contributions from individual species, given by the following expression: Pt ='Epo x, (equation 2) Where po, is the vapor pressure of pure substance i. Provided all the components in the solution, apart from water, have negligible vapor pressure then, regardless of the number of dissolved species, the water vapor concentration in equilibrium with the mixture is directly proportional to the fraction of water in solution. 1 There are many practical examples where knowledge of water vapor concentration in equilibrium with a mixture is important. Such mixtures include complex combinations of inorganic salts used in aqueous electroplating baths, mixtures of mineral acids and water in metal pickling baths, sugar and salt solutions employed in food and beverage manufacture, aqueous sulfuric acid solutions present in lead-acid batteries, salt water baths for quenching of metals during hardening processes and a wide variety of aqueous solutions used in industrial process streams. A detailed practical example has been described by van de Kooi (US Patent 4,049,012 (1977)) in which a method for measuring and keeping constant the water content in a heated salt bath used for the quenching of heat-treated tool steel is developed. This hardening and tempering process is influenced by the chemical composition of the salt bath. For mixtures of water, potassium nitrate and potassium nitrite, a 0.2 vol. % loss of water can lead to a 30% lowering of hardness of the treated steel. Water content is critically important in this process and can be accurately controlled by determining the humidity of the gas in equilibrium with the salt/water mixture. Measurement of water in lead-acid batteries also has considerable practical significance because the average concentration of the water in the battery cells is proportional to the degree of charge of the battery. This is best understood by considering the overall reaction occurring in a lead-acid battery, which may be written as: PbO 2 (s) + Pb(s) + 2H 2

