Abstrict Disclosed is a control apparatus for correcting the measurement
of flow rates for use with a turbine-type flow meter in which the
rotation of a turbine rotating in response to the flow velocity
of fluid to be measured is detected to produce pulses which are,
in turn, integrated to measure the flow rate of the fluid. The apparatus
operates to obtain a value representing the rate of reduction in
the flow rate from the pulse cycle in order to detect a first time
at which the reduction rate increases above a first predetermined
value and a second time at which the reduction rate decreases below
a second predetermined value, produce a correction value on the
basis of the pulse cycle at the second time and the time lag between
the first and second times, and integrate pulses other than those
generated in the period of time between the first and second times,
adding the correction value from the correction value producing
means to the integrated value.
Claims What is claimed is:
1. A control apparatus for correcting measured values of a rate
of flow for use with a turbine-type flow meter comprising:
a turbine rotor which rotates at a speed corresponding to the flow
velocity of a fluid to be measured;
a pulse producing means for detecting the rotational speed of said
turbine rotor and for producing pulses in response thereto having
a pulse repetition rate corresponding to the rotational speed of
said turbine rotor;
a count means for receiving said pulses from said pulse producing
means and for calculating the pulse repetition rate and for obtaining
a value representing the rate of reduction in the flow rate from
the pulse repetition rate and for providing an output signal corresponding
thereto;
a comparing means for receiving said output signal from said count
means and for detecting a first time at which the reduction rate
increases above a first predetermined value and a second time at
which the reduction rate decreases below a second predetermined
value and for providing output signals corresponding thereto;
a correction value producing means for receiving said output signals
from said count means and said comparing means and for producing
a correction value on the basis of the pulse repetition rate at
said second time and the time lag between said first and second
times and for providing an output signal corresponding thereto,
and
an integration means for receiving and integrating pulses from
the pulse producing means other than those produced during the period
of time between said first and second times and for receiving said
output signal from said correction value producing means and for
adding the correction value produced by the correction value producing
means to the integrated value to output a signal corresponding to
the corrected measured value of the rate of flow of the fluid to
be measured.
2. An apparatus according to claim 1 wherein said count means
counts pulses output from the pulse producing means during a predetermined
period of time to calculate said pulse repetition rate.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a control apparatus for a turbine-type
flow meter and, more particularly, to a control apparatus for correcting
measured values of a rate of flow through a turbine-type flow meter.
2. Prior Art
Known turbine-type flow meters for metering the rate of a flow
of fluid to be measured utilize an arrangement which involves detecting
the rotational speed of a turbine rotating in response to the velocity
of the fluid flow such as to produce pulses and integrating the
pulses either to indicate the integrated value or output a signal
representing a unit amount of the fluid when the integration reaches
or exceeds a predetermined value.
In a pipe-line for supplying a fluid to be measured, such as municipal
gas or the like, to a house or other premises, a flow meter is provided
for every house which is adapted to measure the flow rate of the
fluid over a wide range between extremely low and high quantities.
(Since the fluid to be measured is generally municipal gas, it will
hereinafter be referred to as a "gas".) An example of
such a conventional flow meter is described in Japanese Utility
Model Laid-Open Application No. 15520/1986.
In such conventional turbine-type flow meters, problems are encountered
in that when the flow rate of the gas abruptly drops, the inertia
of the turbine rotor means that a relatively long time is required
for the rotational speed of the turbine rotor to decelerate to a
speed which corresponds to the lower flow rate and during this period
of time the measured value of the flow rate may exceed the actual
figure.
SUMMARY OF THE INVENTION
The present invention solves the above-mentioned problems and provides
a control apparatus for correcting the rate of flow measured through
a turbine-type flow meter in order to prevent the rate of flow from
being excessively integrated.
According to the present invention, there is provided an apparatus
for use with a turbine-type flow meter in which the rotational speed
of a turbine rotating in response to the flow velocity of fluid
to be measured is detected to produce pulses which are, in turn,
integrated so as to measure the flow rate of the fluid.
The apparatus comprises a detection means for obtaining a value
for the rate of reduction of the flow rate from the pulse which
corresponds to the rotational speed of the turbine, to detect a
first time at which the reduction rate increases above a first predetermined
value and a second time at which the reduction rate decreases below
a second predetermined value.
