Abstrict A volume flow meter for flowing media that works on the transit-time
principle includes a conduit measurement section carrying the flowing
medium, two ultrasonic transducers arranged on opposite ends of
the conduit measurement section sending ultrasonic signals into
the flowing medium and/or receiving them from the flowing medium
and a control and evaluation circuit. The ultrasonic transducers
send ultrasonic signals into the flowing medium via thermally insulating
ultrasonic waveguides. The control and evaluation circuit determines
the volume flow based on the difference between the total transit
time of the ultrasonic signals between the ultrasonic transducers
and the sum of the transit times of the ultrasonic signals in the
ultrasonic waveguies.
Claims What is claimed is:
1. A method of determining volume flow according to the transit-time
principle, using a volume flow meter with a conduit measurement
section carrying the flowing medium, two ultrasonic transducers
arranged on opposite ends of the conduit measurement section sending
ultrasonic signals into the flowing medium and/or receiving them
from the flowing medium and a control and evaluation circuit and
wherein the ultrasonic transducers send ultrasonic signals into
the flowing medium via ultrasonic waveguides and the control and
evaluation circuit measures the transit time of the ultrasonic signals
between the ultrasonic transducers and the transit time of the ultrasonic
signals in the ultrasonic waveguides to determine the volume flow
through the conduit measurement section, the improvement wherein
the ultrasonic waveguides are in contact with the flowing medium
and are thermally insulating and the control and evaluation circuit
determines the volume flow based on the difference between the transit
time of the ultrasonic signals between the ultrasonic transducers
and the sum of the transit times of the ultrasonic signals in the
ultrasonic waveguides.
2. The method of claim 1 wherein the control and evaluation circuit
determines the transit time of the ultrasonic signals in the ultrasonic
waveguides from the parts of the ultrasonic signals reflecting on
the ends of the ultrasonic waveguides facing the flowing medium.
3. The method of claim 1 or 2 wherein the control and evaluation
circuit determines the temperature of the flowing medium from the
transit time of the ultrasonic signals in the ultrasonic waveguides
and corrects the volume flow according to the determined temperature
of the flowing medium.
Description BACKGROUND OF THE INVENTION
The invention concerns a volume flow meter for flowing media that
works on the transit time principle, with a conduit measurement
section, two ultrasonic transducers arranged at opposite ends of
the conduit measurement section that send ultrasonic signals into
the flowing medium and/or receive them from the flowing medium and
a control and evaluation circuit, wherein the control and evaluation
circuit measures the transit time of the ultrasonic signals between
the ultrasonic transducers to determine the volume flow through
the conduit measurement section.
Since the known volume flow meters on which the invention is based
have proven very good when used in the industrial sector, many other
applications have recently been developed for these volume flow
meters, particularly applications in which the flowing medium has
clearly high temperatures. Here, their use in the area of oil extraction
is mentioned only as an example. Since piezo crystals are generally
used as the basic component of the ultrasonic transducer in the
known volume flow meters, the possibilities of using the known volume
flow meters are limited to a temperature range up to 120.degree.
C., but at the most 150.degree. C. At higher temperatures, the ultrasonic
transducers regularly used are subject to problems or are completely
dysfunctional.
SUMMARY OF THE INVENTION
The task of the invention is, therefore, to develop and design
the known volume flow meters in such a way that they can be used
even when the flowing medium is at high temperatures and in such
a way that they have superior measurement precision.
The volume flow meter in the invention for flowing media that works
on the transit time principle, in which the task introduced and
described above is solved, is characterized by the fact that the
ultrasonic transducers send ultrasonic signals into the flowing
medium via thermally insulating ultrasonic waveguides. The invention
guarantees that the high to very high temperatures in the flowing
medium are far from being reached on the ultrasonic transducer because
of the thermally insulating ultrasonic waveguide placed between
the flowing medium and the ultrasonic transducers. In one suitable
design of the thermally insulating ultrasonic waveguide, for example,
temperatures of up to 1000.degree. C. are possible for the flowing
medium without affecting conventionally--and hence inexpensively--designed
ultrasonic transducers.
One first advantageous embodiment of the invention of the volume
flow meter is that the ultrasonic waveguide is designed in the form
of a rod. The rod-shaped design of the ultrasonic waveguide guarantees
that even materials with higher heat-conducting coefficients, but
even better other material parameters, particularly with regard
to the required ultrasonic waveguide properties, can be used as
the starting material to produce ultrasonic waveguides.
A particularly big difference between the temperature of the flowing
medium and the temperature of the ultrasonic transducer can be guaranteed
in the volume flow meter according to the invention by the fact
that a cooling device is provided to cool the ultrasonic waveguide.
Here, both active cooling devices, like water or air cooling, and
passive cooling devices are conceivable.
An especially advantageous design for the volume flow meter in
the invention is one in which the ultrasonic waveguides have at
least one cooling rib. This passive cooling device is particularly
advantageous since it requires no other installation expense and
is extremely trouble-free.
