U.S. patent number 3,727,627 [Application Number 05/054,934] was granted by the patent office on 1973-04-17 for apparatus for mixing gases.
Invention is credited to Forrest Morton Bird, Henry Louis Pohndorf.
United States Patent |
3,727,627 |
Bird , et al. |
April 17, 1973 |
APPARATUS FOR MIXING GASES
Abstract
A method and apparatus for mixing gases from separate
pressurized gas circuits for use in applications such as medical
respirators. The pressures of the gases in the two circuits are
accurately balanced to provide an equal pressure drop across a
differential mixing valve for precise control of the ratio of flow
rates of the gases being mixed. In one embodiment, a double state
pressure balancing mechanism is provided for increased accuracy of
pressure balance.
Inventors: |
Bird; Forrest Morton
(Sandpoint, ID), Pohndorf; Henry Louis (Sandpoint, ID) |
Family
ID: |
21994480 |
Appl.
No.: |
05/054,934 |
Filed: |
July 15, 1970 |
Current U.S.
Class: |
137/100;
128/205.11 |
Current CPC
Class: |
A61M
16/20 (20130101); A61M 16/206 (20140204); A61M
16/207 (20140204); B01F 3/028 (20130101); Y10T
137/2521 (20150401) |
Current International
Class: |
A61M
16/20 (20060101); A61M 16/10 (20060101); A61M
16/12 (20060101); B01F 3/00 (20060101); B01F
3/02 (20060101); G05d 011/02 () |
Field of
Search: |
;137/7,100,98,101,111
;128/142.2,142.3,145.8 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cline; William R.
Claims
We claim:
1. Apparatus for the controlled mixing of gasses from separate
pressurized gas circuits for delivery through a common flowpath,
including the combination of pressure balance means to establish a
substantially equal gas pressure in a downstream segment of each
circuit, the pressure balance means including first control valve
means controlling pressure in the downstream segment of the gas
circuit and second control valve means controlling pressure in the
downstream segment of the other gas circuit, valve operating means
opening and closing the first valve means while conjointly closing
and opening the second valve means responsive to a differential in
pressure between the gasses in said downstream segments of the gas
circuits to vary the pressures in said downstream segments for
reducing said pressure differential to establish an equal pressure
balance in said downstream segments, and mixing valve means to mix
the gas from each downstream segment into said common flowpath,
said mixing valve means including means forming a mixing chamber in
communication with said common flowpath, means forming a pair of
orifices each communicating between a respective one of said
downstream segments and with said mixing chamber, means forming a
pair of valve elements each adapted for movement within a
respective one of said orifices to control the gas flow area
thereof, and means to conjointly move said valve elements with
respect to said orifices to reciprocally vary said flow areas to
vary the ratio of the gas mixture.
2. Apparatus as in claim 1 in which the valve operating means
includes movable wall means having opposed sides in communication
with respective downstream segments of the gas circuits, and means
mounting the movable wall means in operating connection with said
first and second valve means for said conjoint movement thereof
responsive to an imbalance in gas pressures acting on said opposite
sides of said wall means.
3. Apparatus as in claim 1 in which the first and second valve
means each include a valve opening in a respective gas circuit
together with a valve provided with a pair of valve closure
elements movable to open and close a respective opening, and said
valve operating means including means to conjointly move said valve
closure elements to and from respective openings.
4. Apparatus for the controlled mixing of gasses from separate
pressurized gas circuits for delivery through a common flow path,
including the combination of pressure balance means to balance the
gas pressure in a segment of each circuit, the pressure balance
means including first control valve means in one gas circuit,
second control valve means in the other gas circuit, first valve
operating means opening and closing the first valve means while
conjointly closing and opening the second valve means responsive to
a differential in pressure between the gasses downstream of the
first and second valve means to establish an equal pressure balance
in said downstream gas pressures, third control valve means in said
one gas circuit, fourth control valve means in said other gas
circuit, each of the third and fourth valve means located
downstream of the first valve operating means, a second valve
operating means opening and closing the third control valve means
while conjointly closing and opening the fourth control valve
means, the second valve operating means operating responsive to a
differential in pressure between the gasses downstream of the third
and fourth valve means to establish an equal pressure balance in
said last mentioned downstream gas pressures, and mixing valve
means to mix the gas from each circuit downstream of the third and
fourth control valve means into said common flow path.
