Measurement of fluid properties

Abstract

And measuring fluid characteristics. A method and apparatus for measuring characteristics of a fluid using an integrator connected to a central electrode of a contact conductivity sensor surrounded by an external electrode.

Description

Technical Field

[0001] This disclosure relates to methods for measuring properties of fluids such as resistance, conductivity, and the like. Background Technology

[0002] This section provides background information relevant to this disclosure, which is not necessarily prior art.

[0003] Contact conductivity sensors are in direct contact with the medium. Non-contact conductivity sensors also exist, also known as toroidal conductivity sensors or inductive conductivity sensors.

[0004] Contact conductivity sensors are ideally suited for measuring the resistivity / conductivity of fluids ranging from pure and ultrapure water to seawater, rinse water, and chemical solutions. One of the challenges of conductivity sensors in the past has been their ability to measure fluids with a wide dynamic range. For example, a preferred dynamic range for measuring conductivity is:

[0005] Electrical conductivity: 0.01 uS / cm to 1,000,000 uS / cm

[0006] Although the preferred range for resistivity can be:

[0007] Resistivity ranges from 100 MOhm*cm to 1 Ohm*cm

[0008] According to Ohm's law, if a voltage of V = 1V (volt) is applied across the two ends of a conductor and a current of 1A (ampere) flows through the conductor, then the conductor has a resistance of R = 10hm.

[0009] When 1 = V / R, the higher the resistance, the lower the current, or in order to establish a certain current with R fixed, the correct voltage level must be applied.

[0010] The reciprocal of the resistance that impedes the flow of current through a conductor is the conductance C = 1 / R, which represents the ease with which current flows through the conductor.

[0011] Resistance or conductance is also specific to the material that constitutes a “conductor or resistor”.

[0012] Resistance R = (rho) * L / S, where:

[0013] Rho = ρ is a Greek letter that represents a material-specific property of electrical resistance, called resistivity.

[0014] L represents the length of the conductor.

[0015] S represents the area of ​​the conductor's cross-section.

[0016] A simple rearrangement of the terms in the equation gives us:

[0017] ρ = R / [L / S]. If the dimension is defined in cm, then the unit used for ρ will be [Ohm*cm].

[0018] As the reciprocal of resistance, and also a specific property of materials, conductivity is:

[0019] C = c * S / L, where:

[0020] Conductivity is measured in Siemens units [S].

[0021] c is called electrical conductivity, and its unit of measurement is [S*1 / cm].

[0022] When attempting to measure the resistance or conductance of a fluid medium, the challenge becomes mechanically defining the body of the fluid whose resistance or conductance will be measured.

[0023] If two 1 sq. cm plates are provided facing each other with a distance of 1 cm, the resistance or conductance of the fluid can be measured by applying 1 V and measuring the current flowing through it. Once the current is measured, the resistance or conductance can be determined.

[0024] Once R is known, the resistivity of the fluid can be characterized as

[0025] ρ=R / [L / S]=R / [cm / cm*cm]=Ohm*cm

[0026] In inverse applications of conductivity and electrical conductivity:

[0027] C=C / [S / L]=C / [cm*cm / cm]=S*1 / cm

[0028] In practical applications, due to the wide range of dielectric conductivity / resistivity, the units used will be:

[0029] For conductivity: μS / cm; mS / cm; S / cm, suitable for most applications.

[0030] For resistivity: MOhm*cm, used for ultrapure water, where 18.18 MOhm*cm is used as the standard value for ultrapure water at 25°C.

[0031] Conductivity can be used in ultrapure water measurements because the c value used for UPW is 0.055 uS / cm at 25°C, but resistivity is more readily accepted by the UPW industry and is easier to express numerically.

[0032] To address the difficulty of dynamic range, the concept of a cell constant can be used, which reduces the dynamic range of the actual resistance input value:

[0033] Unit K = 0.01; Conductivity range: 0.01 μS / cm to 100 μS / cm; R input = 1 MOhm to 100 Ohm

[0034] Unit K = 0.1; Conductivity range: 1 μS / cm to 1000 μS / cm; R input = 100 KOhm to 100 Ohm

[0035] Unit K = 1; Conductivity range: 10 μS / cm to 10000 μS / cm; R input = 100 KOhm to 100 Ohm

[0036] Unit K = 10; Conductivity range: 100 μS / cm to 200,000 μS / cm; Input R = 100 KOhm to 50 Ohm

[0037] These unit constants are a portion of the ratio of the distance between the two electrodes to the size of the surfaces they face.

