Magnetic flowmeter for measuring flow

A magnetic flowmeter for measuring flow of a flow of process fluid, includes a flow tube arranged to receive the flow of process fluid therethrough. A magnetic drive coil proximate the flow tube is arranged to apply a magnetic field to the flow in response to a drive signal. At least one electrode is arranged to sense an electrical potential of the process fluid which related to the applied magnetic field and flow rate of the process fluid. Temperature measurement circuitry is coupled to the magnetic drive coil and is configured to provide a temperature output indicative of temperature of the drive coil based upon an electrical parameter of the drive coil. Flow measurement circuitry coupled to the at least one electrode is configured to provide a flow output based upon sensed electrical potential.

BACKGROUND OF THE INVENTION

The present invention relates to magnetic flowmeters that sense the flow of process fluid in industrial process plants. More specifically, the present invention relates to measurement of flow using a magnetic flowmeter.

Magnetic flowmeters are known in the art and utilized electrically insulated flow tube that carries a flow of process fluid past a coil of an electric magnet and past electrodes. The electrode magnet applies electromagnetic field to the flowing process fluid. Due to Faraday's Law of electromagnetic induction, a voltage or Electro Mode of Force (EMF) is generated between a pair of electrodes in the fluid. This voltage is a function of the strength of the applied magnetic field and is proportional to the rate of flow of the fluid.

The sensed voltage is proportional to the volumetric flow rate of process fluid through the flow tube. However, mass flow rate is related to both the density of the fluid as well as its velocity. Mass flow rate can be calculated by multiplying the density of the fluid velocity and the cross sectional area of the flow tube. However, for many fluids, fluid density is related to the temperature of the fluid. In order to use a typical magnetic flowmeter to measure mass flow, a separate temperature sensor must be utilized to perform the mass flow calculation.

SUMMARY

A magnetic flowmeter for measuring flow of a flow of process fluid, includes a flow tube arranged to receive the flow of process fluid. A magnetic drive coil proximate the flow tube is arranged to apply a magnetic field to the flow in response to a drive signal. At least one electrode is arranged to sense an electrical potential of the process fluid which related to the applied magnetic field and flow rate of the process fluid. Temperature measurement circuitry is coupled to the magnetic drive coil and is configured to provide a temperature output indicative of temperature of the drive coil based upon an electrical parameter of the drive coil. Flow measurement circuitry coupled to the at least one electrode is configured to provide a flow output based upon sensed electrical potential.

DETAILED DESCRIPTION

The present invention provides a magnetic flowmeter for measuring mass flow in which an electrical parameter of a coil of the flowmeter is measured and used to provide temperature compensation to the mass flow measurement.

InFIG. 1, a typical environmental for magnetic flowmeter102is illustrated at100. InFIG. 1, magnetic flowmeter102is shown coupled to process piping104which also couples to control valve112. Magnetic flowmeter102is an example of one type of process variable transmitter which can be configured to monitor one or more process variables associated with fluids in a process plant such as slurries, liquids, vapors and gases in chemicals, pulp, petroleum, gas, pharmaceutical, food and other fluid processing plants. In a magnetic flowmeter, the monitored process variable relates to velocity of process fluid through flow tube108. Magnetic flowmeter102outputs are configured for transmission over long distances to a controller or indicator via communication bus106. In typical processing plants, communication bus106can be a 4-20 mA current loop, a fieldbus connection, a pulse output/frequency output, a HART protocol communication, a wireless communication connection, ethernet or a fiberoptic connection to a controller such as system controller/monitor110or other device. System controller110is programmed as a process monitor, to display flow information for a human operator or as a process controller to control the process using control valve112over communication bus106.

InFIG. 2, a perspective cutaway view of magnetic flowmeter102is shown generally. Flowmeter102includes electronics housing120connected to flow tube108. Flow tube108includes electromagnetic coils122which are used to induce a magnetic field in fluid flowing through flow tube108. The electrodes124in flow tube108provide an EMF sensor which senses the EMF generated in the fluid due to the velocity of the flow and the applied magnetic field and which are also sensitive to noise. Coil driver circuitry130(shown inFIG. 3) in electronic housing120provides a drive signal to electromagnetic coils122and electrodes124provide EMF output134to EMF signal amplifier132(also shown inFIG. 3).

