Method for dehydrating a porous material

A method for dehydrating a porous material using electro-osmosis, includes applying a pattern of D.C. voltage pulses to an anode system embedded in the porous material, wherein the anode system is electrically interactive with a cathode structure embedded in earth, and wherein the pattern of D.C. voltage pulses, has a pulse period T in a range from about 3 to about 60 seconds, and each pulse period T includes a positive pulse duration of T+ from about 0.7 T to about 0.97 T, a negative pulse duration of T− from about 0.01 T to about 0.05 T, and a neutral pulse duration of Tp from about 0.02 T to about 0.25 T. In another embodiment, each pulse period includes two or more positive pulses separated by at least one of either a negative pulse or a neutral pulse, with the positive pulse duration being the combined duration of the two positive pulses.

BACKGROUND OF THE INVENTION

The present invention generally relates to a method for dehydrating porous materials, such as moist walls and/or floors of a masonry or concrete structure, as well as the clay containment structure of a landfill, through the use of electro-osmosis. More particularly, the present invention relates to applying a D.C. voltage of a specific pulse pattern to an anode embedded in a porous material, and allowing the voltage to travel through the porous material to a cathode embedded in earth.

Moisture problems in containment structures are common, particularly in building structures located at least partially underground. In addition, modern day efforts to minimize building erection time often do not allow adequate drying time for concrete, leading to moisture problems in the building structure.

A number of methods have been developed for dehydrating porous materials such as concrete and masonry structures. While marginally effective, a number of these methods require significant energy and time to achieve the marginal results.

The principle of electro-osmosis was discovered by Professor F. F. Reuss in Moscow in 1807. Electro-osmosis employs a voltage potential difference between two points on opposite sides of a porous material. If the porous structure of the material has been saturated by water, the porous materials assume a negative potential. This causes positive ions in the water to locate around the porous materials to form a so-called electrical double layer. The positive ions will move towards a region having a lower voltage potential. The positive ions are hydrated, and therefore each ion carries a small amount of water, resulting in a water flow toward the lower potential.

There have been a number of commercial attempts to dehydrate building structures using electro-osmosis. In some European countries, a so-called “passive” electro-osmosis system has been employed, wherein a natural potential difference which is created between a moist material and its surroundings is used. This type of installation has yielded marginal results.

In other types of electro-osmosis systems, a direct current or conventional alternating current has been used to generate a potential difference. With these systems, it is only possible to carry water between the anode and cathode over a short period of time, because the forces after some period will reverse such that the electrolyte (water) is transported back to its origin. Efforts were thus directed toward developing a system capable of functioning over an extended period of time, without the so-called “zeta potential” being reversed, meaning that the water returns back to the capillary material. A number of system have been developed utilizing a pulsating direct current, that is where the current is switched between positive and negative potentials. Such systems are described in U.S. Pat. Nos. 5,368,709; 4,600,486; and 5,015,351; as well as in Swedish patent applications 8106785-2 and 8601888-4 (P. Eliassen), Swedish Patent No. 450264 and Polish Patent No. 140265 (Basinsky et al.). These known systems have problems relating to the durability of the electrodes on the anode side of the system because the anodes are easily corroded due to a reduction-oxidation phenomenon. In addition, these known systems have not balanced the energy of the positive and negative pulses in voltage-seconds, also denoted as magnetic flux, so that a maximum water flow out of porous material of the structure is obtained without having a further moisturizing of the porous material at a later time.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a novel method and apparatus for dehydrating porous materials which sustains the transport of liquid in the desired direction.

It is a further object of the invention to provide an improved electro-osmotic method and apparatus for dehydrating porous materials which reduces the drying time and lowers the relative humidity compared to known methods.

It is yet another object of the present invention to provide a novel electro-osmotic method and apparatus for dehydrating porous materials to a lower relative humidity level than prior art methods and apparatus.

The above and other objects are accomplished according to the invention by the provision of a method and apparatus for dehydrating a porous material using electro-osmosis, including the steps of: applying a pattern of D.C. voltage pulses to an anode system embedded in the material, the anode system being electrically interactive with a cathode structure embedded in earth, wherein the pattern of D.C. voltage pulses has a pulse period T in a range from about 3 to about 60 seconds, and each pulse period includes a positive pulse duration T+ from about 0.7 T to about 0.97 T, a negative pulse duration T− from about 0.01 T to about 0.05 T and a neutral pulse duration Tpfrom about 0.02 T to about 0.25 T.

