Bias current control for potentiometric measurements

Abstract: Apparatus and method are provided to ensure accuracy of a measurement signal obtained using a potentiometric sensor (10) placed in a solution (12). The apparatus includes an interface amplifier (26) that generates a relatively large turn-on bias current whenever the amplifier is initially powered on. To avoid polarization of the potentiometric sensor (10) by the turn-on bias current, a switch device (40) is used to direct the turn-on bias current along a path away from the potentiometric sensor (10). The switch device (40) is controlled using a timer circuit (42) that causes a change in position of the switch device (40) after a predetermined time delay. During the time delay, the bias current reduces to a normal value and polarization of the potentiometric sensor (10) has been avoided. An accurate measurement of a physical parameter associated with the solution can then be taken without the detrimental effect of the turn-on bias current on the potentiometric sensor. The apparatus has particular utility in obtaining water quality data, such as hydrogen potential, specific ion potential and oxygen reduction potential.

Description: The present invention relates to an apparatus for taking measurements of physical parameters and particularly to taking measurements using a potentiometric sensor located in a solution while avoiding inaccuracies due to bias currents that could polarize the sensor when power is applied to the apparatus. It is often desirable to be able to obtain information directed to the quality or physical characteristics of a solution. In one application, it is worthwhile to obtain water quality data for surface and/or ground water environments. Such data might include hydrogen potential (pH), specific ion potential (pIon) and oxygen reduction potential (ORP), and other information of that nature. In connection with obtaining such data, potentiometric sensors can be used to measure the data. Potentiometric sensors are a class of sensors with characteristically high source impedances (in the range of about 10⁸ ohms) and which measure galvanic potential based on the chemical activity in the solution. In connection with developing a measurement signal representative of the physical parameter using the potentiometric sensor, interface amplifiers are available for amplifying the measurement signal and which have input impedances of 10¹³ ohms or greater and bias currents of 10⁻¹² amperes or less. A high input impedance is required for the interface amplifier because of the high source impedance of the potentiometric sensor. It is also important that bias currents be minimal to avoid polarizing the solution in the vicinity of the potentiometric sensor. Polarization occurs when a direct current is applied to the solution at a rate greater than the ion migration rate within the solution. Polarization causes a cloud of ions of opposite polarity to form about the potentiometric sensor. Because of such ion formation, the measurement signal would have a value different from the true or actual value of the physical parameter of the solution being measured. Typically, potentimetric measurements are made in laboratory environments in which the measuring instrument or equipment power is applied prior to taking measurements and a "warm-up" time of 5-10 minutes does not detrimentally impact system performance. Such a warm-up period would result in the stabilization of the potentiometric sensor wherein any polarization thereof, which would affect the accuracy of the measurement signal, would be dissipated and no longer present. Recently, however, newer developments of instrumentation involving potentiometric sensors have evolved where such instruments are battery powered. Such instruments include potentiometric sensors that are located in the field or in an environment in which the sensor or equipment need only be powered on for a short time interval within which to obtain the measurement and then is powered down for the remaining time. In such a case, it is detrimental to system performance, particularly battery life, to require or utilize a warm-up period of 5-10 minutes. Such a warm-up period, would result in an Unacceptable drain on the battery energy and no accurate and useful measurement would be obtainable during this warm-up period. There is known already from SU-A1-1469431 an apparatus that includes a potentiometric sensor suitable for measuring a physical parameter associated with a solution. An amplifier is responsive to the potentiometric sensor. The amplifier outputs a signal relating to the physical parameter. The amplifier generates a bias current. The apparatus also includes a voltage setter for powering the amplifier. In order to reduce the effect of polarising current on the potentiometric sensor, the polarisation current source is enveloped by negative feedback through a comparator and an adder. There is also known from GB-A-1138518 a meter for making pH and/or Redox measurements in which the pH or Redox potential to be measured is applied to a field effect transistor device. In using the field effect transistor device, warm-up drift is substantially eliminated and thermal drift can be reduced to virtually zero by a suitable choice of the operating point of the device. The field-effect transistor Q1 is arranged to communicate with an electrode unit from which pH and/or Redox measurements can be made. An object of the present invention is to provide an apparatus for eliminating, or at least substantially reducing, polarisation of the potentiometric sensor utilized in providing a measurement signal. In accordance with the present invention, this is accomplished by controlling the path of bias current generated when the apparatus is powered on. More particularly, in addition to the potentiometric sensor, the apparatus includes an interface amplifier communicating with the potentiometric sensor. The interface amplifier amplifies the measurement signal, which is obtained using the potentiometric sensor. The amplified measurement