SO

4 (l) <-> 2PbSO4(s) + 2H 2 0(0) As the battery discharges, H 2

SO

4 (sulfuric acid) is consumed and water (H 2 0) is produced. This process reverses when charging occurs. The weight percent water concentration in battery electrolyte may vary from about 60% (fully charged) to 95% (discharged state) and the measured value relative to this range 2 can be used as an indication of the fraction of available energy stored in the battery (i.e. state-of-charge). Winsel (West German Patent 2,254,207 (1973)) describes the use of a strip of ion exchange membrane bonded to a second polymer membrane which is placed in the space above the sulfuric acid-water solution in a battery electrolyte. Differential swelling of the two films in the presence of moisture creates a mechanical deformation that may be related to humidity. A device based on a ceramic ionic conducting humidity sensor has also been published by Weininger and Briant, Journal of the Electrochemical Society, 129, 2409 (1982). This solid state sensor is sealed in an envelope constructed from a membrane of porous polypropylene which is claimed to be permeable to water vapor and, due to its hydrophobic nature, impervious to aqueous liquid. The polypropylene envelope containing the sensor is immersed in the electrolyte and the vapor pressure of the water that diffuses through the pores is determined by the change in electrical impedance of the sensor. A shortcoming of this design is that polypropylene on its own has deficient water repelling characteristics which leads to the gradual migration of the acid solution through the pores resulting in the destruction of the humidity sensor by corrosion. The response time of the sensor to humidity changes was observed to be 30 minutes in some cases. Fujita et al (United States Patent 5,206,615 (1993)) describe an approach for measuring the concentration of sulfuric acid in water solutions. The sensor design and principle of operation is similar to that described by Weininger except that the materials used to fabricate the envelope containing the humidity sensor are modified. In particular Fujita proposes the use of a hydrophobic porous fluorine resin film as the vapor permeable envelope, heat-sealed at the edges around the humidity sensor and the connecting wires. These fluorine resin films are manufactured such that their pore diameter is no larger than 0.5pm, and preferably 0.01 to 0.2 min. For sulfuric acid concentration measurements the preferred design of Fujita is a membrane that has a selective vapor permeable material such as perfluorocarbon sulfonic acid attached, and a material such as 3 calcium carbonate coated on the inside surface which irreversibly reacts with acid. It is claimed that the acid reactive layer assists in removal of sulfuric acid vapors that permeate through the membrane, by formation of carbon dioxide gas and non-corrosive solid calcium sulfate. According to the methods of Fujita et al, films which do not contain the acid-reactive coating show unacceptable degradation in sensing characteristics when exposed to sulfuric acid solutions for only 5 days at 30 0 C. It is to be anticipated that continuous exposure of the films containing the acid reactive layer for long periods of time should lead to eventual depletion of the calcium carbonate layer resulting in eventual sensor degradation. It is generally recognized that in order to achieve reasonable water vapor diffusion rates, hydrophobic films with a thickness of less than approximately 0.5 mm must be used. At these dimensions the films are fragile and problems with mechanical instability can occur. Fluoropolymer membranes, typically polytetrafluoroethylene, have less than half the yield strength of polypropylene membranes and are subject to mechanical creep. Thin films of polyolefins and fluoropolymers are easily ruptured by pressure changes. Devices which do not have a temperature sensor in close proximity to the water vapor sensing element will not permit allowance for the temperature dependency of the partial pressure of the water component in the mixture, as given by equation 1. For example, the most commonly used method of water vapor sensing is the measurement of relative humidity; however the variation in relative humidity with temperature of a given mixture in not easily predicted. For instance, over the temperature range 0 to 50 0 C a saturated solution of potassium nitrate has a linear decrease in relative humidity from 97% to 85%. In the case of sodium chloride it has been found that the relative humidity of a saturated solution is approximately constant (75±1%) over this temperature range (Wexler and Hasegawa J.Res.Nat.Bur.Stds 53, 19 (1953)). Equally significant and unpredictable is the intrinsic variation in response of the humidity sensor as temperature varies. Application of a temperature sensor also enables the final displayed result to be standardized to a common reference temperature. 4 Without prior calibration and appropriate correction for temperature variations any humidity-based sensing technique for measuring water concentration is extremely limited in its usefulness. Objects of the invention. It is an object of the invention to provide a water concentration-measuring device for general use in aqueous solutions containing one or more non-volatile solutes. Additional objects are to provide a water concentration sensor that has rapid response characteristics, is stable in operation, is simple to operate and is able to continuously and automatically measure water concentration in a range of aqueous solutions. It is a further object to provide a sensor that is able to measure water concentration in a lead acid battery, particularly as charging or discharging takes place. Another object of the invention is to provide a device to indicate lead-acid battery state-of-charge that is easily incorporated into some commercial lead-acid battery configurations. Another object of this invention is to provide an electrical signal that may be used to decide on the most appropriate times to charge and discharge a lead-acid battery. 