[0038] In the techniques for designing these electrodes with various unit constants, it will be observed that adjusting the distance / surface ratio becomes challenging when the conductivity is high.

[0039] The need to use AC (alternating voltage / current) is due to the potential for dielectric dissociation, also known as electrolysis, if DC is applied for an extended period of time. This would lead to dielectric alteration and electrode corrosion caused by metal migration resulting from the electrolysis phenomenon.

[0040] The industry employs a very common method for measuring contact conductivity to address the aforementioned problems with the DC method. For example, two electrodes are driven by a well-controlled and known AC voltage, and by measuring the current generated by that AC voltage, the conductivity c is calculated as:

[0041] Conductivity C = I / V [uS]

[0042] Electrical conductivity c = C * unit K [µS / cm]

[0043] However, the industry has not yet provided a conductivity sensor that can measure a wide range of fluid conductivity or resistivity with great accuracy. Summary of the Invention

[0044] This section provides a general overview of this disclosure and is not a full disclosure of its entire scope or all its features.

[0045] According to the teachings of the present invention, a method and apparatus for measuring the properties of a fluid are provided. The apparatus includes a sensor having an external electrode and an internal electrode. The external electrode has an opening to allow fluid to pass through and contact a central electrode and the external electrode. An integrator serves as both positive and negative inputs and outputs. The central electrode is connected to the negative input of the integrator. Furthermore, the output of the integrator is a function of the fluid properties.

[0046] Further areas of application will become clear from the description provided herein. The descriptions and specific examples in this invention are intended for illustrative purposes only and are not intended to limit the scope of this disclosure. Attached Figure Description

[0047] The accompanying drawings described herein are for illustrative purposes only and are not intended to illustrate all possible implementations, nor are they intended to limit the scope of this disclosure.

[0048] Several views are shown throughout the accompanying drawings, with corresponding reference numbers indicating the relevant parts.

[0049] Figure 1 This is a cross-sectional view of an exemplary sensor;

[0050] Figure 2 This is a partial cross-sectional view showing the connections leading to the electrodes in the sensor;

[0051] Figure 3 This is a schematic diagram of a conceptual measurement circuit;

[0052] Figure 4A This is a schematic diagram of a measurement circuit according to a preferred embodiment of the present invention;

[0053] Figure 4B yes Figure 4A The continuation, Figure 4B This is a schematic diagram of a measurement circuit according to a preferred embodiment of the present invention; and

[0054] Figure 5 It is a waveform diagram. Detailed Implementation

[0055] Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0056] Measuring the conductivity of a fluid is a continuous operation, and it can be recognized that the method described above, which applies a DC voltage over a metal plate, may produce an electrochemical reaction (i.e., electrolysis) in the fluid, which would be harmful and unacceptable as an analytical measurement method.

[0057] It is understandable that the optimal dynamic range for measuring conductivity is very large.

[0058] Figure 1A type of conductivity sensor 10 is shown, in which the methods and apparatus of the present invention find particular application. The sensor 10 includes an external metal electrode 12. A central metal electrode rod 14 is centered within the middle of the electrode 12 by means of an insulator 16. Figure 2 As shown, external electrode 12 is connected to ground (sometimes referred to herein as SOL_GND "solution ground"). Central electrode 14 is coupled to the integrator input, as will be explained. The fluid F to be measured flows from the bottom opening of the external tube and exits through openings 18 and 20 in the external electrode 12 via sensor 10 around the central electrode 14. It may also be mentioned that the central electrode rod also serves as a thermal well for the RTD (resistivity-temperature device), since the conductivity / resistivity values ​​are strongly temperature-dependent, and the measured T is used for compensation. Furthermore, the RTD value can be measured using a precision ADC (typically 1000 Ohms, and 100 Ohms at 0°C), but the RTD can be fed as an additional R input to the integrator MUX input.

[0059] According to the teachings of the preferred embodiment, a measuring device for sensor 10 is provided, which utilizes a high-quality operational amplifier configured as an integrator, such as... Figure 3 Described, of which:

[0060] R = = = represents the resistance of the bulk fluid.

[0061] U1 is a high-quality operational amplifier.

[0062] C is a precision capacitor (typically COG / NPO type, designed for high precision and temperature stability).