signal is an accurate analog representation of the physical parameter or parameters being measured and can then be further processed so that it is understandable to the user of the present invention. In one embodiment, such information relates to water quality data to assist the user in evaluating the characteristics of a particular water supply or reservoir. With regard to ensuring the accuracy of the measurement signal, the key aspect of the present invention involves the control of bias currents that are generated when the apparatus is initially turned on. That is, it has been observed that, when the apparatus is first powered on, a turn-on bias current is generated which is several orders of magnitude larger than the normal bias current of the amplifier. Such a bias current tends to polarize the potentiometric sensor. If such polarization is permitted to occur and insufficient time is allowed to elapse after such polarization has occurred and before the measurement signal is obtained, an inaccurate measurement signal results. More specifically, the turn-on bias current introduces inaccuracies into the measurement signal whereby the measurement signal does not accurately reflect the actual or true value of the physical parameter being measured. Instead of the potentiometric sensor providing a signal that is accurately indicative of the physical parameter present in the solution, it outputs a signal that has been affected by the polarization of the potentiometric sensor due to the turn-on bias current. In accordance with a first aspect of the present invention, there is provided an apparatus for measuring a physical parameter associated with a solution comprising a potentiometric sensor means suitable for measuring a physical parameter associated with the solution, an amplifier means responsive to said potentiometric sensor means for outputting a signal relating to the physical parameter, said amplifier means generating a bias current, and power supply means connected to said amplifier means for powering said amplifier means, wherein a current diverting means is connected between said potentiometric sensor means and the input of said amplifier means, with said current diverting means having a first position and a second position, and in that a timer circuit controls said current diverting means such that, when the apparatus is initially turned on, said current diverting means attains said first position wherein it separates said potentiometric sensor means from the amplifier input and diverts said bias current away from said potentiometric sensor means but, after a predetermined time, said timer circuit causes said current diverting means to be changed to said second position in which said potentiometric sensor means is coupled to the input of said amplifier means so that an accurate measurement can be obtained. In a preferred embodiment, the amplifier means has a non-inverting input and, when said current diverting means attains said first position, said non-inverting input is connected via said current diverting means to earth potential, and when said current diverting means attains said second position, said non-inverting input is connected via said current diverting means to said potentiometric sensor means. Advantageously, the current diverting means comprises a switch. The potentiometric sensor means advantageously includes first and second probes and in said second position of said current diverting means for said amplifier means communicates with said first probe obtaining of a measurement of the physical parameter. The amplifier means preferably includes gain means which include a resistor means. In accordance with a second aspect of the present invention, there is provided a method for reducing measurement inaccuracy in a measuring system which uses a potentiometric sensor to measure a physical parameter associated with a solution, the sensor being coupled to the input of an amplifier means powered by a power supply means, such that in use a bias current flows from the amplifier means to the sensor, characterised in that, when the supply of power from the power supply means to the amplifier means is initially turned on, the connection between the sensor and the amplifier means is not made until after a predetermined time has elapsed following the turning on operation, the input of the amplifier means, and hence the bias current, being arranged to be diverted during this period whereby such bias current is prevented from flowing from the amplifier to the sensor. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing, which schematically illustrates one embodiment of hardware for obtaining the measurement signal, together with the turn-on bias current control for preventing, or at least reducing, polarization of the potentiometric sensor in accordance with the invention. In accordance with the present invention, an apparatus is provided for use in obtaining measurements of a physical parameter. The physical parameter is preferably associated with a solution. In one main application, the solution includes water and the physical parameter being measured includes hydrogren potential (pH), specific ion potential (pIon) or oxygen reduction potential (ORP). Such water-related data is of value in informing the user of the apparatus about underground or surface water quality or characteristics. Based on water quality data, determinations can be made by the user concerning important attributes associated with the water. With reference to the sole figure, the apparatus includes a transducer device 10 for measuring a physical parameter of the solution 12 and outputting a measurement signal. The measurement signal is an electrical signal whose amplitude represents the value or magnitude of the physical parameter being measured. In the preferred embodiment, the transducer device 10 is a potentiometric sensor, which is able to output an accurate measurement signal within seconds of power application to the measurement circuitry. Potentiometric sensors typically have high source impedances (10⁸ ohm) and