Solution-free sensor calibration

Abstract

A calibration device for a conductivity sensor for aqueous solutions, the calibration device including a first connector configured to electrically couple to sensor electrodes of the conductivity sensor. The connector is further configured to provide a controlled voltage application between at least two of the sensor electrodes. A first resistor is connected between the at least two sensor electrodes and coupled to the first connector. The resistor has a value such that a current flow between the at least two sensor electrodes replicates the characteristics of an electrochemical calibration solution. In certain embodiments, the calibration device is configured for a pH sensor or a dissolved oxygen sensor.

Description

[Technical Field]

[0001] The present invention generally relates to systems and methods for the calibration of conductivity sensors for aqueous solutions, or any electrochemical sensor technology that senses and converts changes in potential (voltage) or electron loss / gain into measurements of conductivity, pH, dissolved oxygen (DO), or other applicable response. [Background technology]

[0002] The conductivity of an aqueous solution is a measure of the water's ability to conduct electric current. The more ions present in a solution, the higher its conductivity. Temperature also affects conductivity. As the temperature of a solution increases, there is a corresponding increase in the solubility of materials dissolved in the solution, resulting in an increase in conductivity.

[0003] For electricity to flow, the movement of charged particles (e.g., ions) is necessary. Electrical flow does not occur in solid NaCl or KCl crystals. Although the solids are composed of ions, they are held so tightly in the crystal lattice that no current can flow. However, in aqueous solutions, these ions can move, allowing electrical flow.

[0004] Conventional manufacturing processes for aqueous conductivity sensors require that the conductivity solution be within a predetermined acceptable conductivity and temperature range in order to calibrate the sensor. Ensuring that these requirements are met during sensor calibration presents several challenges. For example, the conductivity of the conductivity solution can be highly variable, and in some cases, conductivity fluctuations are observed after replenishing the conductivity solution between sets. Furthermore, maintaining a relatively constant solution temperature during calibration without fluctuations (i.e., due to adjustments in water bath temperature) can be difficult.

[0005] Furthermore, good calibration of the sensor can depend on the accuracy of reference third-party conductivity and temperature probes. Routine external calibration of the third-party probes is necessary to maintain specifications. However, in many instances, when multiple third-party probes are compared to each other, they do not agree with each other in terms of temperature or conductivity readings.

[0006] Many conventional calibration devices experience temperature loss along the sensor chain, which can be difficult to control. Typically, the conductivity solution must be virgin and NIST-certified. After calibration, the sensor must be thoroughly cleaned, which increases the cost and manufacturing time of conventional conductivity sensors. Furthermore, a stabilization time is required for the sensor body and conductivity solution to reach the required temperature. This can require the operator to constantly monitor the sensor and solution temperature.

[0007] As a result of the above issues, in some cases, the calibration pass rate for newly manufactured conductivity sensors can be as low as 25-30% on the first calibration attempt. Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION Embodiments of the present invention relate to systems and methods for calibrating conductivity sensors for aqueous solutions that address many of the problems discussed above. These and other advantages of the present invention, as well as further inventive features, will be apparent from the description of the invention provided herein. [Means for solving the problem]

[0009] In one aspect, an embodiment of the present invention provides a calibration device for an aqueous conductivity sensor. The calibration device includes a first connector configured to electrically couple to sensor electrodes of the conductivity sensor. The connector is further configured to provide a controlled voltage application between at least two of the sensor electrodes. A first resistor is connected between the at least two sensor electrodes and coupled to the first connector. The resistor has a value such that a current flow between the at least two sensor electrodes replicates the characteristics of an electrochemical calibration solution.

[0010] In certain embodiments, the calibration device has a second connector connected to the resistor and also connected to a conductivity monitor. In certain embodiments, the first connector is configured to electrically couple to at least four sensor electrodes of the conductivity sensor, and the calibration device further includes a resistor network having the first resistor, a second resistor, and a third resistor, where each of the three resistors is coupled between a different two of the four sensor electrodes. In certain embodiments, the first resistor has a resistance of 40.2 ohms, and the second and third resistors each have a resistance of 221 ohms.