It has been found that by utilizing a pulse pattern according to one preferred embodiment of the invention, an osmotic pressure differential from one side to the other of a structure made of porous material can be sustained which is at least ten times greater than that possible with known electro-osmotic techniques.

In accordance with another preferred embodiment of the invention, each pulse period includes two or more positive pulses separated by at least one of either a negative pulse or a neutral pulse, with the positive pulse duration T+ being the combined duration of the two positive pulses, an even greater osmotic pressure differential is developed.

Other objects, features and advantages of the invention will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, there is shown a structure1with walls1aand a floor1bcomprising a porous material, such as masonry and/or concrete, located in the earth2below ground level. A conventional drain pipe3is shown running from a roof (not shown) close to outer wall1a. Water will therefore likely seep into the porous wall1a. Some capillary absorption represented by the arrows inFIG. 1of the water by the outer wall1aand by the rest of the masonry structure can create high relative humidity in the space enclosed by walls1a. Insufficient ventilation of the enclosed space will create a musty, damp atmosphere rendering the space unsuitable for many uses.

FIG. 2, shows a similar building structure1which is wired for carrying out electro-osmotic dehydration. Specifically, a number of anodes4are provided in the porous walls and/or in the floor of the underground building structure. Preferably, the anodes are comprised of an inert material such as a mixed metal oxide (MMO) ceramic wire anode. Such wire anodes can be obtained from Ceranode Technologies International, which is a division of APS-Material, Inc., Dayton, Ohio, which sells such wire anodes under the trademark CerAnode. Alternatively, the anodes can comprise conventional rubber graphite anodes which conduct current, according to Faraday's law, by movement of electronically conductive carbon ions. One advantage of using a mixed metal oxide anode is that it has a low dissolution rate on the order of one to 10 mg/amp-year. In contrast, conventional rubber graphite anodes have a dissolution rate on the order of 2 kg of carbon/amp-year and therefore have a relatively short life. The MMO wire anodes are preferably installed in thin, {fraction (1/16)}th inch wide grooves cut in the masonry in parallel lines several feet apart as generally shown by the dashed parallel lines in FIG.2.

A common cathode5preferably comprises a copper clad steel rod embedded in the ground as shown in FIG.2. The anodes and cathode are connected to a power control unit6which supplies a D.C. voltage pulse pattern between the anodes4embedded in porous material of the building structure and the cathode5. By applying an appropriate D.C. voltage pulse pattern as discussed below, water travels from a positive potential (+) at the wall to a negative potential (−) at the cathode. Water therefore flows out of the porous material and into the earth2as shown by the arrows in FIG.2.

A simplified schematic of the electro-osmotic system is shown inFIG. 3, wherein like reference numerals are used to denote like components. InFIG. 3, the anodes4′ are in the form of rubberized graphite plugs as opposed to the wire anodes illustrated in FIG.2.

FIG. 4illustrates the power control unit6in more detail. As shown inFIG. 4, the power control unit6preferably includes a control unit6A, a power unit6B and an output unit6C. The power unit6B receives A.C. power at input terminals12. The A.C. supply voltage is down-converted in a transformer13, rectified in a rectifier14, and is suitably stabilized (e.g., by a conventional capacitor) to deliver D.C. voltage, for example, 30 volts D.C. to an input8A of an electronic switch arrangement8in the output unit6C.

The control unit6A preferably includes a programmable microprocessor9, a program setting panel10and a control display11. The microprocessor9is suitably programmed for producing control signals on control lines16which control the operation of electronic switches17,18,19and20in switch arrangement8in output unit6C. The program setting panel10permits adjustments to be made in the parameters of the microprocessor program to adjust the control signals on control line16for producing a desired pulse pattern at the output of control unit6C as will be discussed below.

The direct voltage signal at input8A is preferably coupled to a series connection of electronic switches17and19and another series connection of switches18and20. Switches17,18,19and20are each individually controlled by a respective control line16A,16B,16C and16D. Another control line16E is optionally connected to a common node7which is also connected to a terminal of each of switches19and20. A resistor23is connected between common node7and a ground electrode25.

