Abstract:
A method for measuring the purity of water in which the water is passed through an insulated pipe (1) in a cell spaced between electrodes (2-3) for measuring the tribo-electric electromotive force (e) generated by the flow, and wherein the resistivity of the water is calculated from the measured electromotive force.

Description:
This is a continuation of application Ser. No. 380,674, filed May 10, 1982, which was abandoned upon the filing hereof. 
    
    
     This invention relates to the measurement of water purity, particularly where the purity is required to be measured in fractions of a part per million. 
     BACKGROUND 
     With the enormous expansion of activity in Semiconductor Technology over the last decade or so has come the need for chemicals of hitherto unheard of purity. We see reference to materials which typically have purity levels measured in fractions of a part per million being used routinely in wet-way chemical processes on the silicon wafers. These materials are used in aqueous solutions, which in turn places a very considerable emphasis on the purity of the solvent, water. The present of traces of inorganic contaminants in both the chemicals being used and in the water can lead to the irreversible modification of the semiconductor materials during processing. 
     The normal technique to asses the purity of water is to measure the concentration of the contaminants in their ionised state using standard conductivity methods. This method, while being most satisfactory at normal impurity levels such as in tap water, distilled water, and the like, becomes extremely difficult when the water approaches the ultimate in chemical purity with resistivities of 10&#39;s of MΩcm, the level required in the semiconductor industry. 
     In the course of our research we have found that a tribo-electric e.m.f. which is generated due to friction in flowing water particularly when it is transported through insulating plastic pipes, can be used to measure water purity. In fact, this e.m.f. can cause damage to highly reflecting surfaces in water cooled pumping cavities in the solid laser. According to this invention use is made of this tribo-electric effect to measure the resistivity of the water, a function inversely of the ionic contamination and hence its purity, in a very simple measuring cell. 
     According to this invention the resistivity of water is measured by flowing water through a pipe of insulating material having linearly spaced electrodes, and measuring the tribo-electric e.m.f. generated by the flowing water, and calculating the resistivity of the water from this measurement. 
     In specific applications, such as the semiconductor technology referred to above, where generally the pipes are looped and the two ends are at the same potential, the invention is modified by having electrodes arranged to provide differing characteristics from adjacent sections of the cell so formed. This can be achieved by using materials of different characteristic or by having differing diameters in the cell to give a different velocity and hence achieve a different tribo-electric e.m.f. 
     While the invention has been based on the semiconductor industry, application of this effect could be widespread in many chemical plants and other industries. 
     Thus the invention can be applied as a method for measuring the purity of water which comprises flowing the water through a cell and measuring the tribo-electric e.m.f. generated by the flowing water, and calculating the resistivity of the water, the measuring of a tribo-electric e.m.f. being effected by flowing the water through a cell comprising a pipe of insulating material arranged between electrodes to which the measuring device is attached. 
     In the case where the two ends of the pipe are at a common potential, two dissimilar cell sections are preferably used which vary by wall characteristic, such as by using for instance a material known under the trade mark &#34;Delrin&#34; for one section of the cell and material known under the trade mark &#34;Perspex&#34; for the other section of the cell, arranged coextensive the one with the other with an end electrode at each end of the cell so formed and a further electrode between the two sections. 
     Instead of using two sections of the cell of dissimilar material the two sections can be of similar material but vary from each other by the internal diameter of the pipe sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic central section of a cell formed of a section insulating material having an electrode at each end, B showing the equivalent electrical circuit, and C the measuring circuit, 
     FIG. 2A is a similar view to FIG. 1A but showing the assembly used when the pipe is looped, in which case two insulating sections with electrodes at each end are used, B again showing the equivalent circuit, and C the measuring circuit of this type of cell. 
     FIG. 3 is a further schematic view showing a cell with varying diameters as opposed to varying materials, the equivalent circuits being similar to B and C of FIG. 2. 
     FIG. 4 is a central longitudinal section showing how the device of FIG. 1 can be constructed, and 
