Flowmeter External Magnetic Grading and Quantization Device and Method

Abstract

A Coriolis flowmeter (5) is provided that includes a flow conduit (103A, 103B) having a driver (104) and a pick-off sensor (105, 105') connected thereto. The meter electronics (20) is configured to drive the driver (104) to vibrate the flow conduit (103A, 103B) in a first bending mode and to receive a signal from the pick-off sensor (105, 105'). The meter electronics (20) is configured to indicate the presence of an external magnetic field.

Description

[Technical Field]

[0001] The embodiments described below relate to vibration sensors, and more specifically, to flow meters and related methods capable of detecting external magnetic fields. [Background technology]

[0002] For example, vibrating sensors such as vibrating concentration meters and Coriolis flow meters are commonly known and are used to measure mass flow rate and other information about a material flowing through a conduit in a flow meter. Exemplary Coriolis flow meters are disclosed in U.S. Patent No. 4,109,524, U.S. Patent No. 4,491,025, and reissued Patent No. 31,450. These flow meters have a meter assembly having one or more conduits in a straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter may have a set of natural vibration modes, such as simple bending, twisting, or coupling types. Each conduit can be driven to vibrate in a preferred mode. When there is no flow through the flow meter, the driving force applied to the conduit causes all points along the conduit to vibrate in the same phase or with a small "zero offset," which is a time delay measured at zero flow rate.

[0003] As material begins to flow through a conduit, the Coriolis force causes each point along the conduit to have a different phase. For example, the phase at the inlet end of a flow meter lags behind the phase at the position of the centrally located driver, and the phase at the outlet leads the phase at the position of the centrally located driver. Pickoffs on the conduit generate sinusoidal signals that represent the movement of the conduit. The signals output from the pickoffs are processed to determine the time delay between the pickoffs, known as ΔT. The time delay between two or more pickoffs is proportional to the mass flow rate of the material flowing through the conduit.

[0004] The meter electronics connected to the driver generate a drive signal to operate the driver and to determine the mass flow rate and / or other properties of the process material from the signal received from the pickoff. While the driver can comprise one of many well-known configurations, a magnet and opposing drive coil has achieved great success in the flowmeter industry. An alternating current is passed through the drive coil to cause the conduit to vibrate at a desired conduit amplitude and frequency. It is also known in the art that the pickoff may be provided as a magnet and coil device very similar to this driver device.

[0005] When a powerful external magnet is placed near a pickoff, several effects are observed. Firstly, the pickoff voltage drops or increases sharply. Secondly, the phase shift between pickoffs drops or increases sharply. When the magnet is removed, the sensor voltage and phase shift return to normal. What is needed is an apparatus and method for detecting external magnetic fields and predicting their effects on flow meter readings. [Overview of the project]

[0006] A Coriolis flow meter is provided. In one embodiment, the Coriolis flow meter comprises a flow conduit and a driver and a pick-off sensor connected to the flow conduit. The meter electronics are configured to drive the driver to vibrate the flow conduit in a first bending mode and to receive a signal from the pick-off sensor. The meter electronics are configured to indicate the presence of an external magnetic field when a magnetic field is detected.

[0007] A method for operating a Coriolis flow meter is provided. According to one embodiment, the method includes the steps of: flowing a fluid material through the flow conduit of the flow meter; and driving a driver connected to the flow conduit to vibrate the flow conduit in a first bending mode. A signal is received from a pick-off sensor connected to the flow conduit. The presence of an external magnetic field is indicated if a magnetic field is detected.

[0008] [Aspect] According to one aspect, a Coriolis flow meter is provided that includes a flow conduit, a driver connected to the flow conduit, and a pick-off sensor. The meter electronics are configured to drive the driver to vibrate the flow conduit in a first bending mode and receive a signal from the pick-off sensor. The meter electronics are configured to indicate the presence of an external magnetic field when the magnetic field is detected.

[0009] Preferably, the presence of an external magnetic field is indicated when a step change in voltage is detected in a signal provided by at least one of the pick-off sensors.

[0010] Preferably, the presence of an external magnetic field is indicated when a voltage spike is detected in a signal provided by at least one of the pick-off sensors.

[0011] Preferably, the presence of an external magnetic field is indicated when a voltage spike is detected in a signal supplied by the driver.

[0012] Preferably, the presence of an external magnetic field is indicated when a step change in ΔT is detected.

[0013] Preferably, the phase of each pick-off sensor is measured relative to a third independent signal.

