Abstrict A flow meter arrangement has a venturi-type flow meter having a
tubular flow channel for conveying a gas flow to be measured and
in which there is provided a constriction. The arrangement further
has a hot wire flow meter having a sensing element located within
the flow channel and a measurement system that receives an output
from the venturi-type flow meter and an output from the hot wire
flow meter, and that determines a gas flow rate therefrom.
Claims I claim:
1. A flow meter arrangement comprising: a venturi-type flow meter
having a tubular flow channel for conveying a gas flow to be measured,
said venturi-type flow meter emitting an output; a hot wire flow
meter having a sensing element disposed in said flow channel of
said venturi-type flow meter, said hot wire flow meter generating
an output separate from said output of said venture-type flow meter;
and a measurement system connected to said venturi-type flow meter
and to said hot wire flow meter to receive the respective outputs
therefrom, said measurement system determining a gas flow rate in
said flow channel from said outputs.
2. A flow meter arrangement as claimed in claim 1 wherein said
measurement system comprises a comparator for comparing at least
one of said outputs to a threshold value to obtain a comparison
result and which, dependent on said comparison result switchably
selects one of said venturi-type flow meter and said hot wire flow
meter to provide an output, as a currently selected output, and
wherein said measurement system determines said gas flow rate only
from said currently selected output.
3. A flow meter arrangement as claimed in claim 2 wherein each
of said venturi-type flow meter and said hot wire flow meter has
a sensitivity, and wherein said comparator compares said output
from at least one of said venturi-type flow meter and said hot wire
flow meter to a threshold value representing a sensitivity at which
the respective sensitivities of said venturi-type flow meter and
said hot wire flow meter are substantially equal.
4. A flow meter arrangement as claimed in claim 2 wherein said
threshold value is a first threshold value, and wherein said measurement
system comprises a memory containing a second threshold value associated
with the output of the venturi-type flow meter and a third threshold
value associated with the output of the hot wire flow meter, said
second and third threshold values being respectively indicative
of different gas flow rates, and wherein said comparator has access
to said memory and compares said currently selected output with
the respective second or third threshold from said memory associated
with said currently selected output.
5. A flow meter arrangement as claimed in claim 1 wherein said
measurement system comprises a difference former for forming a difference
value between the output from the venturi-type flow meter and the
output from the hot wire flow meter, and a comparator for determining
if said difference value exceeds a predetermined threshold.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flow meter arrangement, especially
one suitable for the measurement of respiratory gas flow associated
with the breathing of a patient.
2. Description of the Prior Art
In hospitals, during for example intensive care or surgical procedures,
mechanical breathing aids are used to assist or control the breathing
of a patient. It is important to be able to measure gas flow to
and/or from the patient since this typically provides a control
parameter for the mechanical breathing aid and may also be used
in the derivation of parameters descriptive of the operation and
metabolism of the lung. It is also important, particularly with
respect to the measurement of expiratory gas flow, which the flow
meter presents a relatively small resistance to the gas flow being
measured.
Venturi-type or so-called "differential pressure" flow
meters are well known in the art and are particularly suited to
the clinical applications mentioned above. Such a meter generally
has a tubular flow channel for conveying the gas flow to be measured
and having a constriction, reducing the cross-sectional area of
the flow channel and providing a resistance to gas flow. Also provided
is a differential pressure gauge for measuring a pressure drop within
the channel that is caused by the constriction. This pressure drop
is proportional to the gas flow to be measured. For laminar flow
the sensed pressure difference is directly proportional to the volume
flow rate while for the more typical turbulent flow it is proportional
to the square of the volume flow rate.
The venturi-type flow meter has a problem that its sensitivity
and accuracy at low flow rates is relatively poor. This problem
may arise particularly when the meter is employed in the measurement
of gas flow to and from small children or neonates.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a flow meter
arrangement which has a sensitivity and a dynamic range that are
improved compared to known flow meter arrangements. This object
is achieved in accordance with the principles of the present invention
in a flow meter arrangement that includes a venturi-type flow meter
having a tubular flow channel for conveying a gas flow to be measured,
and a hot wire flow meter having a sensing element disposed in the
flow channel of the venturi-type flow meter, and a measurement system
that receives an output from the venturi-type flow meter and an
output from the hot wire flow meter and which determines a gas flow
rate in the channel from these outputs.
By providing a meter arrangement in which the output from a venturi-type
flow meter is augmented at low flow rates by the output from a known
hot-wire flow meter its sensitivity and dynamic range is enhanced
over either one of the individual flow meter devices of which it
is formed.
The output representing a flow rate measured by the venturi-type
meter and the output representing the flow rate measured by the
hot wire meter may be switchably selected as a basis for a determination
of the flow rate by the arrangement. Switching preferably is done
dependent on a comparison of a threshold value associated with each
output in a manner to ensure that the output from the meter having
the better sensitivity is always utilized.