[0011] In another embodiment of the present invention, the first resistor has a resistance value of 38 to 42 ohms. In a more specific embodiment, the first resistor has a resistance value of 40.2 ohms. The calibration device may further include a temperature calibration circuit configured to simulate a 25°C environment for the conductivity sensor being calibrated. In some embodiments, the temperature calibration circuit includes a switching element coupled to a pair of resistors. In a specific embodiment, one of the pair of resistors has a resistance value of 200 to 300 ohms, and a second of the pair of resistors has a resistance value of 1.5 to 2.5 kilohms. In a more specific embodiment, one of the pair of resistors has a resistance value of 226 ohms, and a second of the pair of resistors has a resistance value of 2.1 kilohms. The calibration device may further include terminals configured for connection to an external power source.

[0012] In another aspect, embodiments of the present invention provide a method for calibrating a conductivity sensor for aqueous solutions, the method including applying a voltage between two electrodes of the conductivity sensor to replicate the properties of an electrochemical calibration solution, measuring a current flowing between the two electrodes, and determining whether the conductivity sensor is well calibrated based on whether the measured current is within a predetermined range.

[0013] In some embodiments, the method further includes connecting a resistor between two electrodes. The method may also include providing a resistor network connecting at least one resistor to each of a plurality of electrodes of the conductivity sensor. Further, embodiments of the method require simulating a 25° C. environment for the conductivity sensor to be calibrated.

[0014] Certain embodiments of the method include connecting a calibration device to a plurality of electrodes of the conductivity sensor, the calibration device including one or more resistors coupled to each of the plurality of electrodes. The method may further require connecting the calibration device to a temperature calibration circuit to simulate a 25° C. environment for the conductivity sensor being calibrated.

[0015] In yet another aspect, embodiments of the present invention provide a method for calibrating a conductivity sensor, the method comprising applying a controlled voltage between two electrodes of the sensor, sensing a current between the two electrodes while maintaining the sensor at a controlled temperature, and calculating the conductivity of the sensor based on whether the sensed current is within a predetermined range indicating the sensor is acceptable. This method embodiment includes applying a constant voltage from an external power source.

[0016] Certain embodiments of the present invention include a calibration device for a conductivity sensor for aqueous solutions that applies a controlled external voltage between two sensor electrodes, or in an alternative embodiment, provides a current between the two sensor electrodes with a series of resistors and / or capacitors to replicate the properties of an electrochemical calibration solution, thereby eliminating the need for direct contact between the solution and the sensor electrodes during the calibration process.

[0017] Other aspects, objects and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

[0018] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate several aspects of the invention and, together with the description, serve to explain the principles of the invention. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a perspective view of a calibration device according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating a resistor network used in the calibration device of FIG. 1, according to one embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram illustrating how resistor networks are coupled to first and second connectors according to one embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram of a temperature calibration circuit used in the calibration device of FIG. 1, according to one embodiment of the present invention. [Figure 5] FIG. 5 is a schematic diagram of a calibration device for use in a conductivity sensor according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] While the present invention will be described in terms of specific preferred embodiments, it is not intended to limit the invention to those embodiments, but rather to cover all alternatives, modifications, and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims.

[0021] In certain embodiments, the present invention includes a conductivity sensor for aqueous solutions that supports a solution-free calibration process by applying a controlled external voltage between two of a plurality of sensor electrodes. In embodiments of the present invention, the controlled external voltage is a constant voltage, which can be provided by an external power source. In alternative embodiments, a current can be supplied between the two electrodes. The sensor electrodes can include a series of resistors and / or capacitors to replicate the properties of an electrochemical calibration solution, thereby eliminating the need for direct contact between the sensor electrodes and the electrochemical calibration solution during the calibration process.

[0022] Although the embodiments of the invention described herein relate to conductivity sensors, those skilled in the art will recognize that the teachings and scope of the invention are equally applicable to measuring pH and / or dissolved oxygen, or any other application involving sensing changes in potential (voltage) or loss / gain of electrons.