Finally, it has also proven particularly advantageous to produce
the ultrasonic waveguides of stainless steel. Ultrasonic waveguides
produced in this way are inexpensive, have a relatively low heat-conducting
coefficient and have good ultrasonic wave-conduction properties.
The invention's design of the volume flow meter necessitates conversion
of the method for determining volume flow according to the transit-time
principle, which can be used in a volume flow meter designed according
to the invention.
The method for determining volume flow according to the transit-time
principle, with the help of a volume flow meter designed according
to the invention, is advantageously designed so that the control
and evaluation circuit determines the volume flow based on the difference
between the total transit time of the ultrasonic signals between
the ultrasonic transducers and the sum of the transit times of the
ultrasonic signals in the ultrasonic waveguides. Not considering
the transit times of the ultrasonic signals in the ultrasonic waveguides
would result in a relatively large error in the measurement results
for volume flow. This is avoided by the method of determining volume
flow described.
The transit time for the ultrasonic signals in the ultrasonic waveguides
can now be found, for example, as part of a calibration process
and can be fed to the control and evaluation circuit as a fixed
value. One particularly advantageous embodiment is the method whereby
the control and evaluation circuit determines the transit time of
the ultrasonic signals in the ultrasonic waveguides from the portions
of the ultrasonic signals reflected on the ends of the ultrasonic
waveguides facing the flowing medium. This guarantees that the actual,
temperature-dependent transit time of the ultrasonic signals in
the ultrasonic waveguides is always used for correction. Compared
to finding the transit time of the ultrasonic signals in the ultrasonic
waveguides only once, this step produces an improvement in measurement
precision from approximately .+-.4% to approximately 0.2%.
Another increase in measurement precision is guaranteed by the
fact that the temperature of the flowing medium is determined by
the control and evaluation circuit from the transit time of the
ultrasonic signals in the ultrasonic waveguides and by the fact
that the result for volume flow is corrected by the control and
evaluation circuit using the temperature of the flowing medium.
Since the transit time of the ultrasound signals in the ultrasonic
waveguides is a direct measure of the temperature of the ultrasonic
waveguides, the temperature of the flowing medium can also be deduced
from the transit time of the ultrasonic signals in the ultrasonic
waveguides. The temperature of the flowing medium now has a major
influence on the measurement precision of the volume flow meter.
Depending on the temperature, for example, the pipeline diameter
of the conduit measurement section changes. Since this is a volume
flow meter and not a mass flow meter, an increase in the diameter
of the conduit measurement section leads to a reduction in the measured
velocity of the flowing medium, which, in turn, without correction
produces too low a value for the volume flow. Due to the effect
described, if there is an increase in the temperature of the flowing
medium from 240.degree. C. up to 260.degree. C. without correction,
there is an error of approximately 1.5%. With the correction described,
this error falls to 0.1%.
BRIEF DESCRIPTION OF THE DRAWINGS
Now there are many ways of designing and developing the volume
flow meter for flowing media in the invention that works on the
transit-time principle and the method of determining volume flow
by the transit-time principle using the volume flow meter in the
invention. Please refer, on one hand, to the dependent patent claims
and, on the other hand, to the description of the preferred examples
of embodiment in connection with the drawings, in which:
FIG. 1 is a schematic representation of a known volume flow meter
for flowing media that works on the transit-time principle;
FIG. 2 is a schematic representation of a volume flow meter designed
according to the invention for flowing media that works on the transit-time
principle;
FIG. 3 is a cross section through one example of embodiment of
an ultrasonic waveguide provided according to the invention;
FIG. 4 is a graphical diagram which shows the dependence of the
temperature of the ultrasonic transducer on the temperature of the
flowing medium in a volume flow meter designed according to the
invention;
FIG. 5 is a similar diagram that shows the dependence of the transit
times of an ultrasonic signal in an ultrasonic waveguides on the
temperature in question, and
FIG. 6 is a graphical diagram that shows the dependence of the
propagation speed of an ultrasonic waveguide on the temperature
of the flowing medium.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
FIG. 1 shows a known volume flow meter for flowing media that works
on the transit-time principle. The volume flow meter has a conduit
measurement section 1 carrying the flowing medium. On opposite ends
of the conduit measurement section 1 there are two ultrasonic transducers
23 sending ultrasonic signals into the flowing medium and/or receiving
them from the flowing medium. FIG. 1 does not show the control and
evaluation circuit that measures the transit time of the ultrasonic
signals between the ultrasonic transducers 23 to determine the
volume flow through the conduit measurement section 1.
The conduit measurement section 1 shown in FIG. 1 represents either
an independent part within a conduit system, or it is made up of
one part of an existing conduit system.
As shown in FIG. 1 known volume meters are generally designed
in such a way that the measurement line 4 connecting the ultrasonic
transducers 2 3 cuts the longitudinal axis 5 of the conduit measurement
section 1 at an angle .o slashed.. Under special circumstances,
namely when the ultrasonic transducers 23 are arranged in curves
in the conduit system on the longitudinal axis 5 of the conduit
measurement section 1 the angle .o slashed. is up to 0.degree..
