Measuring device and method for cleansing a measuring electrode in said device

Abstract: Measuring device including at least one measuring electrode (3-6) arranged in a conduit (1, 2) through which a liquid is intended to be flowed which can cause depositions on the electrode. The device is characterized by means (7, 8, 11) for connecting the measuring electrode (3-6) as an electrode in an electrolytic cell including a second electrode (13, 14 or 3-6), and by means for applying a voltage between these electrodes for cleansing of the measuring electrode. The invention also relates to a method of cleansing a measuring electrode, characterized in that the cleansing occurs with the help of said means (7, 8, 11).

Description: AREA OF INVENTION The present invention relates to a measuring device comprising at least one measuring electrode arranged in a liquid conduit through which a liquid is arranged to be flowed and in which the liquid itself can cause depositions on the electrode. The invention further comprises a method for cleansing the measuring electrode of the measuring device. The invention is principally intended to be used in connection with measuring flow rates, and specifically relatively small differences between two proportionally larger flows. The invention can, for example, be used to measure the ultrafiltration in dialysis, preferably haemodialysis. In haemodialysis a flow of pure dialysis fluid in the order of magnitude 0-1000 ml/min flows to the dialyser. This flow is increased from the dialyser with the ultrafiltration which usually is of the order of magnitude 0-40 ml/min or possibly somewhat higher. PRIOR ART In GB-B-2 003 274 and in GB-A-2 056 691 there are described two similar systems for electromagnetic flow measurements. Two parallel conduits of the flows, the differential flow rate of which is to be measured, are used in both systems. According to the first mentioned document, the measuring is effected with the help of three electrodes which are influenced depending on the flowing medium and an external magnetic field. According to the latter document two main electrodes are influenced in the same way, and which cooperate with a plurality of ground electrodes. Both systems suffer from the disadvantage that the flowing mediums can influence the two conduits and the electrodes respectively in different ways. If the constructions are used for example for measuring the ultrafiltration during dialysis, deposits from the impure dialysate are obtained on the electrodes and/or on the conduit walls themselves. A deposit of the order of few µm or even thiner layers can hereby result in considerable reading errors. An improvement to the two above mentioned systems is described in EP-B1-0 106 940, whereby possible reading errors are compansated for by means of a repeated calibration. EP-A2-337 292 discloses a method and an apparatus for cleansing the electrodes of such a measuring device by connecting the measuring electrodes in an electrolytic cell and applying a voltage to the cell. The electrodes will be cleansed by the current passing through the electrolytic cell. DISCLOSURE OF THE INVENTION The object of the present invention is to improved the above described systems. Thus there is provided a measuring device including at least one measuring electrode arranged in a conduit through which a liquid is arranged to be flowed which can cause depositions on the electrode, means being provided for connecting the measuring electrode as an electrode in an electrolytic cell including a second electrode, and by means for applying a voltage between these electrodes for cleansing of the measuring electrode. At the cathode, i.e. the negative pole, the water's hydrogen is reduced to hydrogen gas during simultaneous formation of hydroxide ions. A portion of this hydrogen gas is dissolved in an atomic state in the underlying electrode metal (adsorption) whilst the remaining portion of the produced hydrogen gas forms bubbles on the electrode surfaces. The hydrogen gas bubbles are formed directly on the electrode's outermost atomic layer and therefore effectively drive away dirt from the surface at the same time that the solution is forceably agitated very close to the metal. During the cleansing, the conduit in which the measuring electrode is arranged must be filled with a solution, for example water, dialysis solution or some other electrolyte which can conduct current between the anode and the cathode. The temperature must be at least such that the solution or the electrolyte is in its liquid phase. A temperature higher than this facilitates the cleansing. The pressure on the liquid in the conduit must not be higher than that hydrogen gas bubbles are formed. The second electrode is preferably a ground electrode. It can however, alternatively, be another measuring electrode. Hereby, use can possibly be made of a low frequency alternating current so that both the measuring electrodes are cleansed alternately. The device can also include two or more measuring electrodes. It is hereby suitable to connect these in parallel via the liquid to one or more ground electrodes for simultaneous cleansing. The hydrogen gas which has dissolved in the cathode metal can, together with the oxygen gas which has been precipitated at the anode or the anodes, form a galvanic element when the electrolytic voltage has been terminated. This galvanic element can disrupt or disturb the operation of the flow measuring device. According to the invention, the device comprises means for shortcircuiting the connected electrodes after the cleansing for dispelling any hydrogen, respectively oxygen, formed in these, whereby the short-circuit is arranged to be maintained during a time period in the order of magnitude 1-60 seconds, preferably at least 5 seconds. Alternatively, the device comprises a means for applying a voltage to the connected electrodes after the cleansing in opposite direction to the cleansing voltage during a time period in the order of 1-60 seconds, preferably at least 5 seconds. By these measures any hydrogen or oxygen which may be formed during the electrolysis are dispelled. Moreover, there is provided, according to the invention, methods for cleansing the electrodes as mentioned in the claims. Further features appear from the subclaims. Further objects, advantages and features are more clearly explained in the ensuing description of a preferred embodiment with reference to the appended drawings. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a sketch of a flow conduit with two included measuring electrodes and two ground electrodes. Figure 2 shows a block diagram of a more complete device according to the invention with two flow conduits. PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 shows a flow conduit 1 with two measuring electrodes 3 and 4 and two ground electrodes 13 and 14. A similar conduit 1 with measuring electrodes 3 and 4 is also shown in figure 2, further showing a flow measuring device which also includes a second conduit 2 with two measuring electrodes 5 and 6. The measuring process can be performed in the manner described in more detail in the above mentioned patent publications and, thus, does not need to be described in more detail herein. In short, the four electrodes 3-6 are connected to a connecting device 7, the purpose of which is to supply the voltages which are required for the cleansing step, and to connect the electrodes to a measuring device, respectively. The measuring device is represented in figure 2 by the components 9 and 10, which, in turn, are connected to a microprocessor 11 or similar such device which is arranged to control both the measuring and the cleansing steps. Voltages required for the cleansing step are achieved with the help of a control device 8 which is connected to the measuring electrodes 3-6 via the device 7 and to one or both of the ground electrodes 13 and 14. The latter are not shown in figure 2, but connection thereto is represented by reference numeral 15. Reference numeral 12 denotes an external control input to the control device 8. Finally it can be noted that the conduits 1 and 2 are under the influence of an electro-magnetic field in such a way as described in more detail in the above mentioned patent publications which are hereby included in the present description. The invention is naturally not restricted to solely the above described example and the examples which are described in said patent publications, but is defined by the following claims. For example the invention should be suitable for general cleansing of any type of measuring electrode which is in contact with a liquid.