[0014] Preferably, the third independent signal includes a drive signal representing a drive mode other than the first bending mode.

[0015] Preferably, the presence of an external magnetic field is indicated when zero flow is compared to the measured asymmetry between the open loop driver signal i

[0016] , , and the pick-off voltages V LPO2 and V RPO2 where V LPO2 and V RPO2 are the pick-off voltages at the second bending mode frequency.

[0016] Preferably, when the presence of an external magnetic field is detected, an interference correction factor is calculated and applied to the measured flow rate to cancel out the effect of the external magnetic field.

[0017] Preferably, an alarm is issued when the presence of an external magnetic field is detected.

[0018] According to one aspect, a method for operating a Coriolis flowmeter includes flowing a flowing material through a flow conduit of the flowmeter and driving a driver connected to the flow conduit to vibrate the flow conduit in a first bending mode. A signal from a pick-off sensor connected to the flow conduit is received. When a magnetic field is detected, the presence of an external magnetic field is indicated.

[0019] Preferably, the presence of an external magnetic field is indicated when at least one of a voltage spike and a step change is detected in a signal provided by at least one of the pick-off sensors.

[0020] Preferably, the presence of an external magnetic field is indicated when a step change in ΔT is detected.

[0021] Preferably, the phase of each pick-off sensor is measured relative to a third independent signal including a drive signal representing a drive mode other than the first bending mode.

[0022] Preferably, the presence of an external magnetic field is indicated when zero flow rate is compared with the measured asymmetry between the open-loop driver signal i drive2 and the pick-off voltages V LPO2 and V RPO2 , where V LPO2 and V RPO2 are the pick-off voltages at the second bending mode frequency.

[0023] Preferably, when the presence of an external magnetic field is detected, an interference correction factor is calculated and applied to the measured flow rate to cancel out the effect of the external magnetic field.

[0024] Preferably, an alarm is issued when the presence of an external magnetic field is detected. [Brief explanation of the drawing]

[0025] In all drawings, the same reference number represents the same element. [Figure 1] Figure 1 shows a vibration meter according to one embodiment. [Figure 2] Figure 2 shows a meter electronic device according to one embodiment. [Figure 3] Figure 3 shows the effect of a magnetic field on the pick-off voltage of a flow meter sensor according to one embodiment. [Figure 4] Figure 4 shows the effect of a magnetic field on flow rate measurement according to one embodiment. [Figure 5] Figure 5 shows the relationship between the drive current and the pick-off sensor voltage. [Figure 6] Figure 6 shows the first bending mode of the dual U-tube Coriolis sensor. [Figure 7] Figure 7 shows the torsional mode of a dual U-tube Coriolis sensor as a result of fluid flow. [Figure 8] Figure 8 shows the second bending mode of the dual U-tube Coriolis sensor. [Figure 9] Figure 9 shows the phasor diagram of the second bending mode of the flowmeter, and in particular, the symmetrical shift of VLP02 and VRP02 associated with the flow. [Modes for carrying out the invention]

[0026] Figures 1-9 and the following description illustrate specific examples to instruct those skilled in the art on how to construct and use best embodiments of the sensor assembly, brace bar, driver, and pick-off sensor. Some conventional embodiments have been simplified or omitted for the purpose of teaching the principles of the present invention. Those skilled in the art will understand variations from these examples that fall within the scope of this specification. Those skilled in the art will understand that multiple variations of the embodiments can be formed by combining the features described below in various ways. Consequently, the embodiments described below are not limited to the specific examples described below, but are limited only by the claims and their equivalents.

[0027] Figure 1 shows a flow meter 5 according to one embodiment. The flow meter 5 includes a sensor assembly 10 and meter electronics 20. The meter electronics 20 is connected to the sensor assembly 10 via lead wires 100 and is configured to provide one or more measurements or information from density, mass flow rate, volumetric flow rate, total mass flow rate, temperature, or other measurements through a communication path 26. The flow meter 5 may include a Coriolis mass flow meter or other vibrating flow meter. It will be apparent to those skilled in the art that the flow meter 5 can include any type of flow meter 5, regardless of the driver, pick-off sensor, number of flow conduits, or mode of vibration operation.

[0028] The sensor assembly 10 includes a pair of flanges 101 and 101', manifolds 102 and 102', a driver 104, pick-off sensors 105 and 105', and flow conduits 103A and 103B. The driver 104 and pick-off sensors 105 and 105' are connected to flow conduits 103A and 103B.