InFIG. 3, a block diagram shows one embodiment of magnetic flowmeter102for measuring a flow of a conductive process fluid through flow tube assembly108. Coils122are configured to apply an external magnetic field in the fluid flow in response to an applied drive current from coil driver130. EMF sensors (electrodes)124electrically couple to the fluid flow and provide an EMF signal output134to amplifier132related to an EMF generated in the fluid flow due to the applied magnetic field, and fluid velocity. Analog to digital converter142provides a digitized EMF signal to microprocessor system148. A signal processor150is implemented in microprocessor system148of flowmeter electronics140which couples to the EMF output134to provide an output152related to fluid velocity. A memory178can be used to store program instructions or other information as discussed below.

Microprocessor system148calculates velocity through flow tube108in accordance with a relationship between the EMF output134and the flow velocity as set forth in Faraday's law, which states:

V=EkBDEq.⁢1
Where E can be the signal output152which is related to the EMF output134, V is the velocity of the fluid, D is the diameter of flow tube108and B is the strength of the magnetic field in the fluid. k is a constant of proportionality. Microprocessor system148uses velocity to calculate flow of the process fluid in accordance with known techniques. A digital to analog converter158coupled to microprocessor system148generates an analog transmitter output160for coupling to communication bus106. A digital communication circuit162generates a digital transmitter output164. The analog output160and the digital output164can be coupled to process controllers or monitors as desired.

In accordance with the present invention, flowmeter102also includes temperature sense circuitry180. Temperature sense circuitry180is configured to couple to the coil122and provide an output182to microprocessor system148which is related to the temperature of the coil122. Temperature sense circuitry180may operate in accordance with any technique and one example embodiment is discussed below in more detail. Using the sensed temperature as well as the sensed EMF134, microprocessor system148calculates the mass flow as:
M=α·V·AEq. 2
Where M is the mass flow rate, α is density of the fluid, V is the velocity in accordance with Equation 1 and A is the cross-sectional area of the flow tube.

As discussed in the background section and in accordance with Equation 2, calculation of mass flow requires a determination of the density. In some examples, a fixed density value is entered by the operator and used to calculate mass flow regardless of the temperature. However, this can lead to substantial errors. For example, a temperature change from 0° C. to 177° C. will result in more than a 10% change in the density of water. The measurement of temperature of the fluid by the mass flow meter can be used to calculate fluid density and provide improved accuracy in mass flow measurements. Further, the temperature information can be used as a redundant measurement point. Any additional temperature information provided by the mass flow meter can be used to compare or validate a measurement from a separate temperature sensor. Temperature information can also be used to provide an indication that the process had exceeded its expected temperature limits, or the temperature limits of particular components in the process such as the flowtube. Such excessive temperatures may result in a shortened life span and premature failure of components.

With the present invention, the temperature of the flowtube and process fluid is inferred by providing a temperature related output based upon a signal flowing through the coils122of the flowmeter. For example, the resistance and inductance of the coils122adjacent to flowtube108can be measured and are related to coil temperature. This can be used to infer the temperature of process fluid carried in flowtube108.

FIG. 4Bis a simplified electrical equivalent of the thermal conduction process of the flowtube arrangement illustrated inFIG. 4A. WithFIG. 4B, the equivalent circuit200uses the following key:

TAmbient—Ambient Air Temperature

TProcessFluid—Temperature of the Process Fluid

RTube Housing—Thermal Resistance from the ambient air to the coils

RTube—Thermal Resistance of the stainless steel tube

RLiner—Thermal Resistance of the liner

In determining temperature of the process fluid, the ambient air temperature should be taken into account to accurately infer the process temperature. The electronics temperature can be used to infer the ambient air temperature as illustrated inFIG. 4B. For example, the electronics temperature is typically is about 10° C. greater than the ambient temperature. This can be calibrated during the manufacturing process or determined empirically.