[0023] The relationship between current and voltage at a particular nominal conductivity can generally be characterized in the following manner: The sensor resistance is related to the conductivity reading as follows: Conductivity=(Ic-Im) / V; where Ic is the raw current from the Analog to Digital Converter (ADC); Im is the zero-level excitation current; V is the raw voltage from the analog-to-digital converter (ADC).

[0024] As used herein, "Zero Level of Excitation Current" refers to the small amount of current that flows in a transformer primary winding when normal voltage is applied to the primary winding terminals, with the secondary winding terminals open. This current is called the transformer excitation current and flows constantly during transformer operation. The excitation current is necessary to maintain the magnetic field inside the transformer core and is generally independent of the load on the secondary winding.

[0025] In one specific example of conventional calibration of a conductivity sensor, a one-point calibration may be performed using a solution with a conductivity of 12.88 millisiemens (mS). In this example, when the voltage is 33,872±1,069 volts and the current is 394,871±53,662 milliamps, the conductivity will read 12,880 microsiemens (μS) or 12.88 millisiemens (mS). To indicate a well-calibrated sensor, the current level sensed and measured by the conductivity monitor when a voltage is applied should be within the range of 394,871±53,662 milliamps. Voltage is regulated for 12880µS (from table). Nominal conductivity: 12880μS.

[0026]

number

[0027] Current values ​​higher than expected are an indicator that there may be a problem with the calibration equipment or conductivity sensor, and such a problem requires further testing.

[0028] In accordance with the present invention, the conductivity calibration process was designed as an alternative to calibration using a solution with a known conductivity of 12.88 millisiemens (mS) at 25°C. However, as discussed above, variations of this process can be used as an alternative to traditional calibration processes for pH or DO sensors. To accomplish this for conductivity, the claimed system operates to simulate the properties of a solution of known conductivity. The claimed system and method allow for a more controlled and repeatable calibration system process (at ambient room temperature) as opposed to existing calibration system processes that require the use of a known 12.88 mS solution in a temperature-controlled chamber at 25°C.

[0029] FIG. 1 is a perspective view of a calibration device 100. As shown in FIG. 1, calibration device 100 does not include a housing, although it is envisioned that a commercial embodiment of device 100 will be disposed within a housing. The embodiment of calibration device 100 shown in FIG. 1 includes a circuit board 102 configured to connect to a conductivity sensor on a first side of circuit board 102 and to a conductivity monitor (not shown) on a second side of circuit board 102 opposite the first side. However, it is envisioned that in alternative embodiments of the invention, the sensor and monitor may be connectable to the same side of circuit board 102. In alternative embodiments of the invention, circuit board 102 is configured to connect to a pH sensor, a DO sensor, or the like.

[0030] The calibration device 100 achieves the above-mentioned objectives by utilizing two subcircuits: one passive subcircuit for conductivity and one active subcircuit for temperature compensation. The passive conductivity subcircuit consists of a resistive network corresponding to a conductivity of 12.88 mS, which will be described in more detail below. During the calibration process, the resistive network is connected to the electrode signal line of the conductivity sensor. This generates a conductivity feedback response corresponding to 12.88 mS, which is returned to the conductivity monitor.

[0031] In one particular embodiment, the conductivity sensor connects to the circuit board 102 at a first connector 104, and the conductivity monitor connects to the circuit board 102 at a second connector 106. Although the conductivity monitor is not shown, those skilled in the art will recognize that the conductivity monitor is used to enable a determination regarding the successful calibration of the conductivity sensor by detecting the current flowing between the electrodes of the sensor via the connection to the second connector 106. The conductivity sensor also controls the voltage applied to the conductivity sensor electrodes and initiates the operation of a temperature calibration circuit, described below.