FIG. 4additionally shows two different sets +A and +B of anodes4coupled by a line27to a node29between switches17and19. Line27is connected to a terminal of respective controllable relays21and22which are selectively activated via control lines16F and16G for energizing a selected one of the anode sets +A and +B. The common cathode5inFIG. 4is denoted by −A, −B which is connected by a line31to a common node33between electronic switches18and20. Multiple sets +A and +B of anodes are provided to utilize the overall working capacity of power control unit6and its associated circuitry. Different sets of anodes will provide greater operational safety by reducing localized current and also increase dehydration capacity, however, the dehydration process can take longer. Dehydration time can be reduced by increasing the working capacity of the power control unit.

Selectively opening and closing switches17,18,19and20causes a positive, negative or neutral (zero voltage level) pulse to be conducted via line27to one or both of the electrodes sets A and B, depending on whether relays21and22are open or closed. For example, if switches17and20are closed and switches18and19are open, the voltage input +25 volts D.C. at input8A will be applied across anodes +A and/or +B relative to the cathode −A, −B. On the other hand, if switches17and20are open and switches18and19are closed, the anode to cathode potential is −25 volts D.C. A neutral or zero voltage level pulse can be produced at the anodes by opening switches17and18and closing the switches19and20. It will be appreciated by those skilled in the art that control lines16A to16D can be controlled by the microprocessor9and program setting panel10to conduct control signals to switches17to20, thereby creating any desired pulse pattern of positive, negative and neutral pulses at the anodes.

FIG. 5Aillustrates a known pulse pattern which has been utilized in the past for electro-osmotic dehydration. This conventional pattern typically includes a positive pulse duration T+ that is approximately 0.7 T, a negative pulse duration T− of approximately 0.1 T, and a neutral pulse duration Tpof approximately 0.2 T. A pulse pattern of this type is disclosed, for example, in U.S. Pat. No. 5,368,709. The pulse pattern duration disclosed in this patent is 1.4 seconds.

Using this conventional pulse pattern shown inFIG. 5A, with a pulse period of T equal to 3 seconds, it has been found that osmotic pressure gradually increases and levels out at relatively modest levels after a few days.

Referring toFIG. 5B, a laboratory experiment was conducted to measure osmotic pressure produced by different pulse patterns. The laboratory setup included two tanks30and32of different sizes. The smaller tank32, which was bottomless, was supported within the larger tank30with the upper edge32aof the smaller or inner tank32spaced apart from the upper edge30aof the outer tank30, defining a gap34. The interior of the inner tank32was communicated with the interior of the outer tank by a porous material35, for this testing, a brick, which was installed in sealed relationship with the open bottom of the inner tank32. The tanks30and32were filled with water36to the same initial level36aabove the brick35. An anode37was placed in the outer tank30. A cathode38was placed in the water36in the inner tank32. The anode37and the cathode38were connected to outputs of a control unit39. The control unit created direct voltage pulse patterns at the anode37relative to the cathode38.

A pulse pattern having the characteristics ofFIG. 5with the positive, negative and neutral duration periods T+, T−, and Tp, respectively, as discussed above, resulted in a 30 mm rise of the water level in the tank32containing the cathode38. No further water level change was observed after about 48 hours. These test results are represented by curve I in FIG.7.

Surprising and convincing results based on extensive testing and verification of the effectiveness of preferred embodiments of the present invention have established that varying certain pulse parameters increases dehydrating effectiveness.

A series of tests were conducted using a different set of pulse parameters for each test of the series of tests. For the testing, equal positive and negative pulse magnitudes of 20, 40 and 66 volts were used in the testing of each of several pulse patterns. In addition, for each pulse pattern and each voltage magnitude, a test was conducted for pulse pattern durations of 3, 6, 10, 20, 30 and 60 seconds. In the following description, each pulse pattern is represented by a sequence of three numbers separated by a hyphen, wherein the first number is the positive pulse duration T+, the second number is the negative pulse duration T−, and the third number is the neutral pulse duration Tp. For the initial testing, the pulse patterns that were used in the testing were as follows:

60-15-2580-10-1070-10-2075-10-1585-5-590-5-5
In accordance with the convention referred to above, the pulse pattern 60-15-25, for example, has a positive pulse duration T+ of 60, a negative pulse duration T− of 15, and a neutral pulse duration Tpof 25. The testing was conducted for each of these six pulse patterns using the voltage magnitudes and pulse pattern durations stated above. The duration of the test for each set of parameters was 48 hours.