[0063] S is a switch used to set the start and stop (open-close-open). When closed, the R value is very low, which is eliminated through a calibration process.

[0064] t is the duration for which S is closed, determined by the value defined for Vout.

[0065] SOL_GND is the solution ground defined as the potential of the external electrode 12 of the sensor.

[0066] V2 is used to set the non-inverting reference voltage of the OpAMP.

[0067] Vout = = = is the target voltage at which the capacitor should be charged.

[0068] from Figure 3 This generates the following equation:

[0069] Ideal Op Amp === V1 = V2; i is generated by V2, i = 0

[0070] When S is closed: V1 = i1 * R

[0071] When i = 0, the summation of the currents at node V1 becomes: i1 = i2

[0072] ^^^i2=C*(V1-V0) / t=C*(V2-V0) / t

[0073] Combination^^^^^^V1 / R=C*(V2-V0) / t

[0074] According to the final equation: R = (t * V²) / C * ((V² - V₀))

[0075] This equation shows that if “t” can be measured accurately, then R can be measured because R and t are linearly proportional.

[0076] Implementation of a method for accurate "t" measurement

[0077] In order to have continuous measurement of R, “t” must be continuously measured in a controlled loop, and R is calculated by accurately measuring “t”.

[0078] Figure 4A and Figure 4B The diagram illustrates a preferred apparatus for this implementation, wherein:

[0079] • Input: Referencing SOL_GND (23) as further described, via MUXR (21), fluid R alternates with a precision reference resistor (R1ref, Riref, and Ri+1ref). (The RTD value to be measured can be added.)

[0080] • R input integrator (26) with time constant scaling via MUX C (28).

[0081] The system is powered by an isolated DC voltage, Vcc / Vss_GND. Isolation is preferred because the sensor (also known as contact conductivity) is in direct contact with the dielectric. Power is supplied to the IC from the Vcc / Vss power rail.

[0082] To allow for positive and negative integral output voltage, SOL_GND (also known as virtual GND) is placed between the values ​​of Vcc and Vss. Example: Vcc = 5V, Vss = 0, therefore SOL_GND = (5V – 0V) / 2 = 2.5V

[0083] • The threshold settings for the comparator (30) and the reference positive input for the integrator are also set with reference to SOL_GND.

[0084] • R1ref, Riref, and Ri+1ref are precision low TC (10ppm) time bases used for continuous calibration of the integrator (26) because capacitors are less reliable when it comes to accuracy and temperature stability. Calibration is performed continuously using an interpolation method for R input and Rrefi immediately following the higher and lower values.

[0085] The number of Ref resistors and capacitors (Ci+1 and Ci) is selected for optimal calibration and range selection.

[0086] The comparator (30) acts as the start and stop of integration, changing the output state based on the reference input.

[0087] The comparator output also sets the integrator sign and slope.

[0088] The Schmitt flip-flop (Trg) gate (32) improves the edge of the square wave for optimal input to the 32-bit timer (34) in CPU 36.

[0089] · Figure 5 The waveform depicts Figure 4A and 4B Typical behavior of the circuit.

[0090] The two electrodes of the contact conductivity sensor are connected to SOL_GND 23 via external electrode 12 and to the R input of MUX R 21 via internal electrode 14.

[0091] For a given fluid R at the input of integrator OP_AMP 26, combining R_input and C_to integrator 26 will generate a positive integral slope until a value is dictated by the threshold of comparator 30. The duration of this sequence is... Figure 5 The duty cycle is represented by X%.

[0092] When the output voltage of integrator 26 equals the threshold voltage, the output of comparator 30 changes state, and negative slope integration begins. The duration of this sequence is... Figure 5 The duty cycle Y% is represented in the middle.

[0093] A complete integration cycle containing both positive and negative integrals is represented by a full cycle of 100%. A complete cycle corresponds to two consecutive rising edges 22 and 24 of the comparator output.

[0094] 32-bit timer 34 ( Figure 4B The CPU counts the pulses between the two rising edges 22 and 24 at a frequency of 36, and generates variable COUNTS proportional to the R input of the multiplexer 22.

[0095] The CPU detects the falling edge at X% (Y%) and evaluates the value used for DUTY_CYCLE. The ideal DUTY_CYCLE for SOL_GND in the middle of the track is 50%.