The apparatus also comprises a correction value producing means
for producing a correction value in response to the pulse repetition
at the second time and the time lag between the first and second
times.
The apparatus also comprises an integration means for integrating
those pulses other than generated in the period of time between
the first and second times adding the correction value from the
correction value producing means to the integrated value.
With the present invention, the rate of reduction of the flow rate
is compared by the detection means with the predetermined value
to detect the first and second times which respectively represent
the commencement and termination of any abrupt drop in the flow
rate. The integration means serves to interrupt the integration
during the period between the first and second times to prevent
any excessive integration. The correction value producing means
also serves to produce a correction value corresponding to a deficient
integration value during the interruption of the integration performed
by said integration means on the basis of the pulse cycle at the
second time and the time lag between the first and second times,
and to add the correction value to the integration value of the
integration means in order to prevent erroneous integration by the
integration means so as to improve the accuracy of measurement of
the flow rate as indicated by said integration value.
BRIEF DESCRIPTION OF THE DRAWINGS:
FIG. 1 is a block diagram of an embodiment of the circuit of the
present invention;
FIG. 2 is a sectional view of an example of a turbine-type flow
meter to which the circuit of the present invention is applicable;
FIGS. 3(A)-(C), 4 7 and 8 are illustrations explaining the operation
of the circuit of FIG. 1;
FIG. 5 is a flowchart of an example of a microcomputer that may
be used for executing the operation of the circuit of the present
invention; and
FIG. 6 shows waveforms used to explain a modified embodiment of
the circuit of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT:
FIG. 2 shows an example of a turbine-type flow meter to which the
control circuit of the present invention is applicable. In FIG.
2 the flow meter 11 is provided mid-way along a pipe line (not
shown) for supplying a gas, such as municipal gas or the like. The
flow meter 11 has a flow meter body 12 which is provided with a
main flow passage 13 formed therein.
The main flow passage 13 comprises an inlet passage 13a, and a
separate chamber 13b, a metering chamber 13c and an outlet passage
13d. Provided between the inlet passage 13a and the separate chamber
13b is a first valve seat 14 and between the separate chamber 13b
and the metering chamber 13c, a second valve seat 15.
A shut-off valve 16 is provided which includes a valve member 16a
movable to be seated on the valve seat 14 on its upstream side,
a coiled spring 16b for urging the valve member 16a towards the
seat and an electro-magnetic solenoid 16c operable when receiving
a valve-opening signal to drive the valve member 16a away from the
seat.
A change-over valve 17 is also provided which includes a valve
member 17a movable to be seated on the valve seat 15 on its downstream
side, a coiled spring 17b for urging the valve member 17a towards
the seat and an electromagnetic solenoid 17c operable when receiving
a valve-opening signal to drive the valve member 17a away from the
seat. The valve member 17a is formed with a disk-like valve plate
17a.sub.2 securely affixed by a screw or the like to a lower end
of a rod 17a.sub.1 extending into the electromagnetic solenoid 17c.
The valve plate 17a.sub.2 is provided on its lower stepped position
with a ring-like rubber packing 17a.sub.3.
The electromagnetic solenoids 16c and 17c of the shut-off and change-over
valves 16 and 17 are mounted on a flat plate-like mounting base
18 for closing openings 13e and 13f above the valve seat 14 and
15. O-rings 19 and 20 are provided for sealing the gap between the
openings 13e, 13f and the mounting base 18.
The electromagnetic solenoids 16c and 17c will now be described
in more detail. The solenoids 16c and 17c are of a self-holding
type operable so that once their solenoid is energized to attract
the iron core, the core is held attracted even when the solenoid
is de-energized. Solenoids 16c and 17c of this type are advantages
in that only an initial supply of power is required to hold the
core attracted, thereby reducing power consumption.
Upon normal measurement of the flow rate, the solenoid 16c is initially
energized to attract and hold the rod 16a.sub.1 in its upward position
so that the valve member 16a of the shut-off valve 16 is moved away
from its seat. Thus, the shut-off valve 16 will be operated so that
in an emergency and when current is supplied momentarily to eliminate
the magnetic field of the solenoid 16c, the valve member 16a is
urged by the coiled spring 16b into contact with the valve seat
so as to close the valve 16.
