A turbulence conveyor flow meter for medical use includes a turbulence
chamber with variable input and/or output areas, a transducer system
for the pressure drop across the turbulence chamber and connected
between two external points of the chamber, a servomechanism under
control of the transducer system, acting on the flow to be measured,
and a detector of the measured flow.
1. A turbulence conveyor flow meter particularly for monitoring
human respiratory functions, comprising:
a body portion including a cavity having three openings, a first
opening to be connected to a patient's mouth, a second opening forming
an input to said cavity, and a third opening forming an output from
said cavity for conveying the inspired and expired air to different
paths; a movable intercepting element within said cavity with at
least a passage way connecting said first opening to said second
opening during inspiration and to said third opening during expiration,
the movement of said intercepting element causing variations of
the area open to the flow;
a first pressure transducer means connected between said first
opening and said second opening;
a second pressure transducer means connected between said first
opening and said third opening;
said first and second pressure transducer means responsive to variable
pressure drops across the openings they are associated with;
a servo mechanism connected to said first and second pressure transducer
means adapted to respond to variable signals therefrom for moving
said intercepting element to vary the degree of registration of
said passage way with said first and second or first and third openings;
a computing means connected with and receiving signals from said
first and second pressure transducer means and said servo mechanism
for sending signals to said servo mechanism to control the operation
of said servo mechanism and to also provide an indication of flow
2. A turbulence conveyor flow meter as defined in claim 1 wherein
said intercepting element is a rotary element rotated by said servo
mechanism and containing two passage ways which mutually exclusively
connect said first opening to said second opening and said first
opening to said third opening.
3. A turbulence conveyor flow meter as defined in claim 1 and
said moveable intercepting element being a rotary element and said
servomechanism comprising a rotational servomotor.
4. A turbulence conveyor flow meter as defined in claim 3 and
the computing means comprising a microprocessor.
5. A turbulence conveyor flow meter as defined in claim 1 and
the moveable intercepting element comprising a reciprocatory element.
6. A turbulence conveyor flow meter as defined in claim 1 and
breath sampling passage means connected in the body portion in communicating
relationship with said through passage.
7. A turbulence conveyor flow meter as defined in claim 1 and
the through passage comprising a substantially straight passage.
8. A turbulence conveyor flow meter as defined in claim 1 and
said through passage comprising an arcuate passage.
BACKGROUND OF THE INVENTION
The present invention relates to a turbulence conveyor flow meter
for medical use, particularly in physiopathology, in pulmonary and
cardiocirculatory medicine and in anaesthesia.
In the medical field, particularly with reference to the pulmonary
function, there exists a need for exact measurements of flows within
a wide range of values, from zero to hundreds of liters per minute.
To carry out such measurements, two types of devices are known in
the prior art, namely, laminar and turbulence flow meters.
Among laminar flow meters, the most widely used is the Fleisch
pneumotachograph. It comprises an undulated thin plate spirally
wound, flowing a diaphragm or laminar, which obstructs air flow.
The pressure drop at the ends of the diaphragm gives a measure of
the flow. Such a device, however, collects dirt and can be obstructed
when the flows are unclean. Moreover, it cannot be used for high
flow values, which alter the laminar motion of the flow.
To overcome these limitations, turbulence flow meters have been
introduced. They consist of obstacles to the flow which produce
turbulence in the flowing fluid. The pressure drop is proportional
to the flow being tested according substantially to a quadratic
Among turbulence flow meters, the Elliot's device is known (Journal
of Applied Physiology, 1975 pages 456-460) comprising a chamber,
the input and output ducts of which are misaligned. The flow value
is measured by the pressure drop at the two chamber ends. An advantage
of such a device is its working stability and lack of sensitivity
to physiologically polluted flows. On the other hand, a limitation
of the device is its excessive resistance to the flow for high values
of the flow (when it is higher than 120 liters per minute) and its
lack of sensitivity to low values of flow (less than 3 liters per
minute). In fact, for such low values, no appreciable turbulence
Flow meters provided with a resilient membrane are also known (Franetzki,
Ph.D dissertation, 1975 Universitat Friedriciana Karlsruhe). In
them, the resistance to the flow is inversely proportional to the
flow. These flow meters can be built with a convenient material,
and shaped in such a way, so as to establish a laminar relation
between the pressure drop and the flow to be measured. The sensitivity
to respiratory soiling (sputum) and the levity of the membranes
makes these flow meters unreliable.