[0032] 1, first connector 104 and second connector 106 are covered with caps 108 to protect the connector terminals when not in use. Calibration device 100 includes a resistor network having three resistors: R1 110, R3 112, and R5 114 connected between the conductivity monitor and the conductivity sensor. In a specific embodiment of the invention, R1 has a resistance of 221 ohms, R3 has a resistance of 40.2 ohms, and R5 has a resistance of 221 ohms.

[0033] Figures 2 and 3 are schematic diagrams of circuitry used in a calibration device 100 according to one embodiment of the present invention. Figures 2 and 3 show the resistor network described above with three resistors R1 110, R3 112, and R5 114. As shown in Figure 3, the three resistors R1 110, R3 112, and R5 114 are coupled to a first connector 104 connected to a conductivity sensor and also to a second connector 106 connected to a conductivity monitor.

[0034] The resistor networks arranged as shown in Figures 2 and 3 are useful in forming the claimed conductivity calibration system and method, which in this embodiment simulates a 12.88 mS solution with these particular resistors placed across the excitation (EX+ / -) and sense (SN+ / -) EX+, SN+, SN-, and EX- signal lines. In one particular embodiment, these four signal lines are each connected to a sensor electrode 120 of a conductivity sensor 118 (see Figure 5). Figures 2 and 3 show how three resistors R1 110, R3 112, and R5 114 are implemented across the signal lines. Resistor R1 110, having an exemplary value of 221 ohms, is connected between EX- and SNS-. Resistor R3 112, having an exemplary value of 40.2 ohms, is connected between SNS- and SNS+. Resistor R5 114, having an exemplary value of 221 ohms, is connected between SNS+ and EX-.

[0035] Since conductivity is temperature dependent, one of the requirements for good calibration of conductivity sensors relates to the temperature stability of the sensor body. Ideally, conductivity sensors are stored for long periods of time inside a temperature-controlled system at 25°C, and it is observed that the temperature of all conductivity sensors is stable (e.g., a thermal camera shows temperature uniformity).

[0036] FIG. 4 is a schematic diagram of a temperature calibration circuit 140 used in calibration device 100 according to one embodiment of the present invention. Temperature calibration circuit 140 simulates a 25° C. environment by applying a temperature resistor network to serial data (SDA) signal line 142 during calibration. In one specific embodiment of the present invention, the resistor network includes two resistors: R17 144 and R19 146. In one specific embodiment, R17 144 has a resistance of 226 ohms, and R19 146 has a resistance of 2.1 kilohms. Switch component U3 148 shown in FIG. 4 is triggered by the conductivity monitor (see pins 15 and 10 of U3) to apply temperature calibration circuit 140 during calibration. Temperature calibration circuit 140 targets 25° C. to effectively “trick” the sensor into believing it is at 25° C. during calibration.

[0037] In a specific embodiment, during the calibration process, an active subcircuit, the temperature calibration circuit 140, applies a firmware-controlled voltage bias to the temperature calibration circuit 140 of the conductivity sensor under test. This causes the temperature calibration circuit 140 of the conductivity sensor under test to respond with a voltage corresponding to 25° C., which is most commonly associated with ambient room temperature. In one particular embodiment, the voltage bias signal is delivered via the second connector 106, and the voltage is controlled by firmware within the conductivity monitor. This allows for precise timing for applying temperature compensation during sensor calibration and removing temperature compensation before writing the calibration offset value to the conductivity sensor's non-volatile memory.

[0038] In testing, the temperature calibration method described above using the temperature calibration circuit 140 has proven effective, as units calibrated using this method have been found to have comparable or better standard deviation percentages than conventionally calibrated conductivity sensors. After calibration, each conductivity sensor is given a unique calibration factor (CF) and temperature offset (TO) value that is applied to the readout output (conductivity reading). The operator then observes the expected CF and TO values ​​as an indication of whether to classify the conductivity sensor as passing or failing after calibration. Those that fail are typically isolated and inspected to determine their ultimate discard.

