Measuring and testing – Volume or rate of flow – Using rotating member with particular electrical output or...
Patent
1996-10-07
1998-05-05
Dombroske, Geroge M.
Measuring and testing
Volume or rate of flow
Using rotating member with particular electrical output or...
G01F 1500
Patent
active
057477035
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of monitoring variation in the current value of a rate of fluid flow through a flow meter, and also to a flow meter including apparatus for implementing such method.
BACKGROUND OF THE INVENTION
Two main types of flow meter are known (namely turbine meters and positive-displacement meters having membranes) that share a common characteristic of including a measurement unit or "measuring block" through which a volume of fluid flows and to which a shaft is connected that is driven in rotation about its own axis under the effect of the fluid flowing through said measurement unit.
The shaft may be directly connected to the measurement unit, as in a turbine meter, or it may be indirectly connected to said measurement unit, as in a positive displacement meter having membranes.
Each revolution of the rotary shaft represents a given fluid volume which is a function of the meter.
Thus, to determine the current value of the rate of fluid flow through the meter, it suffices to detect accurately the rotary motion of the shaft over time.
Unfortunately, it is very difficult to detect said rotary motion accurately, e.g. because of friction between the various moving elements in the meter and the measurement mechanism of said meter which gives rise to irregularities in the rotary motion and thus to discontinuities in the variation of the speed of rotation of the shaft.
In addition, the shaft may be subject to small amounts of oscillation.
All of these phenomena give rise to inadequate knowledge of the exact instants at which the position of the shaft is sampled, thereby making it very difficult to perform subsequent electronic processing to determine a fluid flow rate.
Also, when a meter of that type is fitted with its own power supply, e.g. a battery, it is appropriate to reduce the power consumption of said meter to as small a value as possible in order to increase the lifetime of the power supply.
However, not only are fluid meters required to determine accurately and at low power cost the current value of the fluid flow rate, but they are also required to monitor variations over time in said value, over a continuum of flow rates and relative to certain predetermined fluid flow rate values lying in the continuum.
For example, when the fluid flow rate reaches the predetermined maximum value of the flow rate measuring range of the meter, it is necessary for safety reasons to respond quickly by closing a valve situated upstream from the fluid meter.
To do this, the current value of the fluid flow rate must be known accurately over the entire range up to the predetermined maximum value.
For example, for a domestic gas meter, accuracy of the order of 2% to 5% is required for determining the maximum value.
Also, while the fluid meter is in operation, the flow rate can vary suddenly and take on a value greater than the predetermined maximum value, i.e. it can take up an "excess rate".
It is then essential firstly to perceive this increase in the current flow rate value quickly and secondly to be able to take immediate action on the valve.
For example, for a domestic gas meter having a maximum flow rate value of 6 m.sup.3 /h, the excess rate can reach values of the order of 50 m.sup.3 /h in 100 ms, giving about 15 seconds in which to close the valve.
It can thus be seen that it is very important to monitor variation in the rate of flow of a fluid relative to said maximum flow rate value and said excess flow rate value, and that it is also essential to be able to follow this variation over the entire continuum of flow rates, i.e. to avoid missing any detections of shaft position representative of an increase in the speed of rotation of the shaft and thus of the flow rate.
The use of magnetic sensors for detecting the rotary motion of a shaft is known. In such a configuration, a magnetic sensor is secured to the rotary shaft, another magnetic sensor is secured to a stationary support, and on each revolution of the shaft, when the two sensors face each other, the stationary sensor delive
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Janet Philippe
Plisson Fran.cedilla.ois
Puybaret Frederic
Amrozowicz Paul D.
Dombroske Geroge M.
Pojunas Leonard W.
Schlumberger Industries S.A.
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