Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
1999-10-15
2001-05-29
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
C073S861110
Reexamination Certificate
active
06237424
ABSTRACT:
The present invention relates to an electromagnetic flowmeter, in particular to a battery-powered electromagnetic flowmeter, primarily but not exclusively designed for domestic applications.
FIELD OF THE INVENTION
An electromagnetic flowmeter is used to measure the flow rate of a conductive medium, such as water, through a flow tube. An electromagnetic flow meter, such as that described, for example, in United Kingdom patent no. 2 081 449, comprises a magnetic circuit for developing a magnetic field vector in a plane normal to the direction of flow of fluid in the flow tube.
As fluid flows through the flow tube, a voltage is induced in the fluid subjected to the magnetic flux and in a direction orthogonal to both the direction of fluid flow and the magnetic flux. The induced voltage V
i
is detected by a pair of electrodes disposed in the flow tube, where V
i
is related to the magnetic field strength B and the velocity v of fluid flowing in the flow tube by the expression
V
i
=B×I.v.k
where I is the separation of the electrodes and k is a sensitivity factor dependent upon the geometry of the flow tube. By measuring the magnitude and polarity of the induced voltage V
i
, the magnitude and direction of the velocity of the flow of fluid through the flow tube, and therefore the flow rate, may be calculated.
DESCRIPTION OF PRIOR ART
Conventionally, various combinations of permanent magnets and electromagnetic systems are used to create either a constant or an alternating magnetic field of known magnitude. Such electromagnetic flow meters have high electrical power requirements for the reproduction of a required high magnetic field vector across the flow tube, and consequently employ a mains driven excitation circuit. An external power supply thus is required, and the cost of running such a flow meter is significant over time.
SUMMARY OF THE INVENTION
The preferred embodiment of the present invention seeks to provide a flow meter with a very low power consumption, which can if desired be driven by an internal battery for many (eg. 8 to 10 ) years. Such a meter is particularly suited for installation in large numbers in domestic water supply networks, and requires only periodic reading of the water consumed either by site visit or preferably by remote data logging or polling.
A first aspect of the present invention provides a method of a operating an electromagnetic flowmeter comprising a flow measurement duct, a magnetic circuit for generating a magnetic field in fluid flowing in the duct across the direction of flow, and an electrical circuit for measuring a voltage thereby induced in the fluid as indicative of the flow, the method comprising measuring an output of said electrical circuit in the absence of said magnetic field and determining therefrom the presence or absence of fluid in the duct.
This aspect of the present invention extends to an electromagnetic flow meter comprising a flow measurement duct, a magnetic circuit for generating a magnetic field across fluid flowing in the duct, an electrical circuit for measuring a voltage thereby induced in the fluid and for deriving a flow measurement therefrom and characterised by the electromagnetic flow meter sensing the presence or absence of fluid in the duct from an output of the electrical circuit when the generating magnetic circuit is inactive. Preferably the electrical circuit is adapted to identify noise signifying that the duct is empty of said fluid.
The detection of flow voids (empty tube events), which often is necessary to meet the requirements of water utilities, thus can be achieved in power-efficient manner.
The electrical circuit may be arranged to measure said voltage for short periods of time separated by relatively long intervals, the frequency of occurrence of such periods being such that the flow measured by the electrical circuit during said periods is representative of the flow during the relatively long intervals. In the example of a battery powered domestic meter, this can substantially reduce the power consumption of the meter.
Additionally, the electrical circuit may be arranged to measure said voltage during some of said periods when the magnetic circuit is inactive, whereby to identify said noise.
The flow meter may additionally comprise a processor for varying the duration of said intervals according to the measured flow rate, and/or according to the variation in the measured flow rate. Thus, in another aspect of the present invention, an electromagnetic flow meter comprises an electromagnetic flow meter comprising a flow measurement duct, a magnetic circuit for generating a magnetic field across fluid flowing in the duct, an electrical circuit for deriving a flow measurement from a voltage thereby induced in the fluid, and characterised in that the magnetic circuit is configured to generate the magnetic field for short periods of time, separated by relatively long intervals, the frequency of occurrence of such periods being such that the flow measured by the electrical circuit during said periods in representative of the flow during the relatively long intervals, the flow meter further comprising a processor for varying the duration of said intervals according to the measured flow rate, and/or according to the variation in the measured flow rate.
Preferably, the electrical circuit is arranged to derive a series of time-spaced flow measurement signals from voltages thereby induced in the fluid flow, each signal comprising a component representative of the flow and a variable DC component unrelated to the flow, the flow meter further comprising a processor for determining the DC component of a said signal and for adjusting the DC level of a subsequent said signal in response thereto. This can ensure that drift in the DC level of the flow measurement signal due to electrochemical effects is corrected so that the input dynamic range of subsequent signal-processing circuitry is not exceeded. Otherwise the signal would be clipped or clamped, resulting in measurement errors.
Preferably, the processor predicts the DC component of the subsequent signal from a plurality of values of the DC component obtained from the previous signal. Thus, in yet another aspect of the present invention, an electromagnetic flow meter comprises a flow measurement duct, a magnetic circuit for generating a magnetic field across fluid flowing in the duct, an electrical circuit for deriving a series of time-spaced flow measurement signals from voltages thereby induced in the fluid flow, the signal comprising a component representative of the flow and a variable DC component unrelated to the flow, and a processor for determining the DC component of a said signal and for adjusting the DC level of a subsequent said signal in response thereto, the processor predicting the DC component of the subsequent signal from a plurality of values of the DC component obtained from the previous signal.
Preferably, the first flow measurement signal comprises a plurality of pulses, the processor predicting the DC component of the subsequent signal by applying an algorithm to values of the DC component each obtained from the respective said pulse.
Conventionally, the magnetic circuits of electromagnetic flowmeters are formed from high impurity mild steels, containing a relatively impurity-contaminated ferrite structure. However, these materials possess extremely poor magnetic properties, such as low initial and incremental permeabilities at low and elevated flux density levels respectively, low or undefined remanence flux density, high or undefined coercivity, undeveloped or random crystalline texture, random or undefined power loss and permeability anisotropy, and high power losses at low induction and excitation frequencies.
Power loss is mainly composed of three components: (1) current loss which depends of the excitation frequency, electrical resistivity, the peak flux density and the material thickness of the magnetised body;
(2) hysteresis loss which depends on the composition, processing history, metallurgical processing and co
Gregg Ronald David
Howarth Craig Timothy
Jin Wu
MacManus Gerard
Salmasi Zareh Soghomonian
ABB Metering Limited
Hodgson & Russ LLP
Patel Harshad
Patel Jagdish
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