Once the ambient temperature is identified, the process temperature can be determined using the following formula:

TProcess⁢⁢Fluid=TCoils*(RTubeHousing+RSST_Tube+RLiner)RTubeHousing+TAmbientEq.⁢3
The thermal resistance can be calculated or empirically measured for each line size of the flowmeter tube108. This can also be calibrated during the manufacturing process to improve accuracy. Flowmeter102can store coefficients for each line size, or for individual flowtubes in transmitter combinations in memory178of microprocessor system148.

FIG. 5is a diagram which illustrates example data showing a relationship between coil resistance and coil inductance over temperature for a 0.15 inch flowtube.FIG. 5is a graph of coil resistance (ohms) versus coil inductance (mH) which illustrates how they change linearly with temperature.

FIG. 6is a graph of coil resistance and coil inductance versus time. In the graph ofFIG. 6, coil inductance and coil resistance are shown for a 3 inch diameter flowtube. To generate the graph ofFIG. 6, hot water was placed into the flowtube. The temperature was allowed to settle and ice was added to the water to decrease the temperature in the tube to approximately 0° C. Note that the temperature will change more rapidly under normal flow conditions.

The effect of the process temperature on the coil measurement is heavily damped due to the large amount of mass that needs to change temperature. As the coil resistance measurement is very clean, it is easy to estimate where the measurement reading should be. For example, a linear curve that can be used to predict resistance. A first order low pass filter can be used to smooth the measurement data. This allows the system to react rapidly to step changes in temperature.

FIG. 7is a graph of resistance (ohms) versus time and shows the raw coil resistance and the predicted coil resistance across a change in temperature. The coil resistance inFIG. 7was predicted by calculating a linear fit of the last five measurements. This fit was then projected fifteen measurement points into the future to provide predicted temperature. This is a relatively simple prediction model but it is capable of providing temperature information. A more complex prediction model can be employed in which a curve fit of a step change in temperature is determined for each particular line size. This information can be stored in memory178.

In this example, circuitry180includes a differential amplifier connected to coil122. The output from amplifier is digitized using an analog to digital converter and provided to microprocessor system148. Software run by microprocessor system148can be configured to determine temperature of coil122, and thereby infer the temperature of process fluid184, using resistance of coil122in the following formula:
TCoils=CoilResistance*(Coil Resistance Temperature Gain)+(Coil Resistance Temperature Offset)  Eq. 4
Similarly, inductance can be used to determine temperature using the formula:
TCoils=Coil Inductance*(Coil Inductance Temperature Gain)+(Coil Inductance Temperature Offset)  Eq. 5

The temperature gain and offset for the coil resistance inductance can be trimmed in the factory by measuring the coil resistance inductance at 2 different temperature points and then calculating the gain and offset. Since the Coil Resistance Inductance is linear with Coil Temperature, we can then calculate the Coil Temperature based on the Coil Resistance Inductance.

Determining temperature based upon inductance of coil122is preferable to determining temperature based upon resistance. This is because coil inductance is indifferent to changes in the resistance of the external wiring due to temperature changes or corrosion at the terminals. A remote mount configuration can have up to 1000 feet of cable. The coil inductance also allows the transmitter temperature reading to be calibrated at the factory with short coil wires, but when use longer wires in the field with very little installation effect. A 1000 feet of 18 AWG wire will have ˜6.5 ohms of resistance over 1000 feet. This would require the ability to zero out any installation affect. This may not be required when using the coil inductance measurement.

Once the temperature of the process fluid is determined by microprocessor system148, this temperature information can be used to compensate for density variations in determining mass flow in accordance with Equation 2.

The temperature information can also be used by microprocessor system148to provide an output if the temperature exceeds certain limits, for example, manufacturer limits, maximum or minimum process temperature limits, etc. Such an alert can be transmitted over communication bus106. In another example configuration, the pressure of the process fluid is used in the mass flow computation. In such a configuration, pressure information can be received from another process device over communication bus106. An additional temperature sensor can also be provided proximate flowtube108, for example, near electrodes124, to measure process temperature. This additional temperature sensor can be used for diagnostics, to provide a faster response time, or more accurate measurements.