measure galvanic potential in a solution due to chemical activity therein. In the embodiment illustrated in the figure, the potentiometric sensor is a dual probe device, which is capable of measuring the hydrogen potential (pH) of the solution 12. That is, the transducer device 10 includes a first or measurement probe 14 and a reference probe 16. In some configurations the measuring probe 14 and the reference probe 16 are physically packaged together as a combination probe. The physical parameter associated with the solution 12 can be measured due to the potential difference between the two probes 14, 16. In the case where the physical parameter is hydrogen potential, the potential difference between the two probes 14, 16 results in the generation of a measurement signal outputted by the measurement probe 14 to be carried by the measurement signal conducting line 18 for further processing. The reference probe 16 is typically in electrical communication with a common or ground line 20. The measurement signal conducting line 18 and the common line 20 are part of a cable connected to the potentiometric sensor 10. The apparatus also includes an interface amplifier circuit 24. The interface amplifier circuit 24 includes an amplifier 26 which communicates with the potentiometric sensor 10. Specifically, the measurement signal conducting line 18 is electrically connectable to the non-inverting input of the amplifier 26. In the embodiment illustrated, the interface amplifier circuit 24 also includes first and second gain resistors 28, 30. The first gain resistor 28 is connected to the output of the amplifier 26 while the second gain resistor 30 is connected to the first gain resistor 28 at one side and to the common or ground line at its opposite side. Feedback to the amplifier 26 is provided by the feedback line 34, which electrically connects the junction of the first and second gain resistors 28, 30 and the inverting input of the amplifier 26. In the preferred embodiment, the resistance values of the gain resistors are chosen to provide the usually desired amplifier gain associated with the interface amplifier circuit 24. The gain of the amplifier circuit is determined by the relationship: Gain = 1 + first gain resistor 28/second gain resistor 30. It should be understood that various and other amplifier circuits could be employed other than the particular amplifier circuit 24 illustrated in the figure. It is important that the amplifier circuit 24, however, have a high input impedance because of the high source impedance of the potentiometric sensor 10. Such interface amplifiers 26 typically have an input impedance of 10¹³ ohms or greater, with normal bias currents of 10⁻¹² amperes or less. A bias current is generated when the amplifier 26 is powered on, which bias current flows in the conducting line 18 between the non-inverting input of the amplifier 26 and the potentiometric sensor 10. The actual direction of bias current flow will depend on the characteristics of amplifier 26. The bias current is identified in the figure as I bias . With regard to powering on the amplifier 26, during experimentation using the apparatus, it was observed that a turn-on bias current in the microampere range was initially generated before this current stabilized to the nanoampere or feptoampere range. The relatively high bias current, in the microampere range, was not expected and the turn-on bias current was maintained at that level during the initial several milliseconds after application of power to the amplifier 26. This observation relating to turn-on bias current was noted in both monolithic and hybrid types of amplifiers. It was also noted that the rate or slope of the power supply application appeared to have little or no effect on the generation of the turn-on bias current in the microampere range. It was further observed that, when insufficient stabilizing or settling time was utilized after powering on the amplifier 26, inaccurate measurement signals were being provided by the potentiometric sensor 10 and outputted by the amplifier 26. If, on the other hand, sufficient time was permitted to elapse before taking a measurement after powering on the amplifier 26, a measurement signal accurately representing the value of the physical parameter being measured in the solution 12 was obtained. The conclusion reached as a result of the investigations and experiments was that the initial pulse of amplifier bias current was polarizing the potentiometric sensor 10 or that it was charging a stray capacitor, or a combination thereof. The capacitor 36 represents stray circuitry capacitance including capacitance associated with the cable carrying the measurement signal. The high source impedance of the potentiometric sensor 10, in conjunction with cable and stray circuitry capacitance 36, create time constants of a magnitude that can require 1-5 minutes for the measurement signal to stabilize to a final, accurate value, after the polarization effect, due to the turn-on bias current, has dissipated. In the preferred embodiment, because it is beneficial that the apparatus, particularly the amplifier 26, be battery powered, it is not satisfactory to permit or require that the apparatus have a relatively long warm-up period, after power is applied to the amplifier 26. If such were the case, there would be relatively extensive battery energy being utilized to obtain the measurement. Consequently, a solution was sought to permit the use of battery power, while preserving or optimizing battery life. With further reference to the figure, the present invention includes further circuit hardware for controlling or directing the bias current so that it does not flow to the potentiometric sensor 10 and cannot cause polarization thereof. More specifically, the apparatus includes a switch device 40 having at least two positions or states. In a first position of the switch device 40, that portion of the conducting line 18 connected to the non-inverting input of the amplifier 26 is electrically connected to a common or ground line. In this