Claims: Measuring device including at least one measuring electrode (3-6) arranged in a conduit (1, 2) through which a liquid is arranged to be flowed which can cause depositions on the electrode, means (7, 8, 11) being provided for connecting the measuring electrode as an electrode in an electrolytic cell including a second electrode (13, 14 or 3-6), and by means (8) for applying a voltage between these electrodes for cleansing of the measuring electrode, characterized by means (7, 8, 11) for shortcircuiting the connected electrodes (3-6, 13, 14) after the cleansing, whereby the shortcircuit is arranged to be maintained during a time period in the order of magnitude 1-60 seconds, preferably at least 5 seconds. Measuring device including at least one measuring electrode (3-6) arranged in a conduit (1, 2) through which a liquid is arranged to be flowed which can cause depositions on the electrode, means (7, 8, 11) being provided for connecting the measuring electrode as an electrode in an electrolytic cell including a second electrode (13, 14 or 3-6), and by means (8) for applying a voltage between these electrodes for cleansing of the measuring electrode, characterized by means (7, 8, 11) for applying a voltage to the connected electrodes (3-6, 13, 14) after the cleansing in opposite direction to the cleansing voltage during a time period in the order of 1-60 seconds, preferably at least 5 seconds. Device according to claim 1 or 2, characterized in that the measuring electrode (3-6) is arranged to be connected as a cathode, i.e. with negative potential, during the cleansing. Device according to claim 1, 2 or 3, characterized in that the second electrode (13, 14) is in the form of an ground electrode. Device according to anyone of the previous claims, including two or more measuring electrodes (3-6), characterized in that said measuring electrodes are arranged to be connected in parallel via the liquid to one or more ground electrodes (13, 14) for simultaneous cleansing. Device according to anyone of the previous claims, characterized in that said means (8) for applying a voltage is arranged to apply a voltage in the order of magnitude of 0,5-15V during at least 0,1-5 seconds. Device according to anyone of the previous claims included in a flow measuring device including two conduits (1, 2) through which different liquid flows are arranged to be flowed and each of which is provided with two measuring electrodes (3, 4 and 5, 6, respectively) which are intended to measure the flow difference between the two conduits (1, 2) which are arranged to be exposed to an electromagnetic field, characterized in that all the measuring electrodes (3-6) are connected in parallel to respective ground electrodes (13, 14) connected to the liquid flows. Device according to anyone of the previous claims, characterized in that it is arranged to measure the differences in the liquid flows to, and from, a dialyser intended for haemodialysis, respectively. Method for cleansing a measuring electrode in a measuring device according to anyone of claims 1,3-8, including the steps of flowing a liquid beyond the measuring electrode, connecting the measuring electrode as an electrode in an electrolytic cell including a second electrode, and applying a voltage between these electrodes for cleansing of the measuring electrode, characterized by shortcircuiting the connected electrodes after the cleansing during a time period in the order of magnitude 1-60 seconds, preferably at least 5 seconds. Method for cleansing a measuring electrode in a measuring device according to anyone of claims 2-8, including the steps of flowing a liquid beyond the measuring electrode connecting the measuring electrode as an electrode in an electrolytic cell including a second electrode, and applying a voltage between these electrodes for cleansing of the measuring electrode, characterized by applying a voltage to the connected electrodes, after the cleansing, in opposite direction to the cleansing voltage during a time period in the order of 1-60 seconds, preferably at least 5 seconds.

Inventors: BERTINSSON GERT-INGE, MATTISSON LEIF, OHLSSON GOERAN, BERTINSSON, GERT-INGE, MATTISSON, LEIF, OHLSSON, GOERAN, Ohlsson, Göran
Applicants: GAMBRO AB
Application year: 1991
Main classification: G01F15
IPC codes: G01F15, G01F1