All of the flow meters referred to above, both turbulence and laminar
flow types, possess some drawbacks in the measurement of flows.
Moreover, they are not suited to the conveyance of expired flows
for various reasons. This feature, though not strictly required
for some tests (spirometry) becomes essential in the rebreathing
technique. As known, the aim of rebreathing techniques is the analysis
and/or the intervention in the respiratory and/or cardiocirculatory
function for clinical or therapeutic purposes (nitrogen wash-out,
oxygen rebreathing, anaesthetics inhalation).
The problem of conveying expired flows is solved at present by
combining together independent flow meters and conveyors. In order
to avoid measurement disturbances, the valve (conveyor) has to be
placed not too near the flow meter. This causes an undesired dead
space when the flow meter is placed between patient and valve; therefore,
the necessity of having more flow meters to avoid the dead space,
or at least a means to measure the flow in one of its passages.
Due to these limitations, some rebreathing techniques proposed
in the last century (Pfueger's School, Germany, 1870-1873) and well
developed by the physiologoists in the sixties (Doehring and Thews,
Pfeugers Archiv. 311 1969pages 326-341) did not find clinical application.
The object of the present invention is to overcome the limitations
and drawbacks above-referred to, by providing a reliable compact
device, of simple construction and easy use, in the medical field,
allowing the measurement and conveyance of respiratory flows, particularly
in physiopathology, in pulmonary and cardiocirculatory medicine
and in anaesthesia.
The above objective is attained in accordance with the invention
with a turbulence conveyance flow meter for medical use comprising
a turbulence chamber with variable input and/or output areas, a
transducer system for the pressure drop across the turbulence chamber
and connected between two external points of the chamber, a servomechanism
under control of the transducer system, acting on the flow to be
measured, and a detector of the measured flow.
According to the invention, the servomechanism may be of the type
acting on the input and/or output areas.
Advantageously, the flow meter may comprise a tap or valve with
an intercepting element, provided with at least a passageway forming,
when it is open to the flow, a turbulence chamber. Still according
to the invention, the flow meter may comprise a microprocessor controlling
the servomechanism, according to the signals of the detector and
to the actual input/output areas of the chamber, and detecting the
The turbulence chamber of the flow meter may be provided with at
least an input duct and an output duct, for the continuous sampling
of the gas to be measured and for its compensation with an equal
volume of gas.
Other objects and advantages of the invention will become apparent
during the course of the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view, partly in section, of a flow meter
according to the invention, including a one-way tap as an interception
FIG. 2 is a sectional view of the flow meter in FIG. 1.
FIG. 3a is a similar sectional view of a flow meter according to
the invention having a two-way tap.
FIGS. 3b and 3c are sectional views showing the two-way tap in
FIG. 3a in different operational modes.
FIG. 4 is a sectional view of a flow meter according to the invention
with one pressure transducer.
FIG. 5 is a sectional view showing a different embodiment of the
FIG. 6 is a sectional view showing a further embodiment of the
tap of the flow meter.