[0039] 5 is a schematic diagram of a calibration device 100 used in a conductivity sensor 118 according to one embodiment of the present invention. A schematic diagram of the calibration device 100 is shown, in which the calibration device 100 is connected to four electrodes 120 of the conductivity sensor 118. Alternative embodiments of the calibration device 100 may have fewer than four or more than four electrodes 120. In the embodiment of FIG. 5, the calibration device 100 is powered by an external power source 152, although it is envisioned that alternative embodiments of the calibration device 100 may be powered by, for example, a battery.

[0040] 5, the connector portion of the calibration device 100 is configured to apply a controlled voltage to at least two of the four electrodes 120 of the conductivity sensor 118. This connector portion may also provide the additional function of straightening the four sensor electrodes 120. This may be a useful feature because electrode spacing (i.e., diameter, height, and distance) is important in the conductivity sensor 118. If the electrodes 120 are too close together, current will arc between the electrodes 120, resulting in erroneous readings. If the electrodes 120 are spaced too far apart, current will not flow effectively between the electrodes 120, resulting in erroneous readings.

[0041] In testing, this conductivity calibration method proved effective. Conductivity sensors 118 calibrated by the solution-free method described herein reported conductivity values ​​that generally agreed with conventionally calibrated sensors. Overall, testing confirmed that the conductivity sensors 118 performed as well as or better than conventionally calibrated conductivity sensors.

[0042] Thus, as described above, by applying a controlled external voltage to at least two of the plurality of sensor electrodes 120 of the conductivity sensor 118 (which comprises a series of resistors), the solution-less calibration device 100 replicates the current carried by a conductivity solution, thereby eliminating the need for an actual solution when calibrating the conductivity sensor 118. The current can then be measured with a conductivity monitor to determine whether the sensor 118 is within a predetermined range of acceptable values.

[0043] Additionally, it can be seen that the claimed calibration device 100 provides certain other advantages related to the manufacture and calibration of conductivity sensors 118 for aqueous solutions. As explained above, in alternative embodiments of the present invention, these advantages can also be realized with respect to the manufacture and calibration of pH or DO sensors. Specifically, the claimed systems and methods reduce calibration costs, improve lead times to customers, increase yields, and reduce the risk of product contamination due to the calibration process. Additionally, the calibration device 100 improves the accuracy and tolerance of sensor specifications through more consistent calibration.

[0044] All references, including publications, patent applications, and patents, cited in this specification are herein incorporated by reference to the same extent as if each reference was individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0045] In the context of describing the present invention (particularly in the context of the claims below), use of the terms "a," "an," "the," "the," and similar referents shall be construed to encompass both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" shall be construed as open-ended terms (i.e., meaning "including, but not limited to"), unless otherwise indicated. The recitation of ranges of values ​​herein is merely intended to serve as a shorthand method of referring individually to each individual value falling within the range, unless otherwise indicated herein, and each individual value is incorporated herein as if it were individually set forth herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. Any and all examples or illustrative phrases (e.g., "such as") provided herein are intended merely to further clarify the invention and do not impose limitations on the scope of the invention unless specifically claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0046] Preferred embodiments of the present invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of these preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect that skilled artisans will adopt these variations as appropriate, and the inventors intend that the invention be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Furthermore, this invention includes any combination of the above-described elements in all possible variations thereof unless otherwise indicated herein or clearly contradicted by context.

Claims

1. 1. A calibration device for a conductivity sensor for aqueous solutions, the calibration device comprising: a first connector configured to electrically couple to sensor electrodes of the conductivity sensor, the first connector further configured to provide application of a voltage between at least two of the sensor electrodes; a first resistor connected between the at least two sensor electrodes and coupled to the first connector, the first resistor having a value such that when the voltage is applied between the at least two sensor electrodes, a current flow between the at least two sensor electrodes replicates the characteristics of an electrochemical calibration solution; and a temperature calibration circuit that applies a voltage bias to a separate temperature calibration circuit of the conductivity sensor to be calibrated via a second connector that is connected to the first resistor and also connected to a conductivity monitor, causing the separate temperature calibration circuit of the conductivity sensor to respond with a voltage corresponding to room temperature of 25°C; A calibration device comprising:

2. 2. The calibration device of claim 1, wherein the first connector is configured to electrically couple to at least four sensor electrodes of the conductivity sensor, and the calibration device further comprises a resistor network having the first resistor, a second resistor, and a third resistor connected in series, wherein the three resistors are each coupled between different two of the four sensor electrodes.