5. Apparatus for the control mixing of gasses from separate
pressurized gas circuits for delivery through a common flow path,
including the combination of pressure balance means to balance the
gas pressure in a segment of each circuit, the pressure balance
means including first control valve means in one gas circuit and
second control valve means in the other gas circuit, valve
operating means opening and closing the first valve means while
conjointly closing and opening the second valve means responsive to
a differential in pressure between the gasses downstream of the
first and second valve means to establish an equal pressure balance
in said downstream gas pressures, the valve operating means
including movable wall means comprising a flexible diaphragm
separating the gas circuits downstream of said control valve means,
means mounting the movable wall means in operating connection with
said first and second valve means for said conjoint movement
thereof responsive to an imbalance in pressure acting on opposite
sides of said wall means, said mounting means comprising a valve
stem mounted for movement with the diaphragm and with said first
and second valve means formed at either end of said valve stem, and
mixing valve means to mix the gas from each circuit downstream of
the control valve means into said common flow path.
6. Apparatus for the controlled mixing of gasses from separate
pressurized gas circuits for delivery through a common flow path,
including the combination of pressure balance means to balance the
gas pressure in a segment of each circuit, the pressure balance
means including first control valve means in one gas circuit and
second control valve means in the other gas circuit, valve
operating means opening and closing the first valve means while
conjointly closing and opening the second valve means responsive to
a differential in pressure between the gasses downstream of the
first and second valve means to establish an equal pressure balance
in said downstream gas pressures, said first and second valve means
each comprising an orifice in a respective gas circuit together
with a valve element movable to open and close a respective orifice
under influence of said valve operating means, said orifices being
spaced apart to define opposed valve seats having adjacent
downstream passages, said valve elements being mounted for movement
in the downstream passages for reciprocal opening and closing of
the flow area through said orifices, and mixing valve means to mix
the gas from each circuit downstream of the control valve means
into said common flow path.
7. Apparatus as in claim 6 in which said downstream passages define
a cavity, and said valve operating means comprises flexible wall
means dividing said cavity into separate chambers each
communicating with a respective downstream passage together with
means mounting said valve elements for conjoint movement with said
wall.
8. Apparatus for the controlled mixing of gasses from separate
pressurized gas circuits for delivery through a common flow path,
including the combination of pressure balance means to balance the
gas pressure in a segment of each circuit, the pressure balance
means including first control valve means in one gas circuit and
second control valve means in the other gas circuit, valve
operating means opening and closing the first valve means while
conjointly closing and opening the second valve means responsive to
a differential in pressure between the gasses downstream of the
first and second valve means to establish an equal pressure balance
in said downstream gas pressures, and mixing valve means to mix the
gas from each circuit downstream of the control valve means into
said common flow path, said mixing valve means comprising first and
second metering valve means each controlling the flow of gas in a
respective circuit downstream of said pressure balance means, each
of said metering valve means being mounted for conjoint movement to
reciprocally open and close flow through its associated circuit to
establish a preselected ratio of flow between the two circuits, and
means combining said gas flows downstream of said metering valves
for combined flow into said common flow path.
9. Apparatus as in claim 8 in which said first and second metering
valves each comprise an orifice in a respective gas circuit
downstream of its associated first or second control valve means
together with a valve element movable to control flow through said
orifice, and means to selectively move said valve element of each
of the metering valves in conjoint adjusting movement to increase
gas flow through one of said orifices while reciprocally decreasing
flow through the other of said orifices for establishing a
preselected ratio of flow between the two gases.
10. Apparatus as in claim 9 in which said orifices are spaced apart
to define opposed valve seats, said valve elements are spaced apart
and mounted for movement between said valve seats, and further
including means to selectively adjust the position of said valve
elements between said valve seats to establish said preselected
ratio of flows.
Description
BACKGROUND OF THE INVENTION
Mechanical respirators are used as pulmonary ventilators in the
theraputic management of cardio-pulmonary or other physiological
conditions of a patient. These respirators are operated by a
compressed gas source, such as the 50 psi compressed oxygen source
which is commonly plumbed in hospital rooms. When plumbed oxygen is
not available, commercial high-pressure oxygen cylinders are used
to provide high-pressure oxygen storage, and reduction regulators
are used to reduce this pressure to the 50 psi driving pressures of
the respirators.
