Patent Application
20200217698
The present invention relates generally to the operation of sensors for measuring the velocity of fluids and more particularly to magnetic flowmeters for performing fluid flow measurements.
Magnetic flow meters measure the velocity of conductive fluids passing through pipes by generating a magnetic field and measuring the resultant voltage. These flowmeters rely upon Faraday's Law in which the flow of a conductive fluid through a magnetic field causes a voltage signal which is sensed by electrodes. The sensed voltage is proportional to the velocity of the fluid.
Although these flowmeters are generally effective, shortfalls exist. For example, one limitation with current flowmeters is that the fluid media being measured must meet a minimum electrical conductivity level. If a fluid media falls below this minimum conductivity value it cannot be accurately measured. Furthermore, for fluids with low conductivity the sensed voltage UE must be given sufficient time to settle so that an accurate voltage measurement can be achieved. This time delay can be significant for low conductivity fluids and adversely affect the magnetic flow meter's sampling rate. This reduced sampling rate can in turn affect the measurement accuracy in applications where the fluid velocity changes rapidly and is non-continuous.
It should, therefore, be appreciated there is a need for a magnetic flowmeter assembly that addresses these concerns. The present invention fulfills these needs and others.
Briefly and in general terms, the present invention provides a system and related method for measuring the conductivity of a fluid media being measured by a magnetic flowmeter.
The system comprises a coil driver for providing a drive current to a coil assembly, an electrode for measuring an electrical signal created by a conductive fluid flowing through a magnetic field created by the coil assembly, and a micro-processor for controlling the magnetic flowmeter. The micro-processor determines the electrical conductivity of the fluid in response to the sensed electrical signal. The micro-processor then modifies the frequency of the coil driver responsive to the electrical conductivity of the fluid in order to optimize the flowmeter's sampling rate. Specifically, the flowmeter modifies the coil driver frequency by either increasing the drive frequency for highly conductive fluids or decreasing the drive frequency for less conductive fluids.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain advantages of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention disclosed herein. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment disclosed.
Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings:
In certain embodiments of the present invention, the magnetic flowmeter assembly can be configured as described and claimed in Applicant's co-pending patent application, entitled “FULL BORE MAGNETIC FLOWMETER ASSEMBLY, U.S. application Ser. No. 16/146,090, filed Sep. 28, 2018, which is hereby incorporated by reference for all purposes.
The conductivity of a fluid is its ability to conduct an electric current. A fluid's electrical conductivity is typically measured in Siemens per meter (S/m). A fluid's conductivity is generally a function of the total dissolved solids (TDS) in the fluid. For example, pure deionized water has a conductivity of approx. 5.5 μS/m while sea water with dissolved salts and other impurities has a conductivity of approx. 5 S/m (i.e., sea water is one million times more conductive than deionized water).
The conductance (C) of a fluid solution depends upon the strength and concentration of the electrolytes in the solution. Strong electrolytes typically include strong acids (e.g., HCI, H2SO4, HNO3), strong bases (e.g., LiOH, NaOH, KOH) and/or salts (e.g., NaCL, KNO3, MgCl2). These electrolytes completely ionize or disassociate in the solution and the suspended ions are good electrical conductors. In contrast, weak electrolytes never fully disassociate in the solution (i.e., form a mixture of ions and molecules in equilibrium). Weak electrolytes generally include weak acids (e.g., acetic acid, CH3COOH and phosphorous acid (H3PO4) and/or weak bases (e.g., NH3). In weak electrolytic solutions the concentration of ions is less than the concentration of the electrolyte itself.
The electrical conductance (C) of a solution is determined by measuring the resistance (R) of the solution between two electrodes which are separated by a fixed distance (l) with a conductivity meter.
As a rule of thumb, the minimum electrical conductivity of a fluid being measured with a magnetic flowmeter is 5 micro S/cm. Solutions with lower conductivities generally have voltage signals (UE) which are challenging to detect and difficult to measure accurately. Alternatively, fluids with high electrical conductivities have voltage signals (UE) which are consistent over time, have well defined voltage signals, and can be accurately measured.
Magnetic flowmeters rely upon Faraday's Law of Electromagnetic Induction to measure the velocity of the conductive fluid in flow path. Specifically, Faraday's Law states that the voltage induced across any conductor that moves at right angles through a magnetic field is proportional to the velocity of the conductor.
U
E
˜V×B×L
Where:
Alternatively, the fluid velocity
The flow of the conductive liquid passing through the magnetic field B creates a voltage signal UE which is sensed by the pair of measuring electrodes and which in-turn can be used to calculate the average velocity V of the fluid. Magnetic flow meters are generally very accurate (e.g., <1% measurement error).
As Faraday's equation illustrates, the average fluid velocity V is directly proportional to the induced voltage UE. We'll see shortly that the induced peak voltage (UE Peak) is a function of the fluid's electrical conductivity C. This relationship between induced peak voltage UE Peak and fluid conductivity C enables a fluid's electrical conductivity to be determined and operation of a magnetic flowmeter to be optimized based on the fluid media's conductivity.