In order to guarantee an intensive ultrasonic signal at the receiving
ultrasonic transducer 23 it makes sense for the ultrasonic transducers
23 to be aligned with one another.
Both ultrasonic transducers 23 are now controlled by the control
and evaluation circuit in such a way that an ultrasonic transducer
23 sends out ultrasonic signals that are received by the other
ultrasonic transducer 32. If the propagation speed c.sub.0 of the
ultrasonic signals in the flowing medium is not known, it is necessary
for the ultrasonic transducers 23 to be controlled alternately
as sender and receiver. The transmit times between the ultrasonic
transducers 23 are thus: ##EQU1##
In these equations, L stands for the distance between the two transducers
23 and V.sub.m is the average velocity of the flowing medium in
the conduit measurement section 1. Because the average velocity
V.sub.m of the flowing medium is different from zero, the transit
time T.sub.1 of the ultrasonic signal from ultrasonic transducer
2 to ultrasonic transducer 3 is smaller than the reverse, i.e.,
the transit time T.sub.2 from ultrasonic transducer 3 to ultrasonic
transducer 2.
Now, from equations 1 and 2 with known transit times T.sub.1 and
T.sub.2 the unknown variables c.sub.0 and V.sub.m can be determined.
The volume flow through the conduit measurement section 1 that is
being found then comes from V.sub.m in connection with the cross
section of the conduit measurement section 1.
The working method described up to this point is for known volume
flow meters.
FIG. 2 of the drawings now shows a volume flow meter designed according
to the invention. This volume flow meter differs from the known
one in that the ultrasonic transducers 23 are not placed in direct
contact with the flowing medium. FIG. 2 shows schematically that
the ultrasonic transducers 23 send ultrasonic signals via thermally
insulating ultrasonic waveguides 67 into the flowing medium. The
synopsis in FIGS. 1 and 2 and Equations 1 and 2 shows the need to
consider the transit times of the ultrasonic signals in the ultrasonic
waveguides 67 before determining volume flow by means of a control
and evaluation circuit 14. FIG. 2 of the drawings also shows for
the ultrasonic waveguide 6 that part of the ultrasonic signal is
reflected on the end of the ultrasonic waveguides 67 facing the
flowing medium. This part can be changed by special designs of the
ultrasonic waveguide 67.
FIG. 3 shows a cross section through an example of embodiment of
a rod-shaped ultrasonic waveguide 8. This rod-shaped ultrasonic
waveguide 8 is connected on the first end 9 to the flowing medium
and on the second end 10 is in contact with one of the ultrasonic
transducers 23. The advantage of the rod-shaped ultrasonic waveguide
8 in the invention is due to the thermally insulating properties
of the insulating section 12 arranged between the second end 10
and the mounting flange 11. This insulating section 12 can, for
example, also be provided with one or more cooling ribs 16 and is
preferably made of stainless steel--but ceramic or plastic are also
conceivable, for example, as a starting material for the ultrasonic
waveguide 8.
FIG. 4 is a diagram showing the dependence of the temperature T.sub.UT
of one of the ultrasonic transducers 23 on the temperature T.sub.M
of the flowing medium. The diagram in FIG. 4 was recorded at an
ambient temperature of 50.degree. C. and a length of 15 cm for the
insulating section 12. It is clear that temperatures up to 800.degree.
C. for the flowing medium are allowed without affecting or damaging
one of the ultrasonic transducers 23.
The dependence of the transit times of the ultrasonic signals in
the ultrasonic waveguides 67 on the temperature in question T.sub.USW
of the ultrasonic waveguides 67 is shown in FIG. 5. The straight
line on top shows the dependence of the transit time of the portions
of the ultrasonic signals reflecting on the ends of the ultrasonic
waveguides 67 facing the flowing medium on the temperature of the
ultrasonic waveguides 67. The lower straight line in FIG. 5 shows
the dependence of the sums of the transit times of an ultrasonic
signal in both ultrasonic waveguides 67 depending on their temperature.
FIG. 5 shows clearly that the determination of the transit time
of the ultrasonic signals in the ultrasonic waveguides 68 from
the portions of the ultrasonic signals reflecting on the ends of
the ultrasonic waveguides 67 facing the flowing medium allows a
very good correction of the sum of the temperature-dependent transit
times of the ultrasonic signals in the individual ultrasonic waveguides
67.
FIG. 6 shows the dependence of the propagation speed c.sub.1 of
the ultrasonic signals in the ultrasonic waveguides 67 on the temperature
T.sub.M of the flowing medium. Here it is clear that the temperature
of the flowing medium T.sub.M can be determined with no problem
from the transit time of the ultrasonic signals in the ultrasonic
waveguides 67 and thus can be used to correct the value for the
volume flow. This correction can be made, for example, by determining
the current diameter of the conduit measurement section 1 based
on a known diameter of the conduit measurement section 1 at a predetermined
temperature and the temperature of the flowing medium, found as
explained, using material constants for the material of the conduit
measurement section 1. |