position, the non-inverting input of the amplifier 26 is not electrically connected to the potentiometric sensor 10 and therefore any bias current that flows from the amplifier 26 towards the potentiometric sensor 10 would not be able to continue along that path; rather, any such bias current is routed to a common or ground line using the switch device 40. The first position of the switch device 40 is illustrated in the figure. With respect to the second position of the switch device 40, a current path is provided between the potentiometric sensor 10 and the non-inverting input of the amplifier 26. In this position, the measurement signal from the potentiometric sensor 10 electrically communicates with the amplifier 26. To achieve the major objective of the present invention, when the apparatus, particularly the amplifier 26, is powered on, the switch device 40 is arranged to be in the first position whereby any bias current from the amplifier 26 passes to the common or ground line by means of the first position of the switch device 40. After the bias current has dissipated, or at least substantially dissipated, the switch device 40 can then be caused to change to its second position whereby a measurement signal can be obtained. With regard to changing the position of the switch device 40 from its first position to a second position, the apparatus also includes a timing circuit 42, which electrically communicates with the switch device 40. The timing circuit 42 monitors time and, after the passage of a predetermined time interval, triggers the changing of the switch device 40 from its first position to its second position. In one embodiment, the switch device 40 is a relay which includes an electrical coil that, when energized by the timing circuit 42, causes the switch to change position. When the electrical coil is de-energized, the switch position changes back to its first or original state. In a similar embodiment, the relay could be a latching relay that would be pulsed to cause the switch to change positions. With respect to providing power to the apparatus, the figure illustrates a DC battery 44 which electrically connects to the amplifier 26 through a switch 46. The switch 46 acts to prevent drain on the battery when no measurement signal is being obtained. When a measurement is being taken, the switch 46 changes state to permit the battery voltage to be applied to the amplifier 26. At the same time the amplifier 26 is turned on, the timing circuit 42 is activated to begin monitoring time, during which time the switch device 40 remains in the first position so that the turn-on bias current has a path away from the potentiometric sensor 10. At the end of the predetermined time interval, typically about 1 millisecond to several milliseconds, the timing circuit 42 causes the switch device 40 to change state whereby the measurement probe 14 once again communicates with the non-inverting input of the amplifier 26. In conjunction with applying the battery power from the battery 44 to the amplifier 26, a separate system, such as a processing system, electrically communicates with the switch 46. The user is able to control the position of the switch 46 using the processing system so that the switch 46 only permits the application of power to the amplifier 26 when a measurement is being taken. In operation, the apparatus is normally powered off and in this state, the switch device 40 is normally in the first position. Consequently, when the apparatus is first turned on so that the battery 44 applies power to the amplifier 26 through the switch 46, the switch device 40 is in its first position. As previously noted, in this state, the bias current that is generated upon the initial application of power to the amplifier 26 flows along that portion of the conducting line 18 to the ground or common line. The bias current that is generated continues to flow along this path until it is dissipated, or at least substantially dissipated. The dissipation time corresponds to the predetermined time interval that is being monitored by the timing circuit 42. At the end of the predetermined time interval, the portion of the timing circuit 42 for controlling the state of the switch device 40 from its first position to its second position is activated. When activated, the switch device 40 is in its second position and the measurement signal from the potentiometric sensor 10 has a path to the non-inverting input of the amplifier 26. The measurement signal is amplified by the interface amplifier circuit 24 and outputted from the amplifier 26 for further processing. In one embodiment, the output of the amplifier 26 is applied to the processing system, which can process the measurement signal to present information in a form suitable and understandable to the user. In one embodiment, such a processing system is described in U.S. Patent Application serial No. 07/150,050, filed January 29, 1988, and entitled "Accurate Analog Measurement System," which application has been assigned to the present Applicants. After the desired measurement or measurements have been taken for a desired or predetermined amount of time, the switch 46 can be caused to change state whereby power is removed from the amplifier 26 and the timing circuit 42. When this occurs, the switch device 40 changes state back from the second position to the first position. The apparatus is now once again in the condition that it was in before enabling it for obtaining a measurement. In view of the foregoing detailed description, a number of advantages of the present invention are readily seen. A battery powered apparatus is provided for obtaining measurements of physical parameters in the field. Battery life is preserved by controlling unwanted bias current so that the turn-on bias current does not polarize a potentiometric sensor resulting in an inaccurate measurement being taken, in the absence of a relatively lengthy warm-up time. As a consequence, there is no battery drain due to any need for a warm-up period before an accurate measurement signal can be obtained.