[0096] The voltage value of SOL_GND 23 is prone to being affected by sudden changes in conductivity or other disturbances such as electrical interference or bubbles. This introduces counting errors, and therefore measurement errors. Figure 4B The circuitry also includes a method for controlling the SOL_GND voltage value so that DUTY_CYCLE is always maintained at 50%. This ensures the integrator operates symmetrically, and there are no errors in the R input attributable to waveform asymmetry.

[0097] The summation point 38 of R1, R2, and R3 controls the SOL_GND value. R1 and R2 are the intermediate values ​​of the divider rails (which are 2.5V or Vcc / 2), while R3 can be injected with a voltage higher or lower than this intermediate value to adjust the SOL_GND value used for 50% duty cycle.

[0098] The voltage injected through R3 is generated by CPU_DAC 40. Control of the DAC output is the result of the CPU-implemented PI(D) controller 42, where the error term is: Error = Duty Cycle X% - Duty Cycle Y%. P and I are the tuning factors, the rate of change, and the timing used for error re-evaluation to achieve optimal response to SOL_GND control.

[0099] For systems with secondary SOL_GND feedback, the SOL_GND voltage value is measured by the CPU's ADC 44, which is used to verify system sanity.

[0100] A 32-bit timer 24 can count: MAX CNT = 2^32 = 4,294,967,296, which represents extremely high resolution.

[0101] Example: For a CPU running at 100MHz, a 36-bit timer will take 42.95 seconds to complete counting.

[0102] The frequency domain of the integrator circuit described above is carefully selected to meet the basic requirements of liquid analysis measurements. These requirements are low detectability, accuracy, repeatability, stability, and the fastest possible response time. The method of the preferred embodiment addresses these requirements.

[0103] As a numerical example:

[0104] Example #1

[0105] Input R = 1K; this value can be measured using any unit K conductivity sensor.

[0106] Assume we have a 10nF capacitor in the integrator.

[0107] CPU operating frequency = 16MHz

[0108] Integrator frequency = 1.784kHz

[0109] The 32-bit timer will count 8967 times every 560.45 uses.

[0110] For 8967 counts, the resolution will be well within a good SNR (signal-to-noise ratio), just as for 1000 counts, 10 CNT represents 1%.

[0111] Example #2

[0112] Input R = 10 Ohm; this value can be measured using only a conductivity sensor with unit K = 10.

[0113] Assume we have a 10nF capacitor in the integrator.

[0114] CPU operating frequency = 16MHz

[0115] Integrator frequency = 178.42kHz

[0116] The 32-bit timer will count 90 times every 5.6 seconds.

[0117] For 9 counts, the resolution will not necessarily be within a good SNR range, just as with 90 counts, 9 CNTs represent 10%. High frequencies will cause the comparator's inherent delay to become equivalent to the total cycle time, thus degrading accuracy. In this case, we connect a Ci+1 capacitor in parallel (C becomes C = Ci + Ci+1) to increase the time constant (reduce the frequency).

[0118] For example, if we add a 90nF parallel capacitor, C = 100nF, the frequency becomes 17.842kHz with 900 counts, and it is easier to achieve the target accuracy of 1% for this extended measurement range.

[0119] Example #3

[0120] Input R = 182 kOhm; this value represents the resistivity of ultrapure water at 25°C when measured using a conductivity sensor with unit K = 0.01.

[0121] Assume we have a 10nF capacitor in the integrator.

[0122] CPU operating frequency = 16MHz

[0123] Integrator frequency = 9.8Hz

[0124] The 32-bit timer will count 1,632,048 times every 102.003 ms.

[0125] Of these, 1,632,048 have very high counting resolution.

[0126] Each cycle takes 102ms, and a typical response time of 500ms is sufficient.

[0127] As another example, sensor 10 is used to measure a conductivity of 1 μS / cm, which, according to existing knowledge, is known to be capable of measuring resistance between 1 MΩ and 100 Ohm.

[0128] For a sensor with a unit constant of 0.01 [1 / cm], the measurement is:

[0129] A conductivity of 1 μS / cm is equivalent to measuring a 10K resistor applied to the input of an integrator.

[0130] • Assume we have Rref_1 = 100K (accuracy of 0.1% and 10ppmTC)

[0131] • Assume we have Rref_2 = 5K (accuracy 0.1% and 10ppmTC)

[0132] The two Rrefs are the two closest values ​​to the 10K to be measured, with their values ​​increasing and decreasing.