The advent of the routine use of oxygen-powered medical respirators
has led to an increased incidence of oxygen toxicity, and the need
has been recognized for a practical and ethical method and
apparatus for accurate control of oxygen delivery to a patient for
reducing the danger of oxygen toxicity.
Physiologically, the average patient receiving pulmonary therapy
does not require oxygen tensions or pressures above ambient 21
percent. There are, however, a sufficient number of medical
respirator patients with diffusion and profusion alterations to
warrant elevated oxygen tensions in the inspired respiratory
gases.
With the advent of a practical clinical means for the rapid
analysis of arterial oxygen tensions, the need for a respirator
delivering exact oxygen tensions is justified. By looking at the
arterial oxygen levels of a patient being ventilated with a
mechanical ventilator, the total theraputic efficiency can be
determined. If the ventilator is maintaining a ventilatory level
sufficient to maintain the normal carbon dioxide tensions, arterial
oxygen tensions can be theoretically adjusted to desired levels by
increasing or decreasing oxygen tensions until desired arterial
titration is achieved. Accordingly, the need has been recognized
for a method and apparatus effective to provide accurate oxygen
tension adjustments.
Existing medical respirators commonly employ a venturi both as an
oxygen-air dilution system and a pneumatic clutch functioning
against pulmonary resistances. The dilutor venturi of these
respirators allows mechanical selection of oxygen concentrations
within the range of 40 to 100 percent, but these calibrations are
not accurate when the variables associated with mechanical
ventilation of the lung are considered. Thus, a medical respirator
rated at 40 percent dilution may delivery oxygen tensions as high
as 100 percent at the top of inspiration. The original 40 percent
index is primarily increased by nebulizer flow and secondarily by
progressive venturi entrainment breakdown as pressures distal from
the lung rise. The mean oxygen concentration would be somewhere
between 40 and 100 percent, dependent upon driving pressures at the
venturi jet, nebulizer flow, and pulmonary resistances.
Medical respirators utilizing the venturi as a pneumatic clutch
create nonlinear gross pulmonary resistance during inspiration as a
result of variables such as elastic and non-elastic resistances
within pulmonary structures. When a venturi with a constant driving
pressure functions against variable pulmonary resistance, a
variable flow results. A venturi functioning as a reciprocal flow
pressure converter serves to enhance the distribution of inspired
gases during mechanical ventilation of the lung.
Conventional means of delivering elevated oxygen tensions in
inspired gases with a venturi stabilized mechanical ventilator
include that of powering the respirator by oxygen in lieu of
compressed air. Compressed air can be used as a primary motivating
gas with supplemental oxygen added into the breathing circuit to
obtain higher tensions. Another method is to pre-mix oxygen and
other respiratory gases and deliver the mixture through the
venturi. The same gas must be supplied on demand to satisfy venturi
entrainment.
Another existing method employs the mixing of oxygen and air in a
metering system which combines the gases in a controlled ratio of
flows of each gas. The metering system of this method provides two
adjustable orifices linked together by mechanical means so that the
orifice area for one gas is increased as the orifice area for the
other gas is decreased. The orifice areas are controlled by needle
valves mounted on independent shafts and mechanically linked by a
gear arrangement to a percent oxygen control. The means used to
balance and control the pressures so that the pressure drop across
the metering system is equal for both gases comprises the use of
two separate flow controllers located downstream from the metering
system and functioning from pressure references from the inlet and
outlet gases to maintain the ratio of flows as selected by the
metering system. The pressure drop across the metering system is
equalized by varying the pressures downstream from the metering
orifices with reference to the inlet and outlet pressures. This
method is not entirely satisfactory as a result of certain design
limitations and inherent sources of error in flow control. A
regulator must be employed for the downstream pressure, and an
alarm system must be employed to indicate low outlet pressures
inasmuch as the system may continue to operate if one of the gas
supplies fails. If the differential in inlet gas pressure varies
more than 5 psi, an ever-increasing error develops. Further,
constant ambient bleed of gas totaling approximately 9
liters/minute must be maintained to obtain a 95 percent accuracy,
and accuracy drifts progressively when constant or intermittent
flows below 15 liters/minute are demanded.
Accordingly, the need has been recognized for a gas mixing method
and apparatus providing a high degree of flow and pressure control
accuracy, which is smaller, lighter, less expensive and more
versatile than existing designs, and which is capable of delivering
controlled oxygen ratios to patients by means of respirators,
ventilators and free flow apparatus.