Referring in detail to FIGS. 1 and 2 wherein like numerals designate
like parts, a flow meter includes a tap 1 consisting of a body 2
out of which aspherical cavity has been grooved, and a spherical
intercepting element 3 arranged in said cavity. The intercepting
element 3 has a diametrical duct or passage 4 which for a given
position of the element 3 relative to the body 2 is axially aligned
with two passageways 5 5', formed in the body 2. The passageways
5 5' extend beyond the body 2 into two external conduits 6 6',
whereof the conduit 6 is in contact with the patient's mouth, and
the conduit 6' is open to external air. In the interception element
3 two coaxial ducts 7 7' are formed. The axis of these coaxial
ducts is othogonal to the axis duct 4 and coincides with the rotational
axis of spherical element 3 with respect to the body 2. The two
ducts 7 7' extend into the body 2 of the tap in further ducts 9
9' and then outwardly into conduits 10 10'.
From the ducts 6 6', start respectively conduits 11 11', communicating
with a pressure transducer 12 which transforms the difference of
pressure .DELTA.p observed into electrical signals. These signals
are sent through a connecting element 13 to a servomechanism 14
which controls the angular position of the intercepting element
13 around the rotational axis 8. The signal generated by the pressure
transducer 21 is also sent, through a connection 15 to a microprocessor
16. Through a further connecting element 17 an electrical signal
related to the angular position of the intercepting element 3 is
sent from the servomechanism 14 to the microprocessor 16. A further
connecting element 30 connects the microprocessor 16 to the servomechanism
14 to control the latter, forming a closed loop as will be further
In this first embodiment of the flow meter, FIGS.1 and 2 the invention
operates as follows:
The duct 4 of intercepting element 3 is the turbulence chamber,
into which the flow of gas enters through passageway 5 (inspiration)
or 5' (expiration).
When no pressure signals are present (flow zero between inspiration
and expiration) the transducer 12 acts on the servomechanism 14
in such a way that the latter maintains the tap 1 closed. The starting
of expiration or inspiration causes a pressure increase in the conduit
6 with respect to conduit 6', and when such an over-pressure exceeds
a given value, the corresponding electrical signal generated by
transducer 12 acts on servomechanism 14 which opens the tap 1.
If the pressure transducer 12 gives a reliable response only within
a very limited pressure range, it will be preferable for it to act
on the servomechanism in such a way that the position of the intercepting
element 3 of tap 1 keeps a constant pressure drop in conduits 6
6', during the whole expiration phase. The angular position of the
intercepting element 3 transformed into an electrical signal within
the same servomechanism 14 is sent to the microprocessor 16. The
latter, keeping into account the value of .DELTA.p, coming from
the transducer 12 gives the legible value of flow Q. The signal
processing of microprocessor 16 is in general of the type:
wherein A.sub.1 A.sub.2 are the areas of the input and output
openings of the turbulence chamber 4. In the given example, of FIGS.1
and 2 if A denotes the common value of A.sub.1 and A.sub.2 the
formula (1) can be written in the following way:
wherein g(A) represents a suitable coefficient, which, for a given
geometrical structure, can be held as a constant, thus simplifying
the measurement. The use of the microprocessor 16 however, allows
the calculation of expressions of type (1) in whole generality.
Moreover, the microprocessor 16 can act on servomechanism 14 in
a wider case than referred to above, namely, in the case in which
during the entire inspiration or expiration phase, the difference
in pressure .DELTA.p in the conduits 6 6' is not kept constant.
The choice of the control policy of the angular displacement of
intercepting element 3 as a function of .DELTA.p, depends upon
the use of the conveyor flow meter, its geometry, and upon the feature
of the transducer.
If, during the operation of the conveyor flow meter, a continuous
collection of samples of the breathed flows has to be carried out,
this can be done through duct 7. At the same time, to avoid any
interference in the measurement, an equal volume of air or gas is
delivered into the duct 7'.
In the embodiment shown by FIGS. 3a through 3c, the intercepting
element 18 of the tap has several ways or passages, connecting,
according to the angular position of the element 18 the conduit
19 at the patient's mouth, with the input conduit 20 or with the
separate output conduit 21.