3. 3. The calibration device of claim 2, wherein the first resistor has a resistance value of 40.2 ohms, and the second and third resistors each have a resistance value of 221 ohms.

4. 2. The calibration device of claim 1, wherein the first resistor has a resistance value of 40.2 ohms.

5. 2. The calibration device of claim 1, wherein the temperature calibration circuit includes a pair of resistors connected in series and a switching element coupled to the pair of resistors, one end of one of the pair of resistors being coupled to a common terminal of the switching element and the other end of the one of the pair of resistors being coupled to the other common terminal of the switching element.

6. 6. The calibration device of claim 5, wherein one of the pair of resistors has a resistance value of 226 ohms and a second of the pair of resistors has a resistance value of 2.1 kilohms.

7. The calibration device of claim 1 , wherein the calibration device is powered by an external power source external to the calibration device.

8. 1. A method for calibrating a conductivity sensor for an aqueous solution, comprising: connecting a resistor between two electrodes of the conductivity sensor; applying a voltage between two electrodes of the conductivity sensor to reproduce the characteristics of the electrochemical calibration solution, the reproducing step including connecting a calibration device having a temperature calibration circuit to the conductivity sensor and applying a voltage bias to a separate temperature calibration circuit of the conductivity sensor via a connector connected to the resistor and also connected to a conductivity monitor, the voltage bias causing the separate temperature calibration circuit of the conductivity sensor to respond with a voltage equivalent to room temperature of 25°C; measuring the current flowing between the two electrodes; and determining whether the conductivity sensor is well calibrated based on whether the measured current is within a predetermined range; A method comprising:

9. The method of claim 8 , further comprising providing a resistor network connecting at least one resistor to each of a plurality of electrodes of the conductivity sensor, the resistor network including a plurality of resistors.

10. 9. The method of claim 8, wherein applying a voltage between two electrodes of the conductivity sensor to replicate the properties of an electrochemical calibration solution comprises connecting a calibration device to a plurality of electrodes of the conductivity sensor, the calibration device including one or more resistors coupled to each of the plurality of electrodes.

11. 1. A method for calibrating a conductivity sensor, comprising: a) connecting a resistor between two electrodes of the conductivity sensor and applying a voltage between the two electrodes of the conductivity sensor to reproduce the characteristics of an electrochemical calibration solution, wherein the step of applying a voltage to reproduce the characteristics of the electrochemical calibration solution comprises connecting a calibration device having a temperature calibration circuit to the conductivity sensor and applying a voltage bias to a separate temperature calibration circuit of the conductivity sensor to be calibrated through a connector connected to the resistor and also connected to a conductivity monitor, the voltage bias causing the separate temperature calibration circuit of the conductivity sensor to respond with a voltage equivalent to room temperature 25°C; b. sensing the current between the two electrodes; and c. determining whether the conductivity sensor is well calibrated based on whether the sensed current is within a predetermined range indicative of a passing conductivity sensor; A method comprising:

12. The method of claim 11 , wherein the step of applying the voltage comprises providing power from an external power source external to the calibration device.

13. The calibration device of claim 1 , wherein the temperature calibration circuit and the first resistor are disposed on the same circuit board.

14. The calibration device of claim 2 , wherein the temperature calibration circuit and the resistor network are located on the same circuit board.

15. 2. The calibration device of claim 1, wherein the temperature calibration circuit includes a network of two or more resistors connected in series and a switch component coupled to the network of resistors, one end of one of the resistors coupled to a common terminal of the switch component and the other end of the one of the resistors coupled to the other common terminal of the switch component.

16. The calibration device of claim 15 , wherein the switch component is configured to be triggered by a conductivity monitor connected to a second connector of the calibration device.

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