The test results showed that changing the voltage magnitude did not significantly impact the amount of increase in the height of the water in the inner tank. Moreover, the testing demonstrated that the best results were obtained when the duration of the pulse pattern was between 6 and 10 seconds. In addition, more favorable results were obtained for pulse patterns in which the positive side pulses represented 90% or more of the duration T of the pulse pattern.

Accordingly, further testing was conducted using pulse patterns having a total duration T of 6 seconds. The tests were conducted for a period of 48 hours. The positive and negative pulse magnitudes were 40 volts. Two tests, Test A and Test B, were conducted for each set of parameters. At the end of each test period, the height of the water in the inner tank32was measured, and the measurement data was recorded. The results of the further testing is summarized in TABLE I, for the two tests, Test A and Test B. Each pulse pattern is represented by the three number sequence of the convention referred to above in which the first number of the sequence is the positive pulse duration T+, the second number of the sequence is the negative pulse duration T−, and the third number of the sequence is the neutral pulse duration Tp.

To confirm the results of these tests, further testing was conducted using pulse patterns selected from the initial tests, including pulse patterns 70-1-29, 80-1-19, 90-1-9 and 95-1-4 along with another pulse pattern 85-1-14. For these tests, each of the pulse patterns had a pulse period T of six seconds. The pulse patterns included equal positive and negative pulse amplitudes of 40 volts D.C. The test for each pulse pattern was conducted for a period of 48 hours. The height of the water in the inner tank was measured at the end of each of the tests. The results of these tests are summarized in TABLE II. The test data, arranged in ascending order for the positive pulse duration, demonstrates that a greater increase in height of the water was provided for longer positive pulse periods. The increase in the height of the water in the inner tank ranged from 30 mm to 45 mm for Test A and from 40 mm to 55 mm for Test B. The greatest increase in height was provided for the pulse pattern 95-1-4, which had the longest positive pulse duration for this series of tests.

In view of the favorable results obtained using the pulse pattern 95-1-4 for the 48 hour test period, further testing was conducted over a longer time period, which was twelve days for these tests. The further testing was conducted using the pulse pattern 95-1-4 and having a pulse period T equal to six seconds, and with equal positive and negative pulse amplitudes of 40 volts D.C. At the end of each test period, the height of the water in the inner tank32was measured and the measurement was recorded.

The results obtained for the further testing are summarized in TABLE III for two tests with the measurement data obtained for the two tests being labeled Test A and Test B. As can be seen, for Test A, the height of the water in the inner tank32(FIG. 5B) increased from 0 to 310 mm in the twelve day test period. For Test B, the height in the water in the inner tank increased from 0 to 245 mm in twelve days. The difference in the results between Test A and Test B can be attributed to different chloride and pH levels in both the water and the brick35.

In contrast, using the known pulse pattern 70-10-20, as disclosed in U.S. Pat. No. 5,368,709, the height of the water in the inner tank increased from 0 to only 30 mm over a period of three days. No measurable increase in height was noted after the first three days.

Further testing was conducted in which the percentage of the positive pulse time, i.e., the positive pulse duration T+, was selected to be 0.90 T and 0.95 T for a pulse pattern duration T of 6 seconds. In the two tests, Test A and Test B, for which T+ was 0.90 T, the negative pulse duration T− was increased to 0.05 T. In the tests, Test A and Test B, for which T+ was 0.95, the negative pulse duration T− was increased to 0.04 T. The results of these tests are summarized in TABLE IV.

For Test B which used the pulse pattern 95-4-1, the increase in the height of the water was 40 mm.

Further testing was conducted in which the percentage of the positive pulse time, i.e., the positive pulse duration T+, was decreased, and the negative pulse duration T− was increased. The results of these tests are summarized in TABLES V and VI.

As can be seen, using a smaller percentage positive pulse duration resulted in a reduced increase in the height of the water in the inner tank. In particular, the pulse pattern 70-10-20, which is similar to that used in the U.S. Pat. No. 5,368,709, provided an increase of only about 28 to 37 mm with the longer pulse pattern duration of 6 seconds, as compared to the 1.4 second pulse pattern which is disclosed in this patent.

