Abstrict In a volume flow meter with a measuring line, a first measuring
head and a second measuring head, the flow volume can be measured
more simply and more reliably using evaluation technology by having
the measuring line made of a material that transmits an acoustic
signal given off by one of the measuring heads at a slower sound
velocity than the fluid.
Claims What is claimed is:
1. A volume flow meter for measuring the flow volume of a fluid
by determining the difference in the travel times of at least two
pulsed acoustic signals, said meter being of the type including
a measuring line (2), a first measuring head (5), and a second measuring
head (6), wherein
a first sharp, precisely definable leading edge of the pulsed acoustic
signal transmitted by one of the measuring heads (56) is used for
the direct time measurement,
the two acoustic signals are transmitted through the fluid as a
measuring signal as well as through the material of the measuring
line (2) as an interfering signal at least partially interfering
with the measuring signal, and
the measuring line (2) is made of a material that transmits an
acoustic signal at a slower sound velocity than the fluid transmits
said signal.
2. The volume flow meter according to claim 1 wherein the measuring
line (2) is of plastic.
3. The volume flow meter according to claim 1 or 2 and further
including a frame-like meter housing (7) for housing the measuring
line (2) and the measuring heads (56).
4. The volume flow meter according to claim 4 wherein the meter
housing (7) is of plastic.
5. In a volume flow meter for measuring the flow volume of a fluid
by determining the difference in the travel times of at least two
signals, said meter being of the type including a measuring line
(2), a first measuring head (5) and a second measuring head (6),
the improvement wherein the measuring line (2) is made of PFA plastic
material that transmits an acoustic signal transmitted by one of
the measuring heads (56) at a lower sound velocity than the fluid
transmits said signal.
6. In a volume flow meter for measuring the flow volume of a fluid
by determining the difference in the travel times of at least two
signals, said meter being of the type including a measuring line
(2), a first measuring head (5) and a second measuring head (6),
the improvement wherein the measuring line (2) is made of a material
that transmits an acoustic signal transmitted by one of the measuring
heads (56) at a lower sound velocity than the fluid transmits said
signal and further including a frame-like meter housing (7) for
housing the measuring line (2) and the measuring heads (56), said
meter housing being of CTFE plastic material.
Description FIELD OF THE INVENTION
The invention concerns a volume flow meter for measuring the flow
volume of fluids, with a measuring line, a first measuring head
and a second measuring head.
BACKGROUND OF THE INVENTION
Many industrial applications require precise determination of the
volume of fluid flowing through a measuring line. The volume of
fluid can be calculated from the cross section of the measuring
line and the flow speed of the fluid. Determining the cross section
of the line is easy, since it is preset. Hence, precise measurement
of flow speed is decisive in accurately determining flow volume.
On the one hand, the measured value of the flow speed must be precise
and, on the other hand, it may be necessary to take a continuous
measurement or repeat measurements at the shortest possible time
intervals, since short-term fluctuations in flow speed can occur.
In order to include these fluctuations in the calculation of the
flow volume over a longer period of time, the total flow volume
is determined by adding the flow volume within short periods of
time.
In the state of the art (H. Bernard, "Ultraschall-Durchflussmessung"
in Das Handbuch fur Ingenieure", Bonfig/Bartz/Wolff, 2nd Edition),
on which the invention is based, the flow speed of the fluid is
measured by sending an acoustic signal along a set measuring path.
In this way, the timing of the acoustic signal on the measuring
path from an acoustic sender to an acoustic receiver in the fluid
is determined by the sound velocity and the flow speed (drag). The
principle of acoustic flow measurement by the time difference method
is derived from this. In the fluid, acoustic signals are sent upstream
and downstream alternately or simultaneously. Because of the different
diffusion speed, the acoustic signals reach the acoustic receiver
after different times t.sub.1 and t.sub.2 upstream and downstream
on the same length measuring path. The difference t.sub.2 -t.sub.1
is a measurement of the average flow speed on the measuring path
formed by the acoustic sender and receiver. The senders are excited
to oscillate by an electrical voltage and give off an acoustic signal
in the fluid. The acoustic receivers receive this signal after it
runs along the measuring path and convert it into an electrical
voltage. The time difference is detected from the acoustic signal
times found in this way, and is proportional to the flow speed of
the fluid in the measuring line.
In the state of the art, two measuring heads are placed on a measuring
line in such a way that their connecting line has a component parallel
to the direction of the flow speed. The measuring heads are either
placed in contact with the fluid in the measuring line or attached
to the measuring line from outside with no contact with the fluid.