Moreover, as a safety precaution, the output from the flow meter
utilized in the determination of the flow rate by the arrangement
may be compared with that output from the other meter and a warning
provided by the arrangement if a difference between the two outputs,
or values dependent on those outputs, exceeds a predetermined threshold.
Most usefully the sensing element of the hot wire flow meter may
be disposed in the flow conduit at a location to measure flow in
the region of increased flow speed caused by the constriction. This
increases the sensitivity of the meter to small changes in the gas
flow rate to be measured.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a meter arrangement according
to the present invention.
FIG. 2a is a diagrammatic representation of a pressure response
characteristic of a typical venturi-type flow meter used in the
arrangement of FIG. 1.
FIG. 2b is a diagrammatic representation of a voltage response
characteristic of a typical hot wire flow meter used in the arrangement
of FIG. 1.
FIG. 3 is a diagrammatic representation of a measurement response
characteristic of an arrangement according to FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the flow meter arrangement 2 of FIG. 1 a tubular flow channel
4 has a constriction 6 formed as a neck, within it and is intended
for conveying a gas flow to be measured through it in a direction
indicated by the arrows. The flow channel 4 is also constructed
with a first pressure port 8 and a second pressure port 10 which
are disposed for pressure communication with gas in the channel
4 at locations respectively before and after an entrance 12 to the
constriction 6 so as to enable a measurement to be made of a pressure
drop within the channel 4 caused by gas flowing through the constriction
6. A first pressure sensor 14 and a second pressure sensor 16 are
positioned within the first pressure port 8 and the second pressure
port 10 respectively to supply a measure of the pressure P.sub.1P.sub.2
respectively at those locations to a difference forming unit 18.
The difference-forming unit 18 of the present embodiment is configured
in a known manner to form a pressure difference value .DELTA.P as:
.DELTA.P=P.sub.1-P.sub.2 (1) which is supplied to a measurement
system 20. Alternatively, a known differential pressure sensor (not
shown) may be connected to the two pressure ports 810 that has
an output indicative of the pressure difference value .DELTA.P that
may be supplied directly to the measurement system 20.
It will be appreciated by those skilled in the art that the configuration
described above operates as a known venturi-type flow meter.
It is well known that the pressure difference .DELTA.P is related
to the gas flow rate within the channel 4 according to the equation:
.DELTA.P=K*(.delta..phi./.delta.t).sup.2 (2) wherein K is a constant
dependent upon, among other things, the cross-sectional areas of
the flow channel 4 in the regions of the pressure ports 810; and
.delta..phi./.delta.t is the volume flow rate of the gas, the flow
rate of which is to be measured.
A typical pressure response characteristic of this venturi-type
flow meter, calculated from equation (2), is illustrated by the
plot of .DELTA.P (mbar) against .delta..phi./.delta.t (l min.sup.-1)
which is shown in FIG. 2a. As can be seen from the characteristic
curve 22 the venturi-type meter provides high sensitivity at higher
flow rates but the resolution is much poorer at lower flow rates.
Returning to the flow meter arrangement 2 of FIG. 1 a sensing
element 24 of a known hot wire flow meter is located within the
flow channel 4 preferably proximal the entrance 12 to the constriction
6 that is, in the region of increased flow velocity in the channel
4. A control unit 26 of the hot wire flow meter is provided in electrical
connection with the sensing element 24 and operates in a known manner
to vary a current supplied to the sensing element 24 so as to maintain
it at a known temperature as gas flows within the flow channel 4
and to provide an output indicative of the supplied current, for
example a measure of output voltage V.sub.s of the sensing element
24 which may be expressed according to the known equation: V.sub.s=[C.sub.0R.sub.sA(T.sub.s-T.sub.f-
)+C.sub.1R.sub.sA(T.sub.s-T.sub.f)(.delta.m/.delta.t).sup.0.5].sup.0.5
(3) where R.sub.s is the resistance of the sensor element 24; T.sub.s
is the temperature of the element 24; T.sub.f is the temperature
of the gas flow; A is the heat transfer area; C.sub.0 and C.sub.1
are constants depending on the properties of the gas; and .delta.m/.delta.t
is the mass flow rate of the gas.
As can be seen from equation (3) the output signal from the control
unit 26 is related to the gas flow rate by the 1/4 power. A typical
voltage response characteristic of this hot wire flow meter, calculated
after equation (3), is illustrated by the plot of V.sub.s (mv) against
.delta.m/.delta.t (l min.sup.-1) that is shown in FIG. 2a. As can
be seen from the characteristic curve 28 the hot wire flow meter
provides high sensitivity at lower flow rates but rapidly looses
sensitivity as the flow rates increase.
Returning now to the flow meter arrangement 2 of FIG. 1 the measurement
system 20 is, in the present embodiment, programmed to employ the
output from either the difference forming unit 18 or the control
unit 26 in making a determination of the gas flow rate in a manner
described in more detail below and to provide an output 30 indicative
thereof.
