AU2010241482B2

AU2010241482B2 - Apparatus for Measuring Water Concentration in Multi-Component Solutions

Info

Publication number: AU2010241482B2
Application number: AU2010241482A
Authority: AU
Inventors: John Murray Charlesworth
Original assignee: 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: 2011-11-10
Anticipated expiration: 2023-09-24
Also published as: AU2003248295A1; AU2010241482A1

Abstract

Apparatus for Measuring Water Concentration in Multi Component Solutions. Abstract The invention comprises an electronic device for measuring the concentration of water in aqueous solutions containing one or more dissolved non-volatile solutes In particular the device consists of a tubular probe with a cavity containing a water vapor sensitive element and a temperature sensitive element shielded by a water vapor permeable and liquid impermeable hydrophobic membrane. The tip of the probe containing the membrane and sensing elements is immersed in an aqueous solution of one or more non-volatile substances. The water vapor sensing element of the probe responds to the gaseous water diffusing through the membrane from the solution. Outputs form the sensing elements are directed to a microprocessor that is programmed with previously determined calibration data relating the temperature and the equilibrium water vapor concentration to the water concentration in the solution. The concentration of water in the solution is then calculated by the microprocessor. Such a device is suitable for a multitude of practical purposes including the determination of water concentration in the electrolyte of a battery.

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. 5 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. It is an object of the invention to provide a specific gravity-measuring device for general 10 use in aqueous solutions containing one or more non-volatile solutes. It is also desirable for embodiments of the invention to provide a specific gravity measuring device that has rapid response characteristics, is stable in operation, is simple to operate and is able to continuously and automatically measure specific is gravity in a range of aqueous solutions. It is desirable for embodiments to be able to measure specific gravity in a lead acid battery, particularly as charging or discharging takes place. 20 Such devices may then be used to indicate lead-acid battery state-of-charge, and may be easily incorporated into some commercial lead-acid battery configurations. Summary of the invention 25 According to the present invention, there is provided a device for measuring the specific gravity of an aqueous solution comprising: an elongate probe body with two ends, one of which in use is immersed in an aqueous solution; 30 a humidity sensor and a temperature sensor housed in a cavity at the immersible end of the probe body, with the temperature sensor located in close proximity to the humidity sensor; 35 a water vapor permeable hydrophobic membrane located at the end of, or in the vicinity of, the immersible end of the probe body, and positioned to provide an aqueous 5 2724939_2 (GHMtters) P87203.AU.1 liquid barrier between the cavity and tlhe aqueous solution when immersed, the water vapor permeable membrane being in contact with the aqueous solution in use and allowing water vapor from the aqueous solution to diffuse across the membrane and be detected by the humidity sensor; and 5 electronic signal processing circuitry within the probe body for processing signals from the humidity sensor and temperature sensor to facilitate calculation of the specific gravity of the aqueous solution. 10 Preferred features of the invention are set out in the dependent claims, which are to be read as forming part of the description for the present application. According to claimed and unclaimed embodiments, the device may have one or more of the following features: 15 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 20 of the end of the probe; a cavity within the probe, isolated by the membrane, which houses the water vapor sensitive element; and/or 25 a temperature sensitive element located in-close proximity to the water vapor sensitive element. Embodiments of the invention may also have one or more of the following: 30 a probe with a tubular shaped body con structed from a chemically resistant material, such as polyethylene, polypropylene, polytretrafluoroethylene, polycarbonate, polyvinylchloride, ceramic or glass; 6 272493t_2 (GHMahers) P87203A.1 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 and/or an electrical circuit with a microprocessor component that uses as an electrical input that 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. 5 Brief description of the drawings Embodiments of the invention will now be described in further detail with reference to the accompanying Figures, which are as follows: 10 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 15 invention. Fig. 3 shows sensor response in water-sulfuric acid solutions as a function of temperature and water concentration. 20 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. 25 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. 30 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 35 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 8 27.14093%2 (GHMatters) P#7203.AL.1 shown for comparison. Detailed Description 5 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 mateial such as polycarbonate, polyvinylchloride, polyolefin, glass or ceramic etc. The choice of the material is dependent on the 1o 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 1s 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 gm should be used. This ensures that the rate of water vapor diffusion is rapid enough to achieve 90% equilibration in less than 5 20 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 25 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 30 probe is to be used. The choice is well sa 2724939_2 (GHMaters) P87203.AU.1 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 5 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 current. Under these conditions the sulfuric acid solution in the battery 1o 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 process. 15 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 20 sulfuric acid solutions as the liquid electrolyte. Any number of water vapor and temperature sensing probes may be used as part of the invention. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary 25 implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 12 2724939_2 (GHMaters) Pa7203AL1