     FIG. 5 is a view similar to FIG. 4 but showing construction of a cell as shown in FIG. 3. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a pipe section 1 of insulating material having electrodes 2 and 3 at the ends connected between pipes 4 and 5 through which the water flow occurs. As shown in FIG. 4 this can be constructed by threading the two ends of the plastic pipe section 1 and screwing to it a pair of electrodes 2 and 3 into which the ends of the pipes 4 and 5 project. Wedge nuts 6 and 7 lock the pipes 4 and 5 in position and effect a seal. 
     The device of FIG. 2 has two pipe sections 10 and 11 of insulating material which differ in their physical characteristic using three electrodes 12, 13 and 14. Electrodes 12 and 14 are connected externally by an electrical short circuit, and the electrodes 12 and 14 are joined to the ends of the pipes 15 and 16. Readings are taken between electrode 13 and either electrode 12 or electrode 14. The readings taken between the center electrode 13 and either of electrodes 12 or 14 are achieved by connecting the measuring means (not shown in FIG. 2A) in the manner described in FIG. 2B. The actual construction of this can again be similar to the form illustrated in FIG. 4 excepting that a central electrode would be used, screwing into both of the pipe sections 10 and 15 of different insulating material. 
     The form illustrated in FIG. 3 uses pipe sections 20 and 21 of different dimension with electrodes 22, 23 and 24 and fitted between the ends of the pipes 25 and 26, could be similar to the form illustrated in FIG. 5, in which two end electrodes 22 and 24, joined by an external sleeve 27 and in which pipe sections 20 and 21 of similar insulating material are disposed with the central electrode 23 between them, the end electrodes 22 and 24 again having sockets to take the ends of the pipes 25 and 26 which are locked to the electrodes 22 and 24 by wedge nuts 28 and 29 which screw on to the electrodes 22 and 24. Insulation 30 isolates the central electrode from the sleeve 27. 
     The method of calculating the water purity is as follows: 
     Referring first to the simple cell of FIG. 1, the equivalent electrical circuit may be considered as shown in FIG. 1B and the measuring circuit FIG. 1C where 
     e=generated voltage 
     R=Resistance of water column, function of ρ 
     l=the length, 
     d=the diameter of the cell 
     R in  =voltmeter input resistance--tends to infinity and hence can be ignored 
     R m1  R m2  =Load resistors 
     To obtain a value for R, the water resistance, two measurements are made using two resistors R m1  and R m2  giving two voltages V 1  and V 2  respectively. 
     R may then be derived from the following relation: ##EQU1## If dissimilar plastics are used for the pipes or if the flow rates are modified in certain areas of the pipe by changing the diameter, then once again the e.m.f. can be measured. 
     When the cell is of dissimilar materials and uniform flow velocity, the two outer electrodes a and c are short circuited electrically and the measurement is made across b and c using the formula: ##EQU2## 
     When using similar materials but different diameters the measuring circuit of FIG. 2C applies and the formula from which R can be calculated is as follows: ##EQU3## 
     From R the water resistivity, ρ can be calculated as: ##EQU4## 
     Cells having an optimum of output voltage for a given system may be designed. A cell can be tailored by choice of cell length, diameter, and insulating materials. 
     Practical results have shown that a simple cell having the configuration as in FIG. 4 and inserted in a working system in which the system &#34;loops&#34; are very large and hence electrically isolated has worked extremely well. Results of checks against a conventional conductivity cell system are shown in table 1. The tribo-electric cell was inserted in a working water treatment plant and the results calculated from the tribo-electric cell are shown against the readings given by the standard conductivity cell system. 
     
         ______________________________________           (MΩcm)Conductivity Meter           By tribo-electric(m cm)          cell readings______________________________________0.12            0.120.57            0.581.49            1.502.7             2.6No calibration  5.7on meterNo calibration  8.0on meterNo calibration  11.0on meter______________________________________ 
    
     Tests on the more complex system, FIG. 5 confirm the theoretical tribo-electric model of the system. The voltages generated by the differences of cell insulating materials, differing diameters and differing cell lengths have been as predicted. Inner wall finish of the plastic tube does not appear to have any marked effect in the first instance on cell voltage; in our measurements any variations are lumped with the &#34;tribo-electric&#34; constant K for that particular cell. In all tests, it is assumed that flow-rates have been maintained constant for the period of the measurement, several seconds only. 
     Although various forms of the invention have been described in some detail it is to be realised that the invention is not to be limited thereto but can include various modifications falling within the spirit and scope of the invention.