Abstrict This flow meter uses two known flow responsive techniques and combines
them in a unique structure and system to form an improved flow meter
whose range extends from low to high flow rates. The meter comprises
a flow tube with a region of reduced diameter between its inlet
and outlet. A microbridge flow sensor of a suitable type known in
the art, which responds to thermal changes caused by changes in
the flow rate, is located in the region of reduced diameter. A post
downstream of the microbridge sensor creates Karman vortices whose
frequencies are a function of flow rate and which are detected through
ports in the tube by a suitable detector. The output of the microbridge
is linear from low flow rates to a mid range and the Karman flow
meter output is linear from the mid range to high flow rates. A
correction factor is applied to the thermal flow meter output. The
correction factor is determined by a ratio of the Karman flow meter
output to the thermal output for a flow rate in the mid range, allowing
a smooth transition from thermal to Karman outputs and vice versa
in the mid range.
Claims What is claimed is:
1. A composite flow meter comprising:
a flow tube with an inlet and an outlet and a region of reduced
diameter between said inlet and outlet;
a thermal flow sensor for measuring flows extending from a low
flow range to a mid flow range located in said reduced diameter
section of said flow tube;
a post and a pair of ports in said flow tube located downstream
of said reduced diameter region forming a Karman vortex flow meter
for measuring flows extending from said mid range to a high flow
range.
2. A composite flow meter comprising:
a flow tube with an inlet and an outlet and a region of reduced
diameter between said inlet and outlet;
a thermal flow sensor for measuring flows extending from a low
flow range to a mid flow range located in said reduced diameter
section of said flow tube;
a post and a pair of ports in said flow tube located downstream
of said reduced diameter region forming a Karman vortex flow sensor
for measuring flows extending from said mid range to a high flow
range;
means for modifying the output of said thermal flow sensor by a
factor determined by the ratio of the output of said Karman flow
sensor to the output of said thermal flow sensor for a flow rate
in said mid range.
3. A composite flow meter comprising:
a Karman vortex flow meter for measuring a high flow range and
producing an output signal indicative of flow;
a thermal flow sensor for measuring a low flow range and producing
an output signal indicative of flow; and
means for correcting said thermal flow output signal by said Karman
vortex flow meter output signal.
Description BAKCGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a composite flow meter, and more particularly,
to a flow meter of this kind which is composed of a Karman vortex
flow meter and a mircobridge flow sensor.
2. Description of the Prior Art
A composite flow meter conventionally employed in the art is such
one that is composed of a microbridge sensor 1 and a fluidics flow
meter 2 as shown in FIG. 1.
Such conventional composite flow meter composed of the microbridge
sensor 1 and the fluidics flow meter 2 has a problem in that the
complicated structure and large size of the fluidics flow makes
it difficult to mount the microbridge sensor 1 at a location in
the fluidics flow meter 2 and also reduce the size of the whole
flow meter.
OBJECTS AND SUMMARY OF THE INVENTION
In view of the above-mentioned problem, it is the object of the
present invention to provide a composite flow meter which is in
a simple and small structure and can be manufactured at a low cost.
To achieve the above object, there is provided a composite flow
meter comprising:
a Karman vortex flow meter for measuring a high flow range; and
a thermal flow sensor for measuring a low flow range.
Preferably, the output signal value derived from the thermal flow
sensor is corrected by the output signal value derived from the
Karman vortex flow meter.
The above and other objects and features of the present invention
will become apparent from the following detailed description of
the preferred embodiments with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a conventional composite flow
meter;
FIG. 2 is a cross-sectional view of an embodiment of the present
invention;
FIG. 3 is a side view taken from the direction shown by an arrow
A; and
FIG. 4 is a graph showing output signal levels of the respective
flow meter and sensor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 2 shows an embodiment of a composite flow meter according
to the present invention. The shown composite flow meter includes
a microbridge sensor 1 a Karman vortex flow meter 2a, a screen
3 for preventing disturbance in flow, a vortex generator 4 for generating
Karman vortex and a pressure measuring bore 5 for detecting vortex.
An arrow F indicates the direction of the flow.
The present embodiment measures a flow in a large amount range
with the Karman vortex flow meter 2a and a flow in a small amount
range with the microbridge sensor 1.
Such combination of the microbridge sensor 1 and the Karman vortex
flow meter 2a makes its structure simpler, its size smaller, the
mounting of the microbridge sensor 1 easier and its manufacturing
cost lower than the conventional composite flow meter formed of
a fluidics flow meter and a microbridge sensor.
FIG. 3 is a side view of the composite flow meter taken from the
direction shown by an arrow A. In the same drawing, reference numeral
6 designates a tube and 7 a pressure/electric converting sensor.
Next, a method of correcting the output signal value from the microbridge
sensor 1 with the output signal value from the Karman vortex flow
meter 2a with reference to FIG. 4.
In FIG. 4 a curve S.sub.FA is a characteristic curve indicative
of the output signal level from the microbridge sensor 1 a curve
S.sub.FB a characteristic curve indicative of the output signal
level from the Karman vortex flow meter 2a, and a curve S.sub.FAC
a characteristic curve indicative of the corrected output signal
level from the microbridge sensor 1. The abscissa of the graph represents
the flow (F) and the ordinate the output signal level. The correction
is effected between output signal levels F1 and F2. If it is assumed,
for example, that the output level of the microbridge sensor 1 is
Fb and that of the Karman vortex flow meter 2a Fa when the flow
is FA, the correction coefficient k is calculated by dividing Fa
by Fb (Fa/Fb). Therefore, the corrected output signal level Fb'
of the microbridge sensor 1 is calculated by the following equation;
By this correction applied to the output of sensor 1 by a suitable
circuit 8 continuity of the output signal can be maintained even
if the microbridge sensor 1 is changed over to the Karman vortex
flow meter 2a, and vice versa.
The output signal levels F1 and F2 for determining a correction
range of the output signal are selected in the following manner.
Assuming that a lower limit value which can be correctly measured
by the Karman vortex flow meter 2a is represented by FO, and an
upper limit value which can be correctly measured by the microbridge
sensor 1 by F3 the levels F1 and F2 are selected so as to satisfy
the following equation:
The change-over between the microbridge sensor 1 and the Karman
vortex flow meter 2a is effected between the output signal levels
F1 and F2. The change-over can be automatically carried out by a
microcomputer (not shown) which is provided with the output signal
level from the Karman vortex flow meter 2a. Also, a signal processing
circuit including a comparator may be used in place of the microcomputer.
Incidentally, since the Karman vortex flow meter is generally adapted
to measure a narrower flow range than the fluidics flow meter, it
is necessary to extend the measuring range of the microbridge sensor
so that the composite flow meter of the present invention is applicable
to the same flow range as the fluidics flow meter.
According to the present invention as described above, the composite
flow meter can be made simple and small at a low cost by employing
a Karman vortex flow meter for measuring a high flow rate range
and a microbridge sensor for measuring a low flow rate range.
Further, the output signal value derived from the microbridge sensor
is corrected by the output signal value from the Karman vortex flow
meter which is more accurate than the microbridge sensor (thermal
flow meter) for a long period, to thereby maintain the continuity
and accuracy of the output signal in the whole range. |