5 Another object is to supply a state-of-charge sensor that will function for the service life of the lead-acid battery, will require minimum recalibration and will allow for dynamic monitoring of battery capacity over the battery lifetime. Further objects and advantages of the invention will become evident as the ensuing description develops. Summary of the invention To this end the first aspect of this invention provides a device with a water vapor, humidity or relative humidity sensor including in combination with the following: a probe with a body constructed from a chemically and thermally resistant material; a membrane capable of allowing water vapor to permeate rapidly whilst providing a barrier to aqueous liquids, the membrane being located at the end of, or in the vicinity of the end of the probe; a cavity within the probe, isolated by the membrane, which houses the water vapor sensitive element; a temperature sensitive element located in close proximity to the water vapor sensitive element; Non-limiting but preferred features of the invention include some or all of the following: a probe with a tubular shaped body constructed from a chemically resistant material, such as polyethylene, polypropylene, polytretrafluoroethylene, polycarbonate, polyvinylohloride, ceramic or glass; 6 a water vapor permeable membrane that is constructed from a composite mixture of polytetrafluoroethylene and a polyolefin, such as polypropylene or polyethylene; a water vapor permeable membrane that is in the thickness range 0.02 to 0.5 mm with a porosity from 20 to 60% and pore sizes in the range 0.6 to 10 tim; a water vapor permeable membrane that has been treated on one surface to improve adhesion; a water vapor permeable membrane that is bonded to the body of the probe; a water vapor sensitive element housed in a cavity constructed from a non-water vapor absorbing non-porous and non-water vapor releasing material such as a fluoropolymer; an end-cap placed over the membrane and in contact with the liquid, providing a means of physical protection for the membrane whilst enabling passage of liquid from the bulk to the membrane surface; a water vapor sensitive element that will produce a change in electrical response such as resistance, impedance, capacitance, resonance frequency or digital encoded signal; a temperature sensitive element that will produce a change in electrical response such as resistance, impedance, capacitance, voltage or digital encoded signal; an electrical circuit that can process the signal from a water vapor sensitive element and a temperature sensitive element to provide an electrical output such as a voltage, digital encoded signal or frequency, related to water vapor contact; 7 an electrical circuit with a microprocessor component that uses as an electrical input the signals from vapor and temperature sensing elements and provides an electrical output which can be used to display a quantitative value of the water concentration of the aqueous solution in which the probe is immersed. Basic embodiments of the invention are explained by referring to Figures 1 and 2 appended. With reference to Figure 1, a humidity sensor and a temperature sensor are positioned in a cavity in a tubular probe made from chemically and thermally resistant material. This may be a material such as polycarbonate, polyvinylchloride, polyolefin, glass or ceramic etc. The choice of the material is dependent on the composition and temperature of the solution in which the probe is to be used. Such a choice is well known to those skilled in the art. The humidity sensor is located in close proximity to a hydrophobic membrane manufactured from a mixture of polytetrafluoroethylene and polyolefin. For the polyolefin component, either polyethylene or polypropylene may be used; however polypropylene is preferred because of its superior mechanical properties. The polytetrafluoroethylene-polyolefin composite membrane is manufactured by hot roll pressing and stretching. A membrane with a thickness in the range 0.02 to 0.5 mm with a porosity from 20 to 60% and pore sizes in the range 0.6 to 10 prm should be used. This ensures that the rate of water vapor diffusion is rapid enough to achieve 90% equilibration in less than 5 minutes The method of fixing the membrane to the body of the probe can be by heat sealing, ultrasonic welding or adhesive bonding. In one embodiment, the inner surface of the membrane is chemically etched either with a solution of sodium naphthalene complex dissolved in 2-methoxyether or by cold plasma treatment. Etching should proceed until a surface energy of 70 dynes/cm 2 is achieved. This treatment facilitates improved bonding of the membrane to the body of the probe. The adhesive employed is preferably a solution of PVC in methyl ethyl ketone, although the choice of the adhesive is dependent on the composition and temperature of the solution in which the probe is to be used. This choice is well 8 known to those skilled in the art. The membrane and sensing elements can also be placed on the side of the body of the probe if needed. The water vapor sensor is positioned close to the membrane so as to ensure that water vapor which diffuses through the membrane is rapidly detected and measured. The cavity in which the sensing elements reside is lined with a non-porous fluoropolymer, such as polytetrafluoroethylene, in order to eliminate moisture sorption and desorption by the walls of the cavity. An outer end-cap, with openings to allow contact between solution and membrane, acts as a protective barrier against damage to the membrane. Figure 1 illustrates one embodiment of the invention in which a perforated series of openings in the end-cap is used. The material selected for the end-cap should be suitably chemically and temperature resistant in the same manner as the probe body. This end-cap is designed to fit over the lower end of the probe body, with a space between the end-cap and the membrane. The dimensions of the probe can be adjusted to meet the requirements of the system in which measurements are to be performed. Also included in the body of the probe is suitable circuitry to enable an output signal that is proportional to the water content of the solution under test. A microprocessor can be included in the signal processing circuitry and the digital output from the probe may be taken to a display. Part or all of the electronic components in the probe, excluding the water vapor sensing element, may be protected from mechanical damage and chemical attack by encapsulation in a suitable curing resin such as silicon rubber, polyurethane or epoxy. For the purpose of deducing a quantitative value of the water concentration of the solution under test, previously established relationships between the water content of the solution and the temperature and humidity sensor response characteristics are used to construct a lookup table or a suitable mathematical relationship that can be stored in memory accessible by the microprocessor. Figure 2 shows a block diagram of the configuration of the electronic components in one embodiment of the invention. 