Although the use of smaller percentage positive pulse durations may result in smaller increases in the height of the water in the inner tank, in preferred embodiments, pulse patterns having 0.70 T<T+≦0.97 T; 0.01 T<T−≦0.05 T; 0.02 T<Tp≦0.25 T can be used to provide better results than prior art methods and apparatus. Moreover, although the most favorable results are obtained when the duration of the pulse pattern is between about 6 and 10 seconds, pulse patterns having a duration of 3 seconds<T≦60 seconds are quite effective.

For porous materials in particular, T+=0.95 T; T−=0.01 T; and Tp=0.04 T, (FIG. 6A) provide a dehydrating efficiency which is significantly better than that yielded by the prior art pulse pattern shown in FIG.5A. The long time laboratory testing with a pulse pattern according to this embodiment of the invention has shown that the present invention provides a method that, even for a long term dehydration process, eliminates reverse travel of the dehydrating fluid. In the above-mentioned laboratory test, the water column level (curve II inFIG. 7) rose steadily over the test period at a rate 10 times as great as that of the prior art pulse pattern (curve I in FIG.7). Although the pulse pattern, according to the invention, can provide positive and negative pulses of substantially equal magnitude, a pulse pattern where the positive and negative pulse amplitudes are unequal can also be used effectively. The positive pulse preferably is selected in the range from about +12 volts to +250 volts D.C. The negative pulse preferably is selected in the range from about −12 volts to about −250 volts D.C.

The total pulse period T can be greater than 3 seconds, but, less than or equal to 60 seconds. In a most highly preferred embodiment, the pulse period T between about 6 and 10 seconds. However, it would be possible to set the duration of the total pulse period T to other values in the range, while retaining the pulse duration ranges for T+, T− and Tpas indicated above.

According to a further aspect of the invention, an even greater osmotic pressure can be developed within the context of the invention by dividing the positive pulse duration of each pulse period T into two or more separate positive pulses T1+ and T2+ separated by at least one of either a short duration (e.g. 0.05 T) neutral pulse Tp1as shown inFIG. 6Bor negative pulse T1− as shown in FIG.6C. In one preferred embodiment, the pulse durations of the pulse pattern ofFIG. 6B, in the order of occurrence in the period T, are as follows:
T1+=0.2 T; Tp1=0.05 T; T2+=0.6; T−=0.01 T; Tp2=0.1 T
where Tp2is the second occurrence of a neutral pulse within the period T.

In a highly preferred embodiment, the pulse durations of the pulse pattern ofFIG. 6B, in the order of occurrence in the pulse period T are as follows:
T1+=0.2 T; Tp1=0.05 T; T2+=0.7; T−=0.02 T; Tp2=0.3 T
where Tp2is the second occurrence of a neutral pulse within the period T having a 10 second duration.

In yet another embodiment, the pulse pattern was:
T1+=0.15 T; Tp1=0.05 T; T2+=0.60; T−=0.05 T; Tp2=0.15 T
where Tp2is the second occurrence of a neutral pulse within the period T. The duration of the period T was 10 seconds.

In one embodiment, the pulse durations of the pulse pattern ofFIG. 6C, in the order of occurrence in the pulse period T are as follows:
T1+=0.2 T; T1−=0.05 T; T2+=0.6; T2−=0.05 T; Tp=0.1 T
where T2− is the second occurrence of a negative pulse within the period T.

Further testing was conducted using the highly preferred pulse pattern in which T1+=0.2 T; Tp1=0.05 T; T2+=0.7; T−=0.02 T; Tp2=0.3 T, and wherein the duration of the period T was 10 seconds. For the test using this pulse pattern, the height of the water in the inner tank increased by 420 mm. This represented a 25% increase over the 310 height increase that was obtained using a pulse pattern 95-1-4, for which the period T was ten seconds. As has been stated above, the height increase provided using the pulse pattern 95-1-4 was ten times greater than that produced using the known pulse pattern of 70-10-20.

The results of the previously discussed laboratory test with pulse durations as shown inFIGS. 6B and 6Cshow an even greater rate of increase in osmotic pressure, the water level reaching 40 mm of after 48 hours and continuing to rise thereafter to 420 mm in a twelve day test period.