The measuring heads each contain a piezoelectric transducer, with
which the acoustic signal necessary for flow measurement is produced
and received (acoustic receiver). If a high-frequency alternating
voltage is applied to the two surfaces of the piezoelectric transducer
that have electrically conductive layers, the piezoelectric transducer
periodically changes its thickness at the same frequency and is
capable of producing sound waves in the surrounding medium. This
process is reversible, so that acoustic signals running from the
sender through the fluid to a piezoelectric transducer of the second
measuring head produce a thickness oscillation therein which produces
an electrical alternating voltage in the piezoelectric transducer
of the second measuring head. This is amplified and further processed
by known electronic elements.
Moreover, in the state of the art, direct time measurement by the
"leading edge" method is used for the time difference
method. For this, a precisely defined, pulsed acoustic signal is
sent from a first measuring head to a second measuring head, wherein
to measure the time t.sub.1 only the first sharp, precisely definable
side of the pulsed acoustic signal is used. At the same time, an
acoustic signal is sent from the second measuring head to the first
measuring head and the time t.sub.2 is measured in the same way.
The time difference t.sub.2 -t.sub.1 is directly linearly proportional
to the average flow speed; other parameters like, for example, the
temperature-dependent density and viscosity, are not included in
the measurement.
In addition to the acoustic signal sent from the fluid as a measuring
signal, an acoustic signal occurs as an interfering signal due to
the transmission of the acoustic signal through the material of
the measuring line. In the known volume flow meters, the measuring
line is made of metal, in which the sound velocity is greater than
in fluid. The sound velocity for metals is in the range of 4000-5000
m/s and for fluids in the range of 1500 m/s. The acoustic signal
to be regarded as an interfering signal is thus received by the
acoustic receiver before the acoustic signal to be evaluated as
a measuring signal, so that the measuring signal is superimposed
over the interfering signal. This superposition thus occurs especially
at the beginning of the measuring signal used as a measurement for
determining the running time, as described above. The intensity
of the interfering signal is generally the same size or greater
than the measuring signal, since the acoustic signal is transmitted
very well through metal. This also makes it difficult to determine
the running time of the measuring signal.
In the state of the art, attempts have been made in various ways
to suppress the interfering signal using evaluation technology.
One possibility consists of the fact that a time window is set up
in which the interfering signal occurs regardless of the flow speed
at constant running time, while the acoustic signal received by
the respective acoustic receiver is suppressed. But it must be guaranteed
that, in each case, there is a sufficient running time difference
between the interfering signal and the measuring signal. In the
state of the art, the known volume flow meters must, therefore,
have a long enough measuring path. Another way of suppressing the
interfering signal is by setting an intensity threshold from which
the output signal produced by the acoustic receiver is evaluated.
Here, however, it must be guaranteed that the intensity of the measuring
signal is greater than that of the interfering signal. But this
is problematic, as already described above.
SUMMARY OF THE INVENTION
The invention thus sets itself the task of designing and further
developing the known volume flow meter so that flow volume can be
measured more simply and reliably using known evaluation technology.
The task indicated above is solved in the invention by having the
measuring line composed of a material that transmits an acoustic
signal given off by one of the measuring heads at a slower sound
velocity than the fluid. According to the invention, it was thus
recognized that by using a suitable material for the measuring line,
the acoustic signal to be regarded as an interfering signal has
a longer running time than the acoustic signal to be evaluated as
a measuring signal. This guarantees that in each case, the measuring
signal to be evaluated is received by the respective acoustic receiver
before the interfering signal. Since evaluation of the measuring
signal consists of determining the start of the measuring signal,
any potential superposition of the measuring signal by the interfering
signal that then occurs is insignificant in determining the running
time of the measuring signal.
Another advantage of the volume flow meter of the invention lies
in the fact that the measuring path can be shortened compared to
the state of the art, since the measuring signal is received before
the interfering signal by the respective acoustic receiver regardless
of the length of the measuring path. The result is thus a more compact
design, which offers advantages when the volume flow meter of the
invention is used in small facilities or equipment.
Plastic is preferably used as the material for the measuring line,
especially PFA. The sound velocity in plastic is in the range of
1000 m/s, hence clearly less than the sound velocity of roughly
1500 m/s for fluids already mentioned above.