9 It should be understood that the general design of the invention as illustrated in Figures 1 and 2 is solely for illustrating one embodiment of the invention and the person skilled in the art will be able to make variations or modifications within the scope of the invention. Examples Specific embodiments of the present invention are illustrated in the following practical examples. These are demonstrated by referring to Figures 3 to 8 appended. In these examples the invention is used to measure the water concentration in the electrolyte of a lead-acid battery. An example of the use of the invention for following the discharge of a lead-acid battery is given, thereby illustrating a continuous means of determining the battery state-of-charge. In these applications the body of the probe is manufactured from polycarbonate. The water vapor sensing element of the probe is a commercially available polymer-type relative humidity sensor that provides a voltage varying with relative humidity. The temperature sensor is a commercially available thermistor that provides a resistance varying with changes in temperature. The polytetrafluoroethylene-polypropylene composite membrane has a thickness of 0.25 mm, a porosity of 40% and a pore size of 1 Im. The polypropylene component is 20% of the total mass and is amalgamated into the membrane by hot rolling. The membrane is chemically etched on the inner surface with sodium naphthalene complex and adhesively bonded to the body of the probe using PVC. Figure 3 shows the variation in sensor output at two temperatures in a series of sulfuric acid/water solutions with water content ranging from approximately 55 to 95 wt %. This is typical of the concentration range encountered in lead-acid batteries undergoing charge and discharge. It can be clearly seen from Figure 3 that there is a strong relationship between water concentration and sensor output. There is also a substantial temperature dependency of the sensor output. By using a series of curves, including those presented in Figure 3, a lookup table is constructed and programmed into the 10 memory of a microprocessor. This allows a direct electronic computation of water concentration using the humidity and temperature sensor responses. For the purpose of providing a general measure of battery electrolyte concentration, the microprocessor is programmed to display in specific gravity (SG) units. Figure 4 shows the time dependency of response after rapidly changing acid solutions between a SG of 1.15 (70% discharged) and a SG of 1.3 (fully charged). It is apparent that attainment of 90% of the equilibrium value occurs in approximately 3 minutes. Some lead-acid battery operators choose to employ any of a variety of commercial additives that are claimed to rejuvenate "sulfated" lead-acid batteries. The typical composition of these additives is an aqueous solution of magnesium sulfate that may be added to the sulfuric acid solution in underperforming cells. Figure 5 shows an example of data obtained from experiments with ternary mixtures produced by adding magnesium sulfate at various concentrations to a sulfuric acid solution with an initial SG of 1.225. It can be seen that the SG measured using the invention is slightly increased by the battery additive. This small increase is expected due to the proportional decrease in the water concentration of the solution as magnesium sulfate is added. An example of data illustrating the typical stability of the invention during prolonged exposure to lead-acid battery electrolyte is shown in Figure 6. These data have been obtained using the sensor immersed in a sulfuric acid solution with a SG of 1.225 for a period of approximately 7 months at 25 *C, with measurements of SG every week. The measured SG is relatively constant with no deterioration in sensor performance over this extended period of operation. Figure 7 illustrates the invention inserted into a lead-acid battery cell compartment through a hole in the top of the battery case and sealed against liquid leakage with a rubber o-ring. The tip of the probe is placed above the top of the battery plates and below the surface of the liquid covering the battery plates; 11 however any other location which provides passage from the top, bottom or sides of the battery case to the battery liquid in any cell can be used. An example of the variation in specific gravity measured by the invention in one cell during discharge of a nominal 12V flooded lead-acid battery with a nominal capacity of 60 amp hours at 100-hour discharge rate is shown in Figure 8. An installation of the type illustrated in Figure 7 was used to obtain these results. The battery was connected to a series of resistive loads to give approximate constant current discharge at the indicated currents. Under these conditions the sulfuric acid solution in the battery cell has a maximum SG of approximately 1.26 at full charge, and is discharged to a SG of 1.08. For the purpose of comparison, the battery voltage is also shown in Figure 8 together with manual readings taken with a hand-held hydrometer at regular intervals. It is apparent that the invention is able to accurately and reliably follow the battery discharge processes. As is believed apparent from the above examples, the invention provides an automated instrument which delivers as an output an electrical characteristic that is precise and accurately reflects the state-of-charge of a lead-acid battery in continuous operation. The sensor may be used with any lead-acid battery that employs aqueous sulfuric acid solutions as the liquid electrolyte. Any number of water vapor and temperature sensing probes may be used as part of the invention. Brief description of the drawings Fig. 1 is an overall axial section with exploded perspective through the sensor comprising the present invention. Fig. 2 is a block diagram of the measuring electronics employed as part of the invention. 12 Fig. 3 shows sensor response in water-sulfuric acid solutions as a function of temperature and water concentration Fig. 4 shows the time dependent response of the sensor following rapid changes between water- sulfuric acid solutions with a SG of 1.15 and 1.30. Fig. 5 shows sensor behavior with magnesium sulfate added in various concentrations to a water-sulfuric acid solution with a SG of 1.225. Fig. 6 shows an example of the typical stability of the sensor immersed in a water-sulfuric acid solution with a SG of 1.225 at 20 *C for a period of approximately 7 months. Fig. 7 shows an installation in which the sensor is inserted through a hole in the top of a lead-acid battery case into the sulfuric acid solution in the cavity between the battery case top and the battery plates in one of the cell compartments. Fig. 8 shows examples of SG measurements obtained using an installation of the invention in a 60 amp hr 12V lead-acid battery discharged at several currents. Battery voltage measurements, and manual readings taken with a hand-held hydrometer, are shown for comparison. 13