An important advantage of using the method and apparatus of the invention for dehydrating porous material is that power consumption is less than that for known electro-osmotic processes. The pulse patterns and pulse pattern durations according to the invention cause a higher level of moisture to be removed from the porous structure in a shorter period of time as compared to results obtained using known techniques. As the porous material becomes dryer, electrical power use decreases automatically due to the lack of moisture and increased resistance of the porous material.

The present invention also reduces the relative humidity of the porous structure to a stable level of about 50 percent, compared to stable levels of about 80 percent or greater left by prior art methods and apparatus. Tests were conducted to determine the reduction in relative humidity levels provided by different pulse patterns. The tests were conducted in a containment structure having porous walls that are normally subjected to very high relative humidity, at levels exceeding 90 percent. In conducting these tests, a humidity sensor was inserted into a hole approximately 1 inch deep in a wall of the containment structure.

One test conducted used the known pulse pattern as disclosed in U.S. Pat. No. 5,368,709, having the characteristic ofFIG. 5, with a positive duration T+ of 70 percent, a negative duration T− of 10 percent and a neutral duration Tpof 20 percent. The pulse pattern was applied to the anode relative to the cathode. The duration of the pulse pattern was 1.3 seconds. This pulse pattern (70-10-20) resulted in a reduction in relative humidity of the porous material to a stable level of about 79 percent.

Further testing was conducted using the pulse pattern in accordance with the invention which had the characteristic ofFIG. 6B, in which the positive pulse duration of each pulse period T is divided into separate positive pulses T1+ and T2+, separated by a short duration pulse neutral pulse Tp1and followed by a negative pulse T− which, in turn, was followed by a further neutral pulse Tp2. For one test that was conducted, the pulse pattern was: T1+=0.15 T; Tp1=0.05 T; T2+=0.60; T−=0.05 T; Tp2=0.15 T. The duration of the period T was 10 seconds. For this testing, the humidity sensor was located in the same hole in the wall of the structure that was used for the test that used the known pulse pattern. The pulse pattern was applied to the anode relative to the cathode. The further testing was conducted after a delay in time, following the test using the known pulse pattern, sufficient to allow the relative humidity of the porous material to be returned to near 90 percent. This pulse pattern (15-5-60-5-15) resulted in a reduction in relative humidity of the porous material to a stable level of about 52 percent. In general, mold growth cannot be supported below 65 percent relative humidity. It is well known that harmful fungus cannot survive or grow at relative humidity levels provided by the present invention. In addition, recent testing has confirmed that electro-osmotic pulsing can be used to deter radon gas penetration. Preliminary results have shown a potential 85% reduction in radon diffusion when the electro-osmotic pulsing system is in operation. Thus, the present invention can be used for reducing radon in homes and other buildings. Accordingly, contaminated structures can now be saved using the present invention.

FIG. 8shows a circuit diagram illustrating another feature of the invention, wherein a sensing system40is connected in the circuit between the anodes4and the cathode5for sensing various parameter values to determine if the electro-osmotic system is functioning properly. If the parameter values sensed exceed predetermined limits, the sensing system40produces an alarm signal on the line42which is input to power control unit6for producing either a visual alarm in the form of a light44or an audible alarm46.

The sensing system40can sense, for example, when there is a loss of output power in power control unit6. Loss of output power would be indicated if the voltage between the anodes4and the cathode5is zero for a predetermined period of time. Additionally, the sensing system40can determine if an upper voltage of lower voltage limit is exceeded, whether an upper current limit is exceeded or whether there is a sudden current drop, all of which can indicate a system error. The sensing system40can be implemented by a microcontroller which can be programmed for sensing the above parameters as well as other important system parameters. The power control unit6can itself have internal sensors for activating visual alarm44or audible alarm46. For example, the power control unit6can include a temperature sensor (not shown) for producing a signal if the temperature within the power control unit6exceeds a predetermined limit indicating a system problem.

The electro-osmotic dehydration methods and apparatus of the invention are not only applicable to conventional masonry and concrete, but can be applied to any containment structure made of porous material including, for example, a clay containment structure of a landfill.

The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications can be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications as to fall within the true spirit of the invention.