A flow rate measuring section 21 is fixedly provided in a mounting
portion 13g downstream of the change-over valve 17. The flow rate
measuring section 21 includes a cylindrical body 21a, a flow passage
21b provided in the body 21a, and upstream and downstream cones
21c and 21d supported by support members 21e within the flow passage
21b.
A turbine rotor or rotatable member 22 having a plurality of blades
22a on the outer periphery of its hub is positioned between the
upstream and downstream cones 21c and 21d. The turbine rotor 22
is supported by bearings 23a, 23b for rotation in response to the
rate of flow of fluid flowing through the flow passage 21b. The
hub of the turbine rotor 22 is provided with magnets 24 embedded
therein.
A pick-up 25 for detecting rotation which is comprised of a magnetic
sensor or the like is embedded in the downstream cone 21c so as
to face the path of the magnets 24 of the turbine rotor 22. The
pick-up 25 detects the rotation of the turbine rotor 22 and outputs
pulses corresponding to the rotation to a control circuit 32.
The control circuit 32 is connected to the electromagnetic solenoids
16c and 17c and detects pulses from the pick-up 25 to integrate
the rate of flow and open or close the change-over valve 17 in response
to the pulse intervals.
Provided in the outer periphery of the body 21a of the flow rate
measuring section 21 are O-rings 26 and 27 for sealing the gap between
the body 21a and the inner wall of the mounting portion 13g. The
mounting portion 13g is provided on its inner wall surface with
first and second steps 13g.sub.1 and 13g.sub.2 and the body 21a
abuts against the first step 13g.sub.1 and defines a nozzle chamber
28 therebetween together with the second step 13g.sub.2.
The body 21a is formed with a nozzle hole 29 having a small diameter
which opens at one end into the flow passage 21b and at the other
end into the nozzle chamber 28. The first end of the nozzle hole
is positioned to face the outer periphery of the blades 22a of the
turbine rotor 22.
Also provided is a separate passage 30 which opens at one end into
the separate chamber 13b and at the other end into the nozzle chamber
28. Thus, the gas flowing through the separate passage 30 is directed
to the nozzle chamber 28 and then through the nozzle hole 29 and
is injected to the blades 22a of the turbine rotor 22. With this
arrangement, the gas is directly injected to the turbine rotor 22
to cause the latter to rotate at a relatively high speed, even when
the rate of gas consumption on the downstream side is very low.
Initially, the control circuit 32 holds the shut-off valve 16 in
its open condition. Thus, the valve member 16a is held in its upward
position to maintain the valve 16 open, as described above, and
this condition is held even when the electric current to the shut-off
valve 16 is interrupted. The closure of this shut-off valve 16 occurs
only in an emergency, and even in such an event a momentary or extremely
brief supply of current to the shut-off valve 16 is sufficient to
immediately close the shut-off valve 16 as described above.
Assuming now that gas consuming equipment connected downstream
of the apparatus starts operating and, for example, a pilot light
of a domestic gas-fired water heater is ignited. The gas consumption
at this time is extremely low and so the change-over valve is held
closed and the gas flows from the separate chamber 13b through the
separate passage 30 the nozzle chamber 28 and the nozzle hole 29
and is injected to the blades 22a of the turbine rotor 22 to rotate
the latter.
Such rotation of the turbine rotor 22 is detected by the pick-up
25 and as the magnets 24 embedded in the turbine rotor 22 pass
thereover, the pick-up 25 outputs pulses having a pulse interval
corresponding to the rotational speed of the turbine rotor 22. The
control circuit 32 then integrates the pulses from the pick-up 25
and calculates the rate of flow of the gas. The control circuit
32 also detects the pulse interval of the pulses input by the pick-up
25.
When the burner of the water heater is ignited and operated to
allow the use of the generated hot water, the gas consumption increases
and so the turbine rotor 22 will rotate at a much higher speed.
Since the control circuit 32 observes the pulse interval of the
pulses output by the pick-up 25 when the gas consumption reaches
the upper limit of the rate of flow through the separate passage
30 and the pulse interval reaches a first predetermined value, the
control circuit 32 outputs a valve-opening signal to the electromagnetic
solenoid 17c.
