Note: Descriptions are shown in the official language in which they were submitted.
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A 34770-1/TO~/JGW
PNEUMOTACHOGRAP~ WITH PITOT-LIKE TUBES
Devices for measuring the flow of respiratory gases are well
known in the art, and one of such devices is described in
applicant's prior issued United States Patent No. 4,083,245,
entitled "Variable Orifice Gas Flow Sensing Head. n Other
such devices include the well known orifice meter pneumotach-
ographs. Although pitot tubes have also been known for many
years as a means for measuring flow of g~ses, pitot tubes
have not been successfully used in the measurement of
respiratory gases since, as is well known, pitot tubes
operate with an orifice directed into the flow of the gas to
be measured. With such a configuration, the normally
present flecks of mucus and drops of moisture found in
respiratory gases have a tendency to enter the pitot tube
orifice and to block it. Such blockage, of course, would
render the pitot tube inoperative. In one prior attempt to
use pitot tubes, a multiplicity of the tubes were employed
on the theory that not all of them would be plugged simul-
taneously. This attempt, however, was unsuccessful.
Both orifice meters and classic pitot tubes, when used for
flow measurement, operate on the common principle of a
constant area and a variable pressure drop. In the case of
orifice meters the pressure drop is created by the resistance
of an orifice and is measured by a pair of tubes or "pressure
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taps." For a respiratory device it is desirable to minimi~e this resistance.
The foregoing patent and an article in Anesthesiology, Vol. 51, No. 2,
August 1979, by Jaklad et al. entitled "Pneumotachography" discusses this
technique. Orifice plates are also discussed in the McGraw-~lill
Encyclopedia of Science and Technology, 1977, under the heading "Flow Measure-
mellt. "
In a classic pitot tube the pressure differential is between an "impact"
or "stagnation" pressure and a static pressure.
It is a general object of the pres~ent invention to provide a pneumotachograph
which utilizes pitot like tubes to provide low resistance with high
sensitivity and yet avoids the problems of tube blockage from mucus, water
or other nongaseous material in the respiratory gases.
According to one aspect of the invention, there is provided a pneumotacho-
graph comprising a housing having a pair of input-output ports defining a
flow path for respiratory gases, first and second pitot tubes disposed
along said flow path and a pair of baffles disposed in said flow path and
below each of said pitot tubes, each of said baffles being disposed at an
angle of gas flow reflection between the axis of its associated pitot tube
and the flow path from a different one of said ports.
~0 According to another aspect of the invention, there is provided a
pneumotachograph comprising a housing having a pair of input-output ports
defining a flow path for respiratory gases, one of said ports defining an
inspiration input port and the other of said ports defining an expiration
input port, first and second pitot tubes disposed along said flow path and
at right angles thereto, and a pair of baffles disposed in said flow path,
one of said baffles being disposed at an angle of reflection between the
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axis of one of said pitot tubes and the direction of said Elow path from
said inspiration input port, the other o~ said baffles being disposed at
an angle of reflection between the axis of the other of said pitot tubes
and the direction of said :Elow path from said expiration input port.
Referring to the drawing, F:[GURE ] is a cross sectional elevation showing
a pneumotachograph in accordance with one embodiment of the invention;
~IGURE 2 is an end elevation of the pneumotachograph shown in EIGURE l;
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FIG~RE 3 is a partial cross-sectional elevation similar
to FIG~RE 1 but showing an alternative embodiment of the
invention;
FIGURE 4 is a cross-sectional elevation showing a pneumo-
tachograph in accordance with another embodiment of the
invention;
FIGURE S is a cross-sectional view taken along the line 5-5
of FIGURE 4;
~IGURE 6 is a top view of a component of FIGURE 4; and
.,
FIGURE 7 is a set of characteristic curves showing the
improvement in sensitivity of the present invention.
The pneumotachograph shown in FIGURES 1 and 2 includes
a housing 11 preferably formed of two sections 13 and 15
connected together by means of mating flanges 17 and 19 and
connecting screws 21. The housing includes a pair of
input-output ports 23 and 25 which may be connected to
flexible tubes 27 and 29, respectively. The tubes 27 and 29
may be connected to a respiratory apparatus and respiratory
mask respectively.
~hus it can be seen that, as the patient breathes, durin~
expiration respiratory gases flow in one direction, and upon
inspiration the gases will flow in the opposite direction.