Claims

1. A water activity/concentration measuring sensor comprising, in combination: a probe, with a body defining a cavity, which can be partially immersed in an aqueous solution containing one or more dissolved non-volatile species; a membrane able to permit water vapor transmission whilst blocking the passage of aqueous liquids, the membrane being located between the cavity of the probe and the liquid whose water activity/concentration is to be determined; a water vapor sensing element, adjacent to the membrane, located within the cavity of the body of the probe; a temperature sensor located within the body of the probe in close proximity or connected to the water vapor sensor; methods for measuring the electrical response of the sensing elements and for deducing the water activity/concentration of the solution in contact with the probe;

2. A membrane according to claim 1 that has a thickness in the range 0.02 to 0.5 mm with a porosity from 20 to 60% and pore sizes in the range 0.6 to 10 Lm, manufactured from a composite mixture of polytetrafluoroethylene, and a polyolefin such as polypropylene or polyethylene.

3. A membrane according to claim 2 that is surface-treated on the non-liquid exposed side to render it capable of being bonded to the body of the probe. 14

4. A sensor according to claim 1 with a tubular shaped body constructed from a chemically and thermally resistant material including polyethylene, polypropylene, polycarbonate, polyvinylchloride, ceramic or glass.

5. A membrane according to claim 1 that is bonded to the body of the probe.

6. A sensor according to claim 1 wherein the cavity is lined with a non-water vapor absorbing, non-water vapor permeable and non-water vapor releasing material such as a fluoropolymer.

7. A sensor according to claim 1 wherein the water vapor sensitive element produces a change in electrical resistance, impedance, capacitance, resonance frequency or digital encoded signal;

8. A sensor according to claim 1 wherein the temperature sensitive element produces a change in electrical resistance, impedance, capacitance, voltage or digital encoded signal;

9. A sensor according to claim 1 wherein a barrier is placed over the membrane and in contact with the aqueous liquid, providing a means of physical protection for the membrane whilst enabling passage of liquid from the bulk of the liquid to the membrane surface

10. A sensor according to claim 1 wherein the means of measuring the electrical response of the sensing elements comprises an electrical circuit for processing the signal from both a water vapor sensitive element and a temperature sensitive element and provides an electrical output such as a voltage, digital encoded signal or frequency which can be used to measure water activity/concentration;

11. A sensor according to claim 1 wherein the means of deducing the water activity/concentration of the solution in contact with the probe includes 15 electronic means for storing water concentration isotherm curves associated with the solutes present in the solution.

12. A sensor according to claim 1 wherein the means of deducing the water activity/concentration of the solution in contact with the probe includes a microprocessor component that uses as an electrical input the signals from vapor and temperature sensing elements and provides an electrical output which can be used to display a quantitative value of the water activity/concentration of the aqueous solution in which it is immersed.

13. An installation for measuring water activity/concentration of aqueous solutions containing one or more non-volatile solutes comprising a plurality of sensors according to claim 1.

14. An installation for measuring water activity/concentration in the cells of a lead-acid battery comprising a sensor or a plurality of sensors according to claim 1 which may be indicative of the state-of-charge of the said lead-acid battery.

15. Methods of computation that may be stored in ROM or RAM associated with the microprocessor recited in claim 12 allowing the electrical output to be converted to specific gravity.

16. A visual display device electrically or wirelessly connected to the microprocessor recited in claim 12. 16