A comparator 32 is provided as a part of the measurement system
20 and may be realized through suitable programming of the microprocessor
of the system 20. The comparator 32 is adapted to compare the outputs
from the units 1826 of the respective venturi-type flow meter and
the hot wire flow meter with an associated threshold value that
may be stored within a memory 34 that is included in the measurement
system 20. The selection of the output from either the difference
forming unit 18 or the control unit 26 as a current output by which
the flow rate is to be determined is made within the measurement
system 20 in dependence of the results of the comparison carried
out by the comparator 32. The measurement system 20 operates to
make the selection, which provides a best sensitivity for the flow
meter arrangement 2 at the prevailing gas flow with the flow channel
4 based on the characteristics of the venturi and the hot wire flow
meters.
In one embodiment of the arrangement 2 of FIG. 1 and with reference
to its measurement response characteristic curve depicted in FIG.
3 the memory 34 stores a threshold value T which is associated
with the output from the control unit 26 indicative of a flow rate
of gas through the channel 4 at which the sensitivities of the venturi-type
flow meter and the hot wire flow meter are substantially the same.
As can be seen from the response characteristic curves 2228 the
sensitivity of the venturi-type flow meter (curve 22) decreases
for flow rates lower than this threshold T whilst the sensitivity
of the hot wire flow meter (curve 28) decreases for flow rates higher
than this threshold T. The comparator 32 of the measurement system
20 is programmed to compare the threshold value T with the output
from one or both of the difference forming unit 18 and the control
unit 26 (in the present example only the output from the control
unit 26 is utilized). From this comparison the measurement system
20 is programmed to employ the output from the difference forming
unit 18 in the determination of the flow rate of gas within the
channel 4 if the output from the control unit 26 is greater than
the threshold value T and to employ the output from the control
unit 26 in the determination of the flow rate of gas within the
channel 4 if the output from the control unit 26 is less than the
threshold value T.
According to a further embodiment of the arrangement 2 of FIG.
1 the measurement system 20 may be adapted so that the comparator
32 employs two different threshold values T.sub.a and T.sub.b, on
each side of the original threshold value T. Instability in the
arrangement 2 when the gas flow to be measured is of the order of
the flow value for which the threshold T is set may thus be avoided.
This is also illustrated in FIG. 3. As shown, the output from the
control unit 26 of the hot wire sensor is used in the range from
zero to the threshold T.sub.a. From this point (curve (a)) and upwards
the output from the difference forming unit 18 of the venturi-type
flow meter is used. When the currently selected output is from the
difference forming unit 18 then the threshold value T.sub.b is employed
in the comparator 32 in order to determine when the currently selected
output should be switched to that from the difference forming unit
18. From this point (curve (b)) and downwards the output from the
control unit 26 of the hot wire flow meter is used.
The memory 34 stores the threshold values T, T.sub.a or T.sub.b
that are required to be accessed by the comparator 32. Look-up tables
that index values of output signals from each of the difference
forming unit 18 and the control unit 26 with actual flow rates also
may be stored within the memory 34. The measurement system 20 may
then be adapted to access the appropriate table within the memory
34 dependent on the result of the comparison within the comparator
32 in order to provide the output 30 that indicates the value of
the flow rate being measured within the tubular flow channel 4.
These look-up tables may be constructed by calibrating the outputs
of the units 1826 of each meter with known gas flow rates through
the channel 4 and the results input into the memory 34. This may
of course be done for each arrangement individually or batch-wise,
employing a common look-up table for theoretically identical meters.
As a safety precaution the measurement system 20 may be further
modified to compare, for example within the comparator 32 values,
such as flow values obtained using the look-up tables, dependent
on the outputs from both units 1826 and to provide, as a component
of the output signal 30 a warning in the event that the difference
between the values exceeds a pre-determined threshold (T.sub.s),
which may also be held in the memory 34.
It will be appreciated that while the difference forming unit 18
the control unit 26 and the measurement system 20 are disclosed
with regard to the arrangement 2 of FIG. 1 as being physically separate
units some or all of the functionality of a plurality of these different
units may be provided in a single device, such as a microprocessor
suitably programmed using known programming techniques, without
departing from the invention.
Moreover, it will be appreciated that the output signals from the
venturi-type flow meter and the hot wire flow meter may be utilized
in a number of different ways, such as by addition and normalization
of the two signals, in order to achieve a flow meter arrangement
having a sensitivity across its measurement range that is enhanced
over the sensitivity of either one of the constituent meters, without
departing from the invention as claimed. Suitable empirically derived
look-up tables or formulae may be constructed without undue effort
in order to index the results of a particular utilisation with the
actual flow rate of gas within the channel 4.
Although modifications and changes may be suggested by those skilled
in the art, it is the intention of the inventor to embody within
the patent warranted hereon all changes and modifications as reasonably
and properly come within the scope of the inventor's contribution
to the art. |