[0029] Flanges 101 and 101' are attached to manifolds 102 and 102'. In some embodiments, manifolds 102 and 102' can be attached to both ends of a spacer 106. The spacer 106 maintains the distance between manifolds 102 and 102'. When the sensor assembly 10 is inserted into a pipeline (not shown) carrying the process fluid to be measured, the process fluid enters the sensor assembly 10 through flange 101, passes through inlet manifold 102, where the entire amount of process fluid is guided into flow conduits 103A and 103B, flows through flow conduits 103A and 103B and back to outlet manifold 102', and exits the sensor assembly 10 through flange 101'.

[0030] The process fluid may include a liquid. The process fluid may include a gas. The process fluid may include a multiphase fluid, such as a liquid containing impurities including gases and / or solids, for example, but not limited to these. Flow conduits 103A and 103B are selected to have substantially the same mass distribution, moment of inertia and modulus of elasticity around the bending axes WW and W'-W', respectively, and are appropriately mounted to the inlet manifold 102 and the outlet manifold 102'. Flow conduits 103A and 103B extend outward from the manifolds 102 and 102' essentially parallel to each other.

[0031] The flow conduits 103A and 103B are driven by the driver 104 in opposite directions around their respective bending axes W and W' in the so-called first out-of-phase bending mode of the flowmeter 5. The driver 104 may comprise one of many well-known configurations, such as a magnet attached to flow conduit 103A and a counter coil attached to flow conduit 103B. An alternating current is passed through the counter coil, causing vibrations in both conduits. An appropriate drive signal is applied to the driver 104 via the lead wire 110 by the meter electronics 20. Other driver devices are conceivable and are included within the scope of this specification and the claims.

[0032] The meter electronic equipment 20 receives sensor signals on lead wires 111 and 111', respectively. The meter electronic equipment 20 generates a drive signal on lead wire 110 that causes the driver 104 to vibrate the flow conduits 103A and 103B. Other sensor devices are conceivable and are included within the scope of this specification and the claims.

[0033] The meter electronics 20 processes left and right velocity signals from pick-off sensors 105 and 105' in particular to calculate the flow rate. A communication path 26 provides input and output means that enable the meter electronics 20 to interface with an operator or other electronic system. The description in Figure 1 is provided merely as an example of flow meter operation and is not intended to limit the teachings of the present invention. In some embodiments, single-tube and multi-tube flow meters having one or more drivers and pick-offs are conceivable.

[0034] In one embodiment, the meter electronic equipment 20 is configured to vibrate flow conduits 103A and 103B. The vibration is induced by a driver 104. The meter electronic equipment 20 further receives the resulting vibration signal from pick-off sensors 105 and 105'. The vibration signal includes the vibration response of flow conduits 103A and 103B. The meter electronic equipment 20 processes the vibration response and determines the response frequency and / or phase difference. The meter electronic equipment 20 processes the vibration response and determines one or more flow measurement values, including the mass flow rate and / or density of the process fluid. Other vibration response characteristics and / or flow measurement values ​​are conceivable and are included within the scope of this specification and the claims.

[0035] In one embodiment, the flow conduits 103A and 103B comprise substantially omega-shaped flow conduits, as shown. Alternatively, in other embodiments, the flow meter may comprise substantially straight flow conduits, U-shaped conduits, delta-shaped conduits, and the like. Additional flow meter shapes and / or configurations may be used, and these are included within the scope of this specification and the claims.

[0036] Figure 2 is a block diagram of the meter electronic equipment 20 of a flow meter 5 according to one embodiment. During operation, the flow meter 5 provides a variety of outputtable measurements, including one or more measured or average values ​​of mass flow rate, volumetric flow rate, mass and volumetric flow rates of individual flow components, and, for example, total flow rate including both volumetric and mass flow rates.

[0037] The flow meter 5 generates a vibration response. The vibration response is received and processed by the meter electronic equipment 20 to generate one or more fluid measurements. The values ​​can be monitored, recorded, stored, totaled, and / or output.

[0038] The meter electronic equipment 20 includes an interface 201, a processing system 203 that communicates with the interface 201, and a storage system 204 that communicates with the processing system 203. Although these components are shown as separate blocks, it should be understood that the meter electronic equipment 20 can be composed of various combinations of integrated and / or separate components.

[0039] Interface 201 is configured to communicate with the sensor assembly 10 of the flow meter 5. Interface 201 is coupled to lead 100 (see Figure 1) and can be configured to exchange signals with, for example, a driver 104, pick-off sensors 105 and 105', and a temperature sensor (not shown). Interface 201 may be further configured to communicate with external devices, etc., via a communication path 26.