As a result, the solenoid 17c is energized to attract the valve
member 17a upwardly away from the valve seat 15 under the action
of the electromagnetic force and against the action of the coiled
spring 17b. Since the coiled spring 17b is set to exert the minimum
spring force necessary to close the valve, as will be described
below, the valve member 17a is upwardly moved by a relatively small
driving force. Furthermore, the spring force is also selected to
ensure that, when a predetermined rate of flow is established on
the upstream side and the fluid pressure generated thereby acts
on the valve member 17a, the valve opens against the action of the
spring 17a even when the solenoid is in the non-operating condition.
In the above-mentioned valve opening operation of the change-over
valve 17 the gas flows through the opening of the valve seat 15
to the metering chamber 13c and then through the flow passage 21b
of the flow rate measuring section 21 and is fed from the outlet
passage 13d to the gas equipment downstream.
During this operation, the rotational speed of the turbine rotor
22 may temporarily decrease. With the increase in gas consumption
due to the operation of the water heater, however, the rate of flow
of the gas will rise to cause an increase in the rotational speed
of the turbine rotor 22. The rotational speed of the turbine rotor
22 while this is occurring is detected by the pick-up 25 as described
above, and the control circuit 32 calculates and integrates the
flow rate based on the pulses output by the pick-up 25.
Assuming that the operation of the water heater is now changed
to the condition where only the pilot light is ignited, the gas
consumption of the water heater will then decrease and the flow
rate of the gas passing through the flow passage 21b of the flow
rate measuring section 21 will also drop. Accordingly, the rotational
speed of the turbine rotor 22 will decrease too.
With the control circuit 32 when the pulse interval becomes larger
than the above-mentioned first predetermined value, the change-over
valve 17 opens, but the rotational speed of the turbine rotor 22
temporarily decreases due to the opening operation of the change-over
valve and the pulse interval again becomes larger than the first
predetermined value. If the predetermined values for the valve opening
and closing operations are set at the same value, therefore, the
change-over valve 17 will repeat its opening and closing operations.
To prevent this, a hysteresis is provided in the opening and closing
operations of the change-over valve 17. Namely, the control circuit
32 detects whether or not the pulse interval of the pulses derived
from the pick-up 25 is larger than a second predetermined value
which is set to be larger than the first predetermined value, and
upon detection of a further drop in the rotational speed of the
turbine rotor 22 and an increase in the pulse interval above the
second predetermined value, a valve-closing signal is output to
the change-over valve 17. As a result, the valve member 17a held
in the open position is urged under the force of the spring to abut
against the valve seat 15 thereby closing the main flow passage
13.
The gas then flows from the separate chamber 13b through the separate
flow passage 30 to the nozzle chamber 28 and is injected from the
nozzle hole 29 to the blades 22a of the turbine rotor 22. Thus,
the turbine rotor 22 is rotated by the gas injected from the nozzle
hole 29.
FIG. 1 shows a block diagram of an embodiment of the circuit of
the invention. The circuit of FIG. 1 is provided in the control
circuit 32 of FIG. 2. A terminal 40 receives the pulses from the
pick-up 25 which are, in turn, amplified by an amplifier 41. Thereafter,
the pulses are supplied to and integrated by an integration circuit
42 used as an integration means. At the same time, the pulses from
the amplifier 41 are also supplied to an over-meter detector 43
an over-meter-terminating detector 44 and an in-over-meter-condition
integration circuit 45.
Even when use of gas is abruptly stopped and reduced to zero at
a time T.sub.1 as shown in FIG. 3(A), the rotational speed of the
turbine rotor 22 i.e., the number of pulses per unit of time reaching
the terminal 40 gradually decreases and becomes zero at a time
T.sub.2. Thus, the rotational speed between the times T.sub.1 and
T.sub.2 does not correctly indicate the rate of flow of gas.
The detectors 43 and 44 shown in FIG. 1 measure the pulse rate
by counting the number of pulses received during a constant period.
The measurement is made to determine the rotational speed, N.sub.0
N.sub.1 . . . at times t.sub.1 t.sub.2 . . . with a constant interval
of time as shown in FIG. 3(C).
Furthermore, the over-meter detector 43 subtracts the presently
measured rotational speed Ni from the previously measured rotational
speed Ni-.sub.1 and when the difference Ni-.sub.1 -Ni exceeds a
constant value .DELTA.N (first time), an over-meter-commencing signal
OMS is produced and supplied to the integration circuit 42 and the
in-over-meter-condition integration circuit 45. The constant value
.DELTA.N is a positive value and is set at a value which shows a
considerable variation in the rotational speed N measured at constant
intervals of time and which also shows an abrupt variation in the
flow rate.