Referring now to the drawings, and in particular
With continued reference to
It's also been discovered that the time delay (TD) which is the time necessary for the induced voltage UE measurement to settle and plateau (i.e., reach steady state) is greater for low conductivity fluids than for high conductivity fluids. The steady state time (TS) is the time during which an accurate flow measurement can be performed. As illustrated in
The two coil drivers (24, 26) are energy management IC's which provide an active power pulse output. The coil drivers can be embodied as H bridge drivers, configured with very low resistance and thus low voltage drop. As such, the coil drivers are capable of alternating the direction of the current passing through each coil assembly, and thereby impacting the direction of the magnetic field emitted from each coil. Alternating the direction of the current, and thus magnetic field, is implemented so as to avoid the electrochemical phenomenon of electrode migration. The coil drivers have an integrated on-chip voltage reference, ultra-low temperature drift (<15 ppm/C°) and are highly reliable.
A pair of electrodes (30, 32) measure the voltage signal UE induced in the conductive fluid 18 by the magnetic field 28. The voltage signal is run through a pair of diodes (42, 44), signal conditioners (46, 48), and an instrumentation amp (50) which measures the induced voltages (UE1, UE2) across the fluid. The instrumentation amp (50) amplifies the signal and maintains a linear relationship between the input current (I1, I2) and output voltage (VE1, VE2). An A to D converter (ADC) (52) receives the analog output from the instrumentation amp (50) and converts it into a digital signal. A micro-processor (36) receives the digital signal, processes the data using instructions stored in memory and determines the conductivity of the fluid media based on the digital signal. The micro-processor then determines the optimal frequency of the drive current (I) based on the conductivity measurement. The two coil drivers (24, 26) then use the optimal drive frequency to energize the two coil assemblies (20, 22) as illustrated in
With reference to
With reference to
I=V/R*[1−e{circumflex over ( )}(−)R/L*t]
Where:
An illustrative time varying magnetic field B generated within the flow field is shown in
{right arrow over (B)}˜{right arrow over (I)}×N
Where:
With reference to
With reference to
T
T
=T
D
+T
S
The delay time TD is strongly dependent upon the fluid media's conductivity since it follows a conventional capacitor discharge profile. We saw earlier that a fluid's resistivity R is a function of the number of charge carriers. So, the lower the resistivity R, the shorter the discharge time (e.g., if R is high in an RC circuit, the time constant T is also high). Accordingly, the greater the conductivity of the fluid the shorter the delay time TD. The steady state time TS is the time during which UE is at a steady state value and the voltage measurement is performed. This time can also be optimized to achieve a greater measurement frequency
With reference to
With reference to
The two electrodes sense the voltage UE created by the conductive fluid flowing perpendicular to the time varying magnetic field B as depicted in
The voltage signal from one or both electrodes passes thru a diode and signal conditioner, and is then converted from an analog to digital signal using the A/D converter. The digital signal is received by a micro-processor which compares the amplitude of the peak voltage values (UE1 Peak, UE2 Peak) to a relationship between peak voltage and fluid electrical conductivity (See
The micro-processor then modifies the drive current frequency based upon the fluid media's electrical conductivity (Step 108). The fluid's electrical conductivity is first compared to a minimum electrical conductivity for achieving accurate fluid velocity measurements (Step 110). If the fluid's electrical conductivity is below this threshold value, operation of the magnetic flow meter is typically paused and an error signal is displayed (Step 112). Alternatively, if the fluid's electrical conductivity is above the threshold value, the processor then determines whether the fluid's conductivity is high enough to warrant increasing the frequency of the drive current (Step 114). For example, if the fluid has a conductivity above 50 μS/cm the drive current frequency is increased to 5 Hz.
Next, if the fluid's electrical conductivity is above the threshold value, but too low to increase the drive current frequency, it's then determined whether a lower drive current frequency is warranted (Step 118). In instances where the induced voltage UE requires additional time to achieve a steady state value the frequency of the drive current I is decreased. This results in an increase in steady state time TS and a more accurate and repeatable voltage measurement (Step 120). For example, if the fluid conductivity is below 15 μS/cm the drive current frequency is decreased to 3 Hz. A fluid media that was previously subject to large fluid velocity measurement errors can now be accurately measured
Once the drive frequency has been optimized for the current fluid media the novel method can be repeated at Step 124 or ended at Step 126 (Step 122). There are many reasons for continuing to perform the optimization process including: batch to batch variations in a fluid conductivity, regular changes in the fluid media being measured, or to ensure very fast and accurate fluid velocity measurements, to name a few.
With our discussion of the magnetic flowmeter and method for measuring the conductivity of a fluid media complete. We'll now shift our attention to a commercial implementation of the flow meter.
With reference now to
The brace 21 further serves as magnetic circuitry for the magnetic field generated by the coils 18, 20. The brace has a generally octagonal shape, which benefits the assembly and operation of the assembly 10. More particularly, the brace 21 is formed of two, generally c-shaped components 29 that slide-ably mates with each other about the pipe, to couple to each other. In this manner, the brace 21 can be used on pipes having different diameters. Attachments (e.g., bolts) couple the coils to the brace along the axis (Az).
The assembly 10 is configured to generate a strong alternating magnetic field (flux) B that is distributed evenly over the pipe's cross-section. Utilizing an alternating magnetic field avoids electrode material migration. Configuration of the brace 21, e.g., including shape and materials, facilitates the resulting magnetic field (flux) B within the pipe 12. In the exemplary embodiment the brace 21 is formed “soft” magnetic materials, which refers to relative permeability, meaning is has no remnant magnetization, when shut down.
With reference now to
The present invention has been described above in terms of presently preferred embodiments so that an understanding of the present invention can be conveyed. However, there are other embodiments not specifically described herein for which the present invention is applicable. Therefore, the present invention should not to be seen as limited to the forms shown, which is to be considered illustrative rather than restrictive.