Claims

1. A device for measuring the specific gravity of an aqueous solution comprising: 5 an elongate probe body with two ends, one of which in use is immersed in an aqueous solution; a humidity sensor and a temperature sensor housed in a cavity at the immersible end of the probe body, with the temperature sensor located in close proximity to the 10 humidity sensor; a water vapor permeable hydrophobic membrane located at the end of, or in the vicinity of, the immersible end of the probe body, and positioned to provide an aqueous liquid barrier between the cavity and the aqueous solution when immersed, the %kater 15 vapor permeable membrane being in contact with the aqueous solution in use and allowing water vapor from the aqueous solution to diffuse across the membrane and be detected by the humidity sensor; a barrier at the immersible end of the probe body providing protection from potential 20 damage to the membrane whilst allowing the aqueous solution to come into contact with the membrane; and electronic signal processing circuitry within the probe body for processing signals from the humidity sensor and temperature sensor to facilitate calculation of the specific 25 gravity of the aqueous solution.

2. The device according to claim 1, wherein the hydrophobic membrane is a composite of a fluoropolymer and a polyolefin. 30

3. The device according to claim 2, wherein the fluoropoymer is polytetrafluoroethylene.

4. The device according to any one of the preceding claims, wherein the membrane is between 0.02 to 0.5mm thick. 35 13 289968_1 (GHMattm) P87203 AU I

5. The device according to any one of the preceding claims, wherein the membrane has a porosity of between 20 and 60%.

6. The device according to any One of the preceding claims, wherein the membrane 5 has a pore size of between 0.6 and 10pm.

7. The device according to any one of the preceding claims, wherein the membrane is surface treated on the non-liquid exposed side to render the membrane capable of being fixed to the probe body. 10

8. The device according to claim 7, wherein the surface treatment comprises chemical etching.

9. The device according to claim 8, wherein the etching results in a surface energy of is 70 dynes/cm 2 .

10. The device according to any one of claims 7 to 9, wherein the membrane is affixed by heat sealing, ultrasonic welding or adhesive bonding. 20

11. The device according to any one of the preceding claims, wherein the device comprises a microprocessor.

12. The device according to claim 11, wherein the microprocessor forms a component of the electronic signal processing circuitry within the probe body. 25

13. The device according to claim 11 or claim 12, wherein the microprocessor uses as an electrical input the signals from the humidity sensor and temperature sensor, and provides an output which can be used to display a specific gravity for the aqueous solution or another quantitative value associated with the specific gravity. 30

14. The device according to claim 13, wherein the signal output is sent to a display.

15. The device according to any one of the preceding claims, wherein the humidity sensor detects a change in electrical resistance, impedance or capacitance. 35 14 28996681 (GHMattm) P87203AU.1

16. The device according to any one of the preceding claims, wherein the probe body is formed from a chemically and thermally resistant material.

17. The device according to claim 16, wherein the probe body is formed from one or a 5 combination of polyethylene, polypropylene, polycarbonate, polyvinylchloride, ceramic or glass.

18. The device according to any one of the preceding claims, wherein the cavity in the probe body is at least partly lined with a non-water vapor absorbing, non-water 10 permeable and non-water releasing material.

19. The device according to claim 18, wherein said material is a non-porous fluoropolymer. is

20. The device according to any one of the preceding claims, wherein barrier is in the form of an end-cap with openings to enable the passage of the aqueous solution to the membrane surface.

21. Use of the device according to any one of the preceding claims to measure 20 specific gravity of a battery electrolyte composition, or to measure state of charge of a battery.

22. The device according to Claim 1, substantially as hereinbefore described with reference to the Figures and/or Examples. 25 15 2899660_1 (GHMatter) P87203AU.1