Claims: An apparatus for measuring a physical parameter associated with a solution comprising a potentiometric sensor means (10) suitable for measuring a physical parameter associated with the solution, an amplifier means (24) responsive to said potentiometric sensor means (10) for outputting a signal relating to the physical parameter, said amplifier means (24) generating a bias current, and power supply means (44) connected to said amplifier means for powering said amplifier means (24), characterized in that a current diverting means (40) is connected between said potentiometric sensor means (10) and the input of said amplifier means (24), with said current diverting means (40) having a first position and a second position, and in that a timer circuit (42) controls said current diverting means (40) such that, when the apparatus is initially turned on, said current diverting means (40) attains said first position wherein it separates said potentiometric sensor means (10) from the amplifier input and diverts said bias current away from said potentiometric sensor means (10) but, after a predetermined time, said timer circuit (42) causes said current diverting means (40) to be changed to said second position in which said potentiometric sensor means (10) is coupled to the input of said amplifier means (24), so that an accurate measurement can be obtained. An apparatus, as claimed in claim 1, wherein: said amplifier means (24) has a non-inverting input and, when said current diverting means (40) attains said first position, said non-inverting input is connected via said current diverting means to earth potential. An apparatus, as claimed in claim 1 or 2, wherein: said amplifier means (24) has a non-inverting input and, when said current diverting means (40) attains said second position, said non-inverting input is connected via said current diverting means to said potentiometric sensor means (10). An apparatus, as claimed in claim 1, 2 or 3, wherein: said current diverting means (40) comprises a switch. An apparatus, as claimed in any of claims 1 to 4, wherein: said potentiometric sensor means (10) includes first and second probes (14,16) and wherein, in said second position of said current diverting means (40), said amplifier means (24) communicates with said first probe (14) for obtaining of a measurement of the physical parameter. An apparatus, as claimed in any of claims 1 to 5, wherein: said amplifier means (24) includes gain means including resistor means (28,30). An apparatus as claimed in any of claims 1 to 6, wherein: said potentiometric sensor means (10) includes at least one probe (14) for measuring hydrogen potential, specific ion potential or oxygen reduction potential. A method for reducing measurement inaccuracy in a measuring system which uses a potentiometric sensor (10) to measure a physical parameter associated with a solution, the sensor (10) being coupled to the input of an amplifier means (24) powered by a power supply means (44), such that in use a bias current flows from the amplifier means to the sensor, characterised in that, when the supply of power from the power supply means (44) to the amplifier means (24) is initially turned on, the connection between the sensor and the amplifier means is not made until after a predetermined time has elapsed following the turning on operation, the input of the amplifier means, and hence the bias current, being arranged to be diverted during this period whereby such bias current is prevented from flowing from the amplifier to the sensor. A method, as claimed in claim 8, wherein: said input of said amplifier means (24) is connected to earth potential during said delay time following the turning on operation. A method, as claimed in claim 1, wherein: said delay time is preselectable by a timing circuit.

Inventors: MCKEE CHARLES B, MCKEE, CHARLES B.
Applicants: IN SITU INC, IN-SITU, INC.
Application year: 1990
Main classification: G01N27
IPC codes: G01N27