[0040] The processing system 203 may include any type of processing system. The processing system 203 is configured to read and execute stored routines in order to operate the flowmeter 5. The storage system 204 may store routines including the flowmeter routine 205, the magnetic field detection routine 209, and the alternative bending mode routine 211. Other measurement / processing routines are conceivable and are included within the scope of this specification and the claims. The storage system 204 may store measured values, received values, operating values, and other information. In some embodiments, the storage system stores mass flow rate (m') 221, density (ρ) 225, viscosity (μ) 223, temperature (T) 224, drive gain 306, transducer voltage 303, and any other variables known in the art.

[0041] The flowmeter routine 205 can generate and store quantified values ​​and flow rate measurements of a fluid. These values ​​may include substantially instantaneous measurements, or they may include sums or cumulative values. For example, the flowmeter routine 205 can generate mass flow rate measurements and store them, for example, in the mass flow rate 221 storage of the memory system 204. The flowmeter routine 205 can also generate density 225 measurements and store them, for example, in the density 225 storage. The values ​​of mass flow rate 221 and density 225 are determined from the vibration response, as described above and as known in the art. The mass flow rate and other measurements may include substantially instantaneous values, may include sample values, may include average values ​​over a time interval, or may include cumulative values ​​over a time interval. The time interval can be selected to correspond to a block of time in which a particular fluid state, e.g., a fluid state containing only liquid, or instead, a fluid state containing liquid and impurities, is detected. In addition, other mass flow rates and associated quantified values ​​are conceivable and are included within the scope of this specification and the claims.

[0042] Referring to Figure 3, by monitoring the meter electronics 20, it can be seen that when magnets and coils are used in the pick-off sensors 105 and 105', an external magnetic field, whether an electromagnetic source or a permanent magnet, affects the reading of the sensor assembly 10. It is clear that there is a relatively steep and symmetrical step change.

[0043] In Figure 3, the area indicated by bracket #1 represents the placement of a magnet near the pick-off sensor 105', which is located closest to the output of the flowmeter. When a magnet is placed there, the output of the flowmeter (PO in Figure 3) OUT A relatively steep and symmetrical step change in voltage is detected in the signal provided by the pick-off sensor 105', which is located closest to the (labeled) point.

[0044] In Figure 3, the area indicated by bracket #2 represents the placement of a magnet near the pick-off sensor 105, which is located closest to the input of the flow meter. When a magnet is placed there, the output of the flow meter (PO in Figure 3) OUT A relatively steep and symmetrical step change in voltage is also detected in the signal provided by the pick-off sensor 105', which is located closest to the input of the flowmeter (labeled as PO in Figure 3). IN Voltage spikes are detected in the signal provided by the pick-off sensor 105, which is located closest to the (labeled) point. Voltage spikes are also detected in the signal provided by the driver 104.

[0045] In Figure 3, the region indicated by bracket #3 represents the location of the magnet near the driver 104. A detectable, relatively steep, and symmetrical step change in voltage is detected in the signal supplied by the driver 104.

[0046] Referring to Figure 4, it can be seen that the external magnet affects the ΔT reading of the flow meter 5. When the driver 104 stimulates the flow conduits 103A and 103B, causing them to vibrate in opposite directions at their natural resonant frequencies, the flow conduits 103A and 103B vibrate, and the voltages generated from each pick-off sensor 105 and 105' generate sine waves. This represents the movement of one conduit relative to the other. The time delay between the two sine waves is called ΔT, which is directly proportional to the mass flow rate. If the phase of either flow conduit 103A or 103B is affected, ΔT changes. Depending on the flow rate, a positive change occurs in the phase of one pick-off sensor, and a negative change equal to the phase of the other pick-off sensor occurs.

[0047] In Figure 4, the region indicated by bracket #1 represents the placement of a magnet near the pick-off sensor 105', which is located closest to the output of the flowmeter. When the magnet is placed there, a relatively steep and symmetrical stepwise decrease in ΔT is detected.

[0048] In Figure 4, the region indicated by bracket #2 represents the placement of a magnet near the pick-off sensor 105, which is located closest to the input of the flowmeter. When the magnet is placed there, a relatively steep and symmetrical stepwise increase in ΔT is detected.

[0049] In Figure 4, the region indicated by bracket #3 represents the presence of a magnet near driver 104. When a magnet is placed there, a relatively steep and symmetrical stepwise decrease in ΔT is detected.