Ports 23 and 25 provide a generally linear path of respira-
tory flow shown generally by the arrow 31. A pair of pitot
tubes 33 and 35 are included and extend thro~gh the housing
walls. Pitot tubes 33 and 35 are connected b~ means of
flexible tubing 37 and 39 to a differential pressure gauge
which is not shown but which is well known in the art.
Below each of the pitot tubes and in general alignment with
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the axes thereof are disposed a pair of baf~les 41 and 43
which are retained upon standards 45 and 47, respectively.
Each of the baffles 41 and 43 is disposed at an angle of gas
flow reflection between its asso~iated pitot tube and the
direction of the flow path from different ones of the ports
23 and 25. With a linear flow path as shown in FIGURE 1,
the baffles are ideally disposed at an angle of from 60 to
70 to the flow path. Thus when the port 23 is serving as
the input port and the direction of flow is as shown by the
arrow 31, a portion of the gases are reflected by the baffle
~ 41 to the pitot`tube 33. At this time the baffle 43 reflects
none or very little of the gases to its associated pitot
tube 35. When the direction of flow reverses, the baffle 43
does reflect a portion of the flow to the pitot tube 35 and
the pitot tube 33 becomes passive since the baffle 41
reflects more or very little of the gases to ito
In the embodiment shown in FIGURE 1 the standards 45 and 47
are relatively rigid whereby the baffles 41 and 43 are held
securely in the position shown in FI~URE 1. As can be seen
more clearly in FIG~RE 2 the baffles 41 and 43 may take the
form of relatively small flat plates and serve essentially
as defelctors. In use, the upstream baffle directs air into
one of the pitot tubes which serves as a positive pressure
sensor proportional to the flow of gases. The downstream
tube would receive none or very little reflected flow an- so
acts as a passive pressure sensor. During the course of
respiration the baffles 41 and 43 alternate as upstream and
downstream baffles and so the pitot tubes alternate as
posi~ive pressure sensors and passive pressure sensors.
In operation, as the respiratory gases are directed toward
the baffles 41 or 43, the gases themselves are directed up
toward the pitot tubes 33 or 35 but the heavier flecks of
mucus, water or other nongaseous material do not rise so
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easily and in reality merely run up to and over the ~op of
the baffle falling across the downstream side thereof. Thus
the gases which actually reach the pitot tube are relatively
cl~an and do not have any clogging effect.
In the operation of the pneumotachograph of FIG~RE 1 con-
nected to a differential pressure gauge, the output is not
linear but rather is exponential such that the measured
differential pressure is an exponential function of velocity
of flow (see FIGURE 7). Since the relationship between the
differential pressure and the velocity of flow is exponential
it is a relatively simple matter to linearize with either a
microprocessor or even by analog techniques both of which
are well ~nown to those skilled in the art.
The pneumotachograph as described above has a substantial
advantage over those prior art devices known as orifice
meter pneumotachographs in that a higher signal and lower
resistance to flow results from the construction. Moreover,
the device has no moving parts and i5 very easy to construct
by molding such that mass production of a pneumotachograph
for respiratory measurements is economically feasible.
In certain instances it is desirable that the output signal
have a linear relationship to the flow without the use of a
microprocessor or analog techniques. This relationship can
be provided by the embodiment shown in FIGURE 3 wherein the
baffles 41 and 43 are supported on flexible standards 49 and
Sl respectively.
Preferably the standards 49 and 51 are formed of Kapton*
plastic having a thickness of 3 mils. With such a construc-
tion the measured differential pressure signal varies
linearly with the flow of fluid through the pneumotachograph
regardless of the velocity. Thus as velocity is increased
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from left to right, as shown in FIGURE 3, the standards ~9
and 51 flex so that they and the baffles supported thereby
move to the position shown in dash-do~ lines thus decreasing
the amount of gases reflected to the pitot tube 33 and
permitting some of the bases to be reflected to the pitot
tube 35. Thus, the positive signal produced by the pitot
tube 33 is decreased whereas, at the same time, the signal
at the pitot tube 35 which is essentially zero when in the
position shown in solid lines in FIGURE 3, shifts toward the
positive. The correction provided by the movement of the
baffles 41 and 43 is itself exponential and thus cancels out
the exponential variation in ~he differential pressure
signal from the two tubes.