Now there are various ways of designing and further developing
the theory of the invention, which may be referred to in the claims
subordinate to patent claim 1 on one hand, and in the explanation
of a preferred embodiment along with the drawing, on the other.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of the invention may be better
understood by referring to the following description in conjunction
with the accompanying drawings, in which:
FIG. 1 is a cross sectional view of a preferred embodiment of a
volume flow meter according to the invention, and
FIG. 2 is a perspective view of the volume flow meter in FIG. 1.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
A preferred embodiment of a volume flow meter 1 in which the theory
of the invention is embodied, is shown in FIGS. 1 and 2. The volume
flow meter 1 has a measuring line 2 in which a fluid flows in through
a supply line 3 and flows out through a drainage line 4. On one
end of the measuring line 2 there is a first measuring head 5 and
on the other end of the measuring line 2 there is a second measuring
head 6.
According to the invention, the measuring line 2 is made of a material
that transmits the acoustic signal at a slower sound velocity than
the fluid. This results in a longer running time of the acoustic
signal transmitted over the measuring line 2 which is the interfering
signal, compared to the acoustic signal transmitted through the
fluid, which is the measuring signal.
As best seen in FIG. 2 in the embodiment shown there, the measuring
line 2 is made of plastic, preferably of PFA. The measuring line
2 and the two measuring heads 5 and 6 are in a frame-like meter
housing 7 which is made up, on one hand, of sensor housings 8 and,
on the other, of side plates 9. The sensor housings 8 are used to
protect the measuring heads 5 and 6 while the side plates 9 are
used to stabilize the volume flow meter 1. The frame-like meter
housing 8 of course, also transmits the acoustic signal, so that
the sensor housing 8 and the side plates 9 are also made of plastic,
preferably of CTFE.
Clamping plates 10 encompassing the measuring line 2 are also provided.
The measuring line 2 and the sensor housings 8 are braced to one
another by clamping screws (not shown) that are supported on the
clamping plates 10 and go into the sensor housings 8. The side plates
10 are connected to the sensor housings 8 with the screws 11 that
attach to the sensor housings 8.
The two measuring heads 5 and 6 each contain conventional piezoelectric
transducers which function both as acoustic senders and receivers
in the way described above. For this, the input and output signals
necessary to operate the measuring head 5 are fed in and out over
electric lines 12 that go into the associated sensor housing 8 via
a cable duct 13. This happens in the same way with the measuring
head 6 via electric lines 14 that go into the sensor housing 8 for
that head through a cable duct 15.
The time difference method is now carried out in the following
way. A pulsed acoustic signal is given off at the same time by both
measuring heads 5 and 6; both measuring heads 5 and 6 thus function
simultaneously as acoustic senders. Then, both measuring heads 5
and 6 are switched over to receive so that they can receive the
acoustic signal given off by the other measuring head 6 and 5.
The evaluation signals used in evaluation electronics, which have
been produced by the measuring heads 5 and 6 are now evaluated
in the following way: the start of production of the acoustic signal
with the measuring heads 5 and 6 is determined using the known "leading-edge"
method described above, and a first and second counter, set back
ahead of time, are set in motion. The first counter is stopped as
soon as the first acoustic signal is received by the measuring head
6. The counter status thus gives a measurement of the running time
t.sub.1 of the acoustic signal that was given off by the measuring
head 5. In the same way, the counter status of the second counter
is used to determine the running time t.sub.2 of the acoustic signal
that was sent out by the measuring head 6. Since the running time
difference t.sub.2 -t.sub.1 is directly proportional to the flow
speed, the flow volume of the fluid can be calculated from it.
In the volume flow meter of the invention, because of the fact
that the measuring signal, i.e., an acoustic signal through the
fluid in the measuring line 2 has a shorter running time than the
interfering signal, i.e., the acoustic signal through the measuring
line 2 and the meter housing 7 each first acoustic signal measured
can be used to determine the running time of the measuring signal.
It is therefore no longer necessary to suppress the interfering
signal, as in the state of the art, using time windows or signal
thresholds. Measurement of the flow speed of the fluid in the measuring
line 2 has therefore been substantially simplified.
Finally, it should be pointed out that the acoustic signals do
not cross the measuring line 2; rather they diffuse in the direction
of the longitudinal axis of the measuring line 2 because the measuring
heads 5 and 6 are provided on both ends of the measuring line 2.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently attained
and, since certain changes may be made in the above construction
without departing from the scope of the invention, it is intended
that all matter contained in the above description or shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
It is also to be understood that the following claims are intended
to cover all of the generic and specific features of the invention
herein described .
|