[0050] As described above, ΔT changes when the phase of either flow conduit 103A or 103B is affected. Furthermore, if the phases of each pick-off sensor 105, 105' are measured relative to a third independent signal, it can be determined whether ΔT is derived from the mass flow rate. For example, the drive current might seem like a good choice for this third signal, but unfortunately, as shown in Figure 5, the drive current is not independent of the two pick-off sensor voltages.

[0051] Figure 5 shows the drive current and pick-off sensor voltage (V) in a typical example of meter electronic equipment. RPO ), Pick-off sensor 105 voltage (V LPO An exemplary phasor diagram is shown illustrating the relationship between (V) and ΔT. In this example, the drive current is generated from the voltage of the pick-off sensor 105. The dashed line shows the fluid flow and the resulting voltage (V) RPO The meter electronic equipment 20 represents the scaled phase change ΔΦ. RP0 It should be clear that it is not possible to distinguish between and ΔT. When a fluid flows through a pipe, the driving current (i drive ) is the pick-off sensor 105 voltage (V LPO ) remains phase-shifted by 0°, and the measured ΔT is solely due to the pick-off sensor 105' phase (Φ RPO It is derived from ).

[0052] Since the drive current cannot be used as an independent signal, in one embodiment a third signal is added to the drive current. In one embodiment, this is the second bending mode of the flow conduits 103A and 103B. In another embodiment, other frequencies / bending modes may be used.

[0053] Figure 6 shows the first bending mode of a dual U-tube Coriolis sensor as an example. The flowing fluid generates a Coriolis force that excites the non-resonant response of the second bending mode at the frequency shown in Figure 7. Figure 8 shows the second bending mode of the dual U-tube as a result of the same exemplary fluid flow shown in Figure 7.

[0054] By adding an additional drive signal, the sensor can excite both the first and second bending modes. These excitation signals are V LPO This represents the LPO voltage at the first bending mode frequency, V LPO2 This represents the LPO voltage at the second bending mode frequency, and so on.

[0055] In order to separate these two signals, in one embodiment, a second signal i drive2 This is generated using an open-loop method. LPO2 or V RPO2 It is not generated by scaling and phase shifting, otherwise it would not provide more information than a normal drive sensor. In one embodiment, i drive2 is, i drive It is generated using scaling factors for the frequency and amplitude, but with arbitrary phase. This allows i drive A phase-unlocked signal is provided. These signals can be generated by the alternative bending mode routine 211.

[0056] In one embodiment, i drive2 , V LPO2 , and V RPO2 It is independent of all other signals. Therefore, i drive2 , V LPO2 , and V RPO2 The phase difference is measurable in all cases. Therefore, it is possible to determine whether the phase change is symmetric (as expected from the flow) or asymmetric (indicating an external magnet).

[0057] In one embodiment, the effect of the external magnet is quantified and corrected. The mass flow rate can still be calculated using the first bending mode, while the second bending mode can be easily used as a check of the external magnet during normal operation.

[0058] At zero flow rate, i drive2 V LPO2 and V RPO2 Ideally, the phase should be shifted by 90° for both. As the flow rate increases, V LPO2 and V RPO2 The phases are as shown in Figure 9, i drive2 V LPO2(FLOWING) and V RPO2(FLOWING) It shifts symmetrically.

[0059] Therefore, in one embodiment, the asymmetry between pick-offs can be calculated. In one embodiment, the following formula is used for the calculation. Φ L,i2 =V LPO2 and i drive2 Phase between (1) Φ R,i2 =V RPO2 and i drive2 Phase between (2) a = asymmetry = 1 - ((Φ L,i2 ) / (Φ R,i2 )) (3)

[0060] Based on this methodology, the asymmetry should be zero under all flowing and non-flowing conditions. This only changes if one pick-off signal behaves differently from other pick-off signals. This indicates the presence of magnetic interference. By comparing the zero flow rate with the measured asymmetry, an interference correction factor that offsets the effects of magnetic interference can be calculated and applied to the measured flow rate. If magnetic interference is detected, in one embodiment, a flag is recorded by the meter electronics. In one embodiment, if magnetic interference is detected, an alarm is issued. The alarm may be audible and / or visible. In one embodiment, the alarm includes a notification delivered to a remote device such as a server, computer, telephone, meter electronics, or other electronic device.

[0061] The detailed description of the embodiments described above is not an exhaustive description of all embodiments that the inventors consider to be within the scope of this description. In fact, those skilled in the art will recognize that further embodiments can be created by combining or excluding certain elements of the embodiments described above in various ways, and that such further embodiments are included within the scope and teachings of this specification. It will also be apparent to those skilled in the art that additional embodiments can be created by combining the embodiments described above, in whole or in part, within the scope and teachings of this specification.