The pneumotachograph shown in FIGURES 4 and 5 includes
a housing 61 formed of left and right sections 62 and 63
connected together by male and female portions at 64. The
housing includes a pair fo input/output ports 66 and 67
which may be connected to flexible tubes 68 and 69, respec-
tively. In the embodiment of FIGURE 1, these tubes may be
connected to a respiratory apparatus and respiratory mask,
respectively. A pair of tubes 68 and 69 are included and
extend through the housing walls so that their axes is
perpendicular to the nominal flow path of gases indicated by
the arrow 71. Tubes 68 and 69 are connected to a differen-
tial pressure gauge as indicated.
A disc-shaped baffle 72 is disposed in the center of the
flow path 71 and perpendicular to it to form an annular-like
opening 73, see especially FIGVRE 5, between the cylindrical
housing 61 and the disc-shaped baffle 72. It is suspended
from the walls of the housing by a fixed rod 77. ~oreover
it is positioned substantially below the bottom openings of the
tubes 68 and 69 so that on the upstream side, that is, at 74
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assuming the flow in in the direction 71, a high pressure is
produced in the proximity to the end of tube 69. In addition,
a low pressure is produced at 75 on this downstream side
because o~ a venturi effect.
In accordance with ~low measurement theory, the pressure
difference between the two tubes in indicative of flow
velocity. This is more specifically indicated in FIGURE 7,
where the pressure differential, ~P, is the horizontal axis
and FLOW the vertical axis.
The non-linear curve labeled "NE~" is very similar to
that produced by the embodiment o~ FIG~RE 4 and is easily
compensated for by the microprocessor techniques. The other
curve labeled "OLD" indicates prior techniques, especially
orifice techniques, where at low flow rates, because of the
steeper curve, the sensitivity of the gauge is greatly
reducedO Thus, FIGURE 7 aptly illustrates the improvement
of the embodiment of FIGURE 4 in increased small signal
sensitivity compared to an equivalent orifice meter with the
same flow resistance.
The present invention also has the advantage of a greater
signal to noise ratio. The venturi effect mentioned as a
probable cause of a lower pressure at the locality 75 in
proximity to tube 69 is perhaps not the total cause of the
low pressure. In general comparing the embodiments of
FIGURES 1 and 4, FIGURE 4 might be considered as a scaled-
down reflected pitot design where the two reflectors 41 and
43 have been merged into one baffle. It is also believed
that if the baffles 41 and 43 of FIGURE 1 are made vertical,
70% of the "reflected" effect will be retained. Thus, the
tubes 68 and 69 of FIGURE 4 might be called pitot-like tubes
in one sense or pressure taps in the sense of standard
orifice meter; or the present invention might be thought of
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as a hybrid between a standard orifice meter and a standard
pitot tube measuring device. ~n any case, it is not intended
that the invention be unduly limited by the standard defini
tion of a pitot tube which extends directly into the flow of
sas to be measured.
In order to insure that the flow of gas is relatively
stable, that is, a misapplied input tube 68 might cause an
unwanted jet effect where the flow is not uniform over its
1~ cross-section, a pair of flow director horizontal plates 78
and 79 are provided. FIGURE 6 is an elevation view of both
of these plates. They are affixed to notches in the side
walls of housing halves 62 and ~3 with an end notch 81 and
82, respectively, ens~ring against slippage out of the
structure and into the breathing way of the patient. The
flow director plates, of course, are horizontal and co-planar
with the flow path 71 and provide stable flow.
An orifice measuring device with a center plate such as 72
has been suggested. For example, see the Fourth Edition of
Perry's Encyclopedia of Chemical Engineering where annular
orifice meters are discussed. These apparently are used
only in industrial applications and the press~re tap on the
downstream side is far enough away from the baffle so as not
to be affected by any venturi action. In comparison in the
present invention which, of course, must be set up for
bi-directional flow, the spacing of the tubes with respect
to their outer diameters is approximately the same as the
annular opening 73. This is believed to cause the pressure
build-up on the upstream side and the lower pressure on the
downstream side which provides a significant differential
pressure. In fact, there is enough of a pressure difference,
which increases sensitivity, that for a readable siqnal
which is obtained from the differential pressure gauge, the
resistance to air flow is only half the amount of an eq~i-
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valent orifice meter. As in the case of FIGURE 1, theembodiment of FIGURE 4 avoid~ the problem of tube blocka~e
by mucus, water or other non-gaseous mater.ial carried by
respiratory gases. In addition the construction of FIGURE 4
is smaller and has less dead-space.