The over-meter-terminating detector 44 produces an over-meter-terminating
signal OME and supplies it to the integration circuit 42 and the
in-over-meter-condition integration circuit 45 when the difference
Ni-.sub.1 -Ni becomes less than the constant value .DELTA.N or less
than zero.
Similar to the over-meter detector 43 the in-over-meter-condition
integration circuit 45 counts the number of pulses reached during
a constant period of time to measure the Ni then subsisting and
also measures the over-meter time between the reception of the signal
OMS and the reception of the signal OME. The product of the rotational
speed at the time of reception of the signal OME, Ni and the over-meter
time is then obtained.
As shown in FIG. 4 when, for example, the signal OMS is produced
at a time T.sub.A and the signal OME at a time T.sub.B, an in-over-meter-condition
integrated value N.sub.B (T.sub.B -T.sub.A) corresponding to the
hatched area in FIG. 4 or a correction value, is obtained by the
in-over-meter-condition integration circuit 45 and supplied to the
integration circuit 42.
The integration circuit 42 integrates the pulses from the amplifier
41 but this integration is interrupted during the period between
the reception of the signal OMS and the reception of the signal
OME. With the reception of the signal OMS, integration is achieved
while adding the in-over-meter-condition integrated value from the
in-over-meter-condition integration circuit 45.
The integrated value output by the integration circuit 42 is supplied
to an indicator 46 which, in turn, indicates the integrated value
as the rate of flow.
In this manner, during the period between the generation of the
signal OMS and the generation of the signal OME, the integration
circuit 42 does not perform integration so as to prevent any excessive
integration of the flow rate, and even when the number of revolutions
Ni at the time of generation of the signal OME is not zero, the
in-over-meter-condition integrated value in the period is obtained
by the in-over-meter-condition integration circuit 45 and added
to the integrated value of the integration circuit 42 so that any
deficient integration of the flow rate is corrected and thus avoided.
In this way, the accuracy of the turbine-type flow meter is enhanced
and it is thus usable at sites where large variations in the flow
rate take place. Thus the potential range of application of the
meter is greatly increased.
After the reception of the signal OME and when the in-over-meter-condition
integrated value is totally added at a time to the integration circuit
42 the value of the flow rate to be indicated on the indicator
46 may abruptly vary. To avoid this, the in-over-meter-condition
integrated value can be divided into small values and those divided
small values may be added to the integration circuit 42 one by one
every time the integration circuit 42 receives pulses from the amplifier
41.
It may be possible to perform the operation of the circuit of FIG.
1 using a microcomputer. FIG. 5 shows a flowchart for such an operation.
After the initial setting (step 50), the rotational speed Ni is
measured (step 51), the difference D from the previous rotational
Ni-.sub.1 is obtained (step 52), and substitution of the rotational
speed Ni for the previous rotational speed Ni-.sub.1 is performed
(step 53).
The difference D is then compared with a constant value .DELTA.N
(step 54), and if the difference D is larger than the constant value
.DELTA.N, the timer OT is incremented (step 55), the over-meter
flag is set (step 56), the value of the integrating counter is displayed
as the flow rate (step 57) and the routine is then returned to step
51.
If the difference D is less than the constant value .DELTA.N a
determination is made as to whether or not the flag OF is set (step
58), and if the flag OF is not set, the rotational speed Ni is added
to the integrating counter SC (step 59) and the value of the counter
SC is displayed as the flow rate (step 60). If the flag OF is set,
the product (in-over-meter-condition integration value) of the rotational
speed Ni and the timer OT is added to the value of the counter SC
(step 61), the value of the counter SC is displayed as the flow
rate (step 62) and the timer OT and the flag OF is reset (step 63).
After the execution of the steps 60 and 63 the routine is then
returned to step 51. In the above-described embodiment, the value
.DELTA.N for the first time and the value .DELTA.N for the second
time are equal to each other.
In the region where the rotational speed of the turbine rotor 22
is small, the pulse repetition of the pulse (rotor pulse) entering
the terminal 40 becomes long, as shown in waveform (A) of FIG. 6.