[0062] Therefore, although certain embodiments are described herein for illustrative purposes, various equivalent modifications are possible within the scope of this specification, as those skilled in the art will recognize. The teachings provided herein can be applied not only to the embodiments described above and shown in the accompanying drawings, but also to other sensors, sensor brackets, and conduits. Accordingly, the scope of the embodiments described above should be determined from the following claims.

Claims

1. Flow conduits (103A, 103B) and A driver (104) and pick-off sensors (105, 105') connected to the flow conduits (103A and 103B), A meter electronic device (20) is configured to drive the driver (104) to vibrate the flow conduits (103A, 103B) in a first bending mode and to receive signals from the pick-off sensors (105, 105'), Equipped with, The meter electronic device (20) is configured to indicate the presence of an external magnetic field when a magnetic field is detected. The presence of the external magnetic field is indicated when at least one of a step change in voltage and a spike is detected in the signal provided by at least one of the pick-off sensors (105, 105').

2. The Coriolis flow meter (5) according to claim 1, wherein the presence of the external magnetic field is indicated when a voltage spike is detected in the signal supplied by the driver (104).

3. The Coriolis flow meter (5) according to claim 1, wherein the presence of the external magnetic field is indicated when a step change of ΔT, which is the time delay between two signals provided by the pick-off sensor (105, 105'), is detected.

4. The Coriolis flow meter (5) according to claim 1, wherein the phase of each pick-off sensor (105, 105') is measured with respect to a third independent signal.

5. The Coriolis flow meter (5) according to claim 4, wherein the third independent signal includes a drive signal representing a drive mode other than the first bending mode.

6. The presence of the external magnetic field is indicated by the fact that the phases of the pick-off voltages V LPO2 and V RPO2 with respect to the phase of the open-loop driver signal i drive2 when the flow rate is zero are asymmetrically shifted from the phases of the pick-off voltages V LPO2 and V RPO2 with respect to the phase of the open-loop driver signal i drive2 when the flow rate is not zero, where V ＬＰＯ２ , and V ＲＰＯ２ The Coriolis flow meter (5) according to claim 5, wherein is the pick-off voltage at the second bending mode frequency.

7. The Coriolis flow meter (5) according to claim 1, wherein, when the presence of the external magnetic field is detected, an interference correction coefficient is calculated and applied to the measured flow rate in order to cancel out the effect of the external magnetic field.

8. The Coriolis flow meter (5) according to claim 1, wherein an alarm is issued when the presence of the external magnetic field is detected.

9. A method for operating a Coriolis flow meter, The steps include: flowing the fluid material through the flow conduit of the flow meter, The steps include: driving a driver connected to the flow conduit to vibrate the flow conduit in a first bending mode; The steps include receiving a signal from a pick-off sensor connected to the flow conduit, A step to indicate the presence of an external magnetic field when a magnetic field is detected, Includes, A method in which the presence of the external magnetic field is indicated when at least one of a voltage spike and a step change is detected in the signal provided by at least one of the pick-off sensors.

10. A method for operating a Coriolis flowmeter according to claim 9, wherein the presence of the external magnetic field is indicated when a step change of ΔT, which is the time delay between two signals provided by the pick-off sensor, is detected.

11. A method for operating a Coriolis flowmeter according to claim 9, wherein the phase of each pick-off sensor is measured with respect to a third independent signal including a drive signal representing a drive mode other than the first bending mode.

12. The presence of the external magnetic field is indicated by the fact that the phases of the pick-off voltages V LPO2 and V RPO2 with respect to the phase of the open-loop driver signal i drive2 when the flow rate is zero are asymmetrically shifted from the phases of the pick-off voltages V LPO2 and V RPO2 with respect to the phase of the open-loop driver signal i drive2 when the flow rate is not zero, where V ＬＰＯ２ and V ＲＰＯ２ A method for operating a Coriolis flow meter according to claim 11, wherein the pick-off voltage is at a second bending mode frequency.

13. A method for operating a Coriolis flowmeter according to claim 9, wherein, when the presence of the external magnetic field is detected, an interference correction coefficient is calculated and applied to the measured flow rate in order to cancel out the effects of the external magnetic field.

14. A method for operating a Coriolis flow meter according to claim 9, wherein an alarm is issued when the presence of the aforementioned external magnetic field is detected.

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