[0133] • Assume C = 1 / nF

[0134] Output value:

[0135] Rref1=100k; t=0.178kHz; CNT=89673

[0136] Rref2=5k; f=3.568kHz; CNT=4484

[0137] Rin presents an output of 8967 CNT.

[0138] Determine the slope and offset of Rref1 and Rref2:

[0139] The calculated R = 1.115167 * CNT value - 0.410851

[0140] Rin=1.115167*8967-0.410851=10.00033K

[0141] Accuracy = 0.0033%

[0142] Conclusion based on the example:

[0143] As mentioned in the functional description, in order to cover the full R input from 100 Ohm to 1 MOhm, the operating frequency of the integrator needs to be carefully selected to meet all requirements.

[0144] This method encompasses a full range of contact conductivity sensors, measuring conductivity / resistivity as well as temperature using RTD elements (resistive temperature devices) with linear R characteristics and values ​​of 100 Ohm, 1000 Ohm, 3000 Ohm, or 10 KOhm as multiplexed inputs.

[0145] The foregoing description of embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or limiting of this disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but are interchangeable and can be used in chosen embodiments where applicable, even if not specifically shown or described. This can also be varied in many ways. Such variations should not be considered as departing from this disclosure, and all such modifications are intended to be included within the scope of this disclosure.

Claims

1. An apparatus for measuring the properties of a fluid, the apparatus comprising: A sensor having at least an external electrode and an internal central electrode, the sensor being configured to allow fluid to pass through and contact the central electrode and the external electrode; An integrator has positive and negative inputs and outputs; One of the electrodes is connected to the negative input of the integrator, and the other electrodes are connected to the solution ground; and The output of the integrator is a function of the fluid properties.

2. The apparatus according to claim 1, further comprising: A comparator is connected to the output of an integrator, the comparator having an output coupled to the positive input of the integrator, the output changing state according to the output of the integrator.

3. The apparatus according to claim 2, further comprising: A Schmitt flip-flop gate is connected to the output of the comparator, and the Schmitt flip-flop generates a pulse for each state change of the comparator. and A timer, connected to the output of a Schmitt flip-flop gate, counts the pulses coming from the Schmitt flip-flop gate; and The pulse count from the timer is a function of the fluid's properties.

4. The apparatus according to claim 3, further comprising: A multiplexer has an output connected to the negative input of an integrator. The multiplexer has multiple inputs, with a first input connected to the central electrode of a sensor and other inputs connected to one or more reference resistors. The multiplexer selectively switches the inputs to the negative input of the integrator.

5. The apparatus according to claim 4, further comprising: The first capacitor is connected between the output of the integrator and the negative input of the integrator.

6. The apparatus according to claim 5, further comprising: Second capacitor; and A multiplexer is connected between the first and second capacitors and the negative input of the integrator.

7. The apparatus according to claim 1, wherein, The integrator output is configured to generate an output with a positive slope, followed by an output with a negative slope, and The counter is configured to count pulses from the Schmitt trigger between the start of a positive slope and the end of a negative slope.

8. The apparatus according to claim 1, further comprising: Adjustment component, used to adjust the potential on the external electrode.

9. The apparatus according to claim 8, wherein, Adjustment components include: The summation point of the first, second, and third resistors; and The central processing unit (CPU) has a digital-to-analog converter (DAC) connected to the first resistor.

10. The apparatus of claim 9, further comprising: An analog-to-digital converter in the CPU that is connected to the voltage associated with the voltage applied to the external electrodes of the sensor.

11. An apparatus for measuring the properties of a fluid, the apparatus comprising: A sensor having an external electrode and an internal central electrode, wherein the external electrode has an opening to allow fluid to pass through and contact the central electrode and the external electrode; An integrator has positive and negative inputs and outputs; The central electrode is connected to the negative input of the integrator; A comparator connected to the output of an integrator, the comparator having an output coupled to the positive input of the integrator, the output changing state according to the output of the integrator; A Schmitt flip-flop gate is connected to the output of the comparator, and the Schmitt flip-flop generates a pulse for each state change of the comparator. A timer, connected to the output of a Schmitt flip-flop gate, counts the pulses coming from the Schmitt flip-flop gate; and The pulse count from the timer is a function of the fluid's properties.

12. The apparatus of claim 11, further comprising: Adjustment component, used to adjust the potential on the external electrode.

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