Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
1998-12-18
2001-08-07
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
C073S861120, C073S861080, C073S861110
Reexamination Certificate
active
06269701
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The preset application relates to electromagnetic flowmeters. Electromagnetic flowmeters are used in a variety of applications. As is well known, they operate by providing a current to a coil to generate a magnetic field in the fluid to be monitored, and detecting a voltage, generated across the fluid, which is generally proportional to the product of the magnetic field strength and the flow velocity.
2. Description of Prior Art
A common method of signalling the flow rate is for the flowmeters to control the current flowing in a current signalling loop (also sometimes referred to as a_sensing loop or simply a current loop), the magnitude of the current in the current sensing loop_indicating the rate of fluid flow, the current being arranged to flow in a current signalling_loop incorporating a display or other metering apparatus remote from the sensor probe of the flowmeter. Commonly, a current output in the current signalling loop in the range of 4-20 mA is used; a current of 4 mA indicating zero rate of flow, and 20 mA indicating a full rate of flow, the scale typically being linear.
It has recently been proposed to power an electromagnetic flowmeter from the output current flowing in the current signalling loop. This has the benefit of two-wire operation; the meter power and output signal are conveyed along the same pair of wires. The power constraints in such a flowmeter are very severe, however.
One recent proposal attempts to make powering a meter from a current signalling loop feasible by reducing power consumption at lower flow rates, when the current flowing in the current signalling loop will be smaller, by cycling measurement, and only measuring for short cycles at low flow rates to conserve power. This requires complicated control circuitry, adding to the cost of the flowmeter.
In addition, the above-described proposal is believed to make use of the fact that a typical voltage available in the current signalling loop is about 24V. However, the inventor considers it better practice to provide a flowmeter which will reliably control the current signalling loop with a voltage drop of only about 10V across the flowmeter, to ensure reliable operation when several devices are connection in series in the same current signalling loop. This however would allow only about 40 mW to operate the sensor at low flow rates, and the above described proposal may not function reliably at such low power levels.
This disadvantage is compounded, as the inventor has realised, by the fact that it is particularly when the flow rate is lowest that it is most difficult to obtain reliable measurements with a high signal-to-noise ratio; the signal generated by such a flowmeter is generally proportional to the coil current applied to the coil and to the flow velocity, and at low velocities, the signal will be small for a given coil current.
Thus, where power is limited, it is difficult to obtain a good signal-to-noise ratio at low flow levels. This problem is particularly pronounced in the prior art current signalling loop-powered flowmeter, in which the available power drops as the rate of fluid flow drops. Although practical improvements on the prior art may be achieved, with attendant cost and complexity, by further attempts to reduce power consumption of the elctronics and optimisation of the switching algorithms, the inventor has appreciated that, owing to the problems at low flow rates, the performance of such current signalling loop powered flowmeters is inherently limited. The inventor has proposed a novel alternative which is both simpler to implement and can provide better results, particularly at low flow rates.
SUMMARY OF THE INVENTION
The invention provides an electromagnetic flowmeter having means for controlling an electrical current flowing in a signalling loop to a value indicative of the rate of flow of fluid at a measuring location, and means for drawing power from said current to operate the flowmeter, characterised in that the control means is arranged to increase the current as the rate of fluid flow decreases whereby more power can be drawn at lower flow rates than at high flow rates.
With this elegant solution, the power available for operation of the flowmeter increases as the measured flow rate decreases. In this way, for example, the flowmeter coil current may be increased at lower flow rates. As an alternative, more measurements may be taken at lower flow rates and the results combined or averaged. (This latter method may be directly contrasted with the fewer measurements taken in the prior art at lower flow rate.) In each case, the resultant measurement should have a higher signal to noise ratio than that obtainable by taking a single measurement at a constant value of coil current. Thus the decrease in the signal to noise ratio caused by drop in flow velocity that would otherwise occur can be compensated for.
At first sight, a potential drawback of the invention is that it is necessary to connect the flowmeter to a modified display or monitoring device, arranged to recognise a low current in the current signalling loop as indicative of a high flow rate and a high current in the current signalling loop as indicative of a low flow rate. However, the inventor has appreciated that most conventional display or monitoring devices can be modified to operate in this manner without undue difficulty; if the monitoring device simply comprises an analogue meter, it is merely necessary to reverse the scale, and in commonplace computerised electronic monitoring equipment, it is typically only necessary to alter the software processing performed on the signal obtained from electronic measuring circuits associated with the current signalling loop of the flowmeter, or failing that, to make minor modifications to the circuitry. Thus, in fact, little difficulties are caused in practice by this reversal of the current charcteristic.
The invention also provides a current loop sensing device adapted for use with an electromagnetic flowmeter having a negative current against flow rate characteristic, the sensing device having means for monitoring current flowing in a current signalling loop and processing means for obtaining a measure of flow rate from the current flowing in the current signalling loop, the apparatus being arranged to produce a measure of flow rate which decreases as the current in the current signalling loop increases.
In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described, by way of example only, with reference to the accompanying drawing, which shows, in schematic outline form, apparatus in accordance with one example of the invention.
REFERENCES:
patent: 3764880 (1973-10-01), Rose
patent: 3906338 (1975-09-01), Grindheim
patent: 3955413 (1976-05-01), Steele
patent: 4059014 (1977-11-01), Torimaru
patent: 4373400 (1983-02-01), Sekiguchi
patent: 4766770 (1988-08-01), Mannherz et al.
patent: 4969363 (1990-11-01), Mochizuki
patent: 5079957 (1992-01-01), Wehrs
patent: 5436824 (1995-07-01), Royner
patent: 5639970 (1997-06-01), Schulz
patent: 0 485 135 A1 (1991-04-01), None
ABB Instrumentation Limited
Fuller Benjamin R.
Larson & Taylor PLC
Patel Jagdish
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