Two pressure transducers 22 and 23 control the pressure variation
between the conduit 19 and conduits 20 and 21 respectively, and
transform such variations into electrical signals, sent to a microprocessor
24 and servomechanism 25. The latter controls the angular position
of intercepting element 18 whereas the microprocessor gives the
flow signal Q.
In this embodiment, the conveyor flow meter operates as follows:
When there is no pressure signal, the tap remains closed, FIG.
3a. When expiration starts, there is an increase in pressure in
conduit 19 with respect to conduits 20 and 21. The two pressure
variations, transformed into electrical signals by transducers 22
and 23 are sent to microprocessor 24 which, acting on servomechanism
25 puts into communication the conduits 19 and 21 FIG. 3b. When
the flow meter has such a configuration, it operates as the flow
meter described above.
At the end of the expiration phase, the signal .DELTA.p=0 brings
the intercepting element 18 back to the closed position of FIG.
When the inspiration phase begins, there is a lowering of pressure
in the conduit 19 relative to conduits 20 and 21. The two pressure
variations, transformed into electrical signals by the transducers
22 and 23 are sent to microprocessor 24 which acting on the servomechanism
25 puts into communication the two conduits 19 and 20 FIG. 3c.
In FIG. 3a, for simplicity, two pressure transducers 22 and 23
are illustrated. It is possible to employ only one pressure transducer
26 FIG. 4. When .DELTA.p=0 and the intercepting element 18 is closed,
the transducer 26 communicates with both ducts 20 and 21 through
valves 27 and 28 each of which is closed due to the action of microprocessor
24 when .DELTA.p>0 or .DELTA.p<0 respectively.
In the embodiment illustrated in FIG. 5 the tap of the flow meter
is of a three-way type. In this case, the intercepting element 29
is built so that the opening connected to the patient is always
open, for whichever angular position of the intercepting element.
In the embodiment of FIG. 6 the tap of the flow meter is of the
piston type, particularly useful when the servomechanism acts with
an axial movement.
From the foregoing, it should be apparent that the conveyor flow
meter according to the invention offers a number of advantages among
which the following are important:
(1) The possibility of measuring flows within a wide range of values,
to answer all the requirements in the medical field. This possibility
is due to the presence of the turbulence chamber and to the servo-adjustment
of the area of the input and/or output openings.
(2) The practical insensitivity to dirtiness, as the turbulence
is not obtained by means of laminars or obstacles of any other type,
but only with simple holes.
(3) The facility of cleaning and sterilizing.
(4) The possibility, thanks to the use of a microprocessor, of
using transducers of every kind of response, including the non linear
ones, so far as they are reliable, and therefore the possibility
of manufacturing low-cost equipment.
(5) A remarkable reduction of the dead space, with respect to the
flow meters coupled to a separate conveyor, currently used, and
therefore the possibility of carrying out measurement and tests
in the medical field, practically without limitations.
(6) The possibility of carrying out flow measurements during rebreathing
tests under the most variable conditions (stress testing, maximum
forced expiration, etc.) due to the coupling in a unique device
of the flow measuring and the conveying, within a wide range of
values, and therefore the possibility of carrying-out from now on
high-level researches and measurements in the physiological and
(7) The possibility to perform with the same unique instrument
the evaluation of the resistance of the airways with the flow interruption
(8) The possibility to sample the gas in the turbulence chamber
even at high sampling rates without affecting the flow measurement:
these gas samples may thus be analyzed by low cost industrial analyzer.
In some applications, it can be useful to compensate the pressure
drop in the turbulent chamber in order to use a zero point detector
as pressure transducer to minimize its dimensions. In this case
an active compensation system of the pressure drop can be easily
realized through a vent or an air jet inside or outside the turbulent
chamber within the measuring points.
It is to be understood that the forms of the invention herewith
shown and described are to be taken as preferred examples of the
same, and that various changes in the shape, size and arrangements
of parts may be resorted to, without departing from the spirit of
the invention or scope of the subjoined claims.