In this case, a gate pulse shown in waveform (B) of FIG. 6 is produced
from the rotor pulse and the H-level of the gate pulse [shown in
waveform (C) of FIG. 6] can be used in place of the rotor pulse.
In such a case, the pulse interval T.sub.P of the gate pulses is
set to become larger than the pulse width T.sub.W, so that the difference
between the number of clock pulses n.sub.i-l and n.sub.i corresponding
to the gate pulses GP.sub.i-l and GP.sub.i, respectively, become
large and the over-meter detection and the over-meter-termination
detection can be easily accomplished.
In the region where the flow rate is large, when the flow rate
abruptly increases the rise in the rotational speed tends to be
delayed. This is, however, cancelled with the delayed fall in the
rotational speed at the time of an abrupt decrease in the flow rate.
Thus, the correction circuit of the present invention may be arranged
to operate only when the flow rate is less than a predetermined
value.
Since the time T.sub.A of the over-meter detection is delayed from
the time T.sub.AA when the abrupt decrease in the flow rate takes
place, as shown in FIG. 7 the rotational speed during this delayed
period is integrated by the integration circuit. In order to correct
this measurement, the signal OME may be output at a time T.sub.BB
which is delayed from the time T.sub.B of the over-metering-termination
detection by a time .DELTA.T sufficient to cancel the measurement.
With this, the over meter time is T.sub.BB -T.sub.A so that excessive
integration of the flow rate between the times T.sub.AA and T.sub.A
(the above-mentioned measurement) can be cancelled with the deficient
integration of the flow rate due to the extended stoppage of integration
of the time .DELTA.T.
When the flow rate is abruptly reduced to a considerable extent
such as to become extremely low, the amount of correction of the
in-over-meter-condition integration is extremely small and there
is practically no need for correction. Therefore, when the rotational
speed becomes less than a predetermined value N.sub.TH, as shown
in FIG. 8 the rotational speed can be regarded as a constant value
k so that the value of the in-over-meter-condition integration can
be k(T.sub.B -T.sub.A) and, further, the constant value K may be
zero.
In a specific application, when the flow rate abruptly becomes
zero, the indication of the value may be stopped, as will be described
below. When the flow rate abruptly becomes zero, the rotor pulse
in the low speed rotation obtained by the subsequent inertial rotation
would be deemed as over-meter termination and the in-over-meter-condition
integration value based on the inertial rotation would be abruptly
integrated into the measured value, so the rotor pulses obtained
by the subsequent inertial rotation might be integrated as measured
values. Even when the flow rate is abruptly decreased to an extremely
small value, the in-over-meter-condition integration value would
be abruptly integrated into the measured value when the over-meter
at low speed rotation ceases, and thus the indication of the measured
value would greatly vary with irregularity, nevertheless the actual
flow rate at that time is extremely small.
In order to correctly distinguish the decision of over-meter-termination
made in a case where the rotation completely ceases from that made
in a case where there is still a low flow rate, as shown in FIG.
8 a time .DELTA.Te equal to the time period of rotation of one
the rotor immediately before stoppage is set as a reference. A waiting
time .DELTA.Tz is set by employing the period of time from when
the rotation becomes lower than a predetermined value N.sub.TH until
when the rotation completely stops thereafter in the case where
the flow rate is changed to zero. When a flow rate pulse is detected
within the time .DELTA.Te following the lapse of the waiting time
.DELTA.Tz after the rotation had become lower than the value N.sub.TH,
the correction value is transferred to the integration circuit 42
and the content of the integration circuit 42 is transferred to
the indicator 46 by utilizing a predetermined rule. If there is
no pulse within the time period .DELTA.Te following the time .DELTA.Tz,
the content of the in-over-meter-condition integration circuit 45
becomes zero and the operation is returned to the normal mode. This
will pose no problem in practice, because the correction value has
already become extremely small. Thus, the above-mentioned irregular
variation in the indication can be avoided.
As described above, the correction circuit for the turbine-type
flow meter in accordance with the present invention is advantageous
in that when the flow rate abruptly decreases, excessive integration
of the flow rate can be prevented and the accuracy of the flow rate
measurement is enhanced to expand the application of the turbine-type
flow meter and the practical utility thereof. |