SUMMARY OF THE INVENTION AND OBJECTS
This invention relates generally to methods and apparatus for
mixing the flow from separate pressurized gas sources for use in
applications such as medical respirators and the like.
It is an object of the invention to provide a method and apparatus
for controlling the mixing of separate pressurized gases.
Another object is to provide a method and apparatus for the
differential mixing of gases in separate pressurized gas circuits
for delivery into a flow path at pre-selected, accurately
controlled ratios.
Another object is to provide a method and apparatus of the type
described to provide an equal pressure drop in each of two separate
pressurized gas circuits across differential mixing valve means
controlling the ratio of flows of the two gases for mixing into a
delivery flow path.
Another object is to provide a method and apparatus for mixing
oxygen with respirator, theraputic, or anesthetic gases with
accurate and selectively adjustable pressures and flow rates for
use in applications such as a mechanical respirator for theraputic
management of physiological conditions.
The invention broadly provides a method and apparatus for
controlling the mixing of gases in separate pressurized gas
circuits in which pressure balance means are employed as the first
stage, or alternatively, a pair of pressure balance means in series
are employed, to provide a highly accurate pressure equilization in
the two circuits for delivery to a differential mixing valve
arrangement adapted to selectively control the ratio of flows in
the two circuits for mixing into a delivery flow path.
Additional objects and features of the invention will appear from
the following description in which the preferred embodiments of the
invention have been set forth in detail in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of a preferred embodiment of the gas
mixing apparatus of the invention shown in use with a mechanical
respirator;
FIG. 2 is a top-plan view of the apparatus of FIG. 1;
FIG. 3 is a cross-section elevational view to an enlarged scale
taken along the line 3--3 of FIG. 1;
FIG. 4 is a cross-section elevational view taken along the line
4--4 FIG. 3;
FIG. 5 is a cross-section elevational view taken along the line
5--5 of FIG. 3; and
FIG. 6 is a partially schematic view of the apparatus of the
invention illustrating the operation thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawings, FIG. 1 illustrates generally at 10 an apparatus
for controlling the mixing of gases in accordance with the method
of the invention. Apparatus 10 is shown in operating relationship
for blending a mixture of gases such as oxygen and air for delivery
to a mechanical respirator indicated generally at 11. While the
preferred embodiment is illustrated in conjunction with a
respirator ventilating a patient's lungs it is understood that the
invention will find broad application in the controlled mixing of
disparate gases from separate pressurized sources for delivery to a
common flow path.
The ventilating apparatus or respirator 11 may broadly be of the
type disclosed in the Bird et al. U.S. Pat. No. 3,331,368 issued
July 18, 1967. Suffice it to say here that a respirator of the type
disclosed in the Bird et al patent would in general comprise a
controller 12 adapted to receive the mixture of gases under
pressure through inlet 13. The controller is connected through
outlet tubes 14 and 16 with a suitable patient adapter 17 provided
with a nebulizer 18. Controller 12 includes an inspiratory flow
rate control 19, negative pressure control 21, expiratory time for
apnea control 22, inspiratory pressure control 24, sensitivity
pressure control 26, gauge 27 measuring pressure in the breathing
chamber, and a compliance bag or test lung 28 connected through
bleeder tube 29 and connector adapter 31 with the inlet gas flow
into controller 12.
Gas mixing apparatus 10 is mounted above respirator 11 and is
provided with a pair of inlet fittings 32,33 adapted for connection
with gas inlet tubing, not shown, from separate gas circuits or
pressurized gas sources which are to be mixed or blended. In the
preferred embodiment oxygen pressurized at, for example, 50 psi
would be introduced into apparatus 10 through fitting 33 for
blending with the desired theraputic, anesthetic or respiratory
gas, such as air, introduced under pressure through inlet fitting
32.
Apparatus 10 comprises a modular construction providing a first
pressure balance stage 34, a second pressure balance stage 36
downstream of the first stage, and a differential gas mixing device
37 adapted to control the ratio of flows of the two gases during
mixing for delivery through outlet fitting 38 into connector
adapter 31 of the respirator. The first and second pressure balance
stages are connected in series to provide a highly accurate
pressure balance between the two inlet gases so that the delivery
pressures of both gases to differential gas mixing device 37 are
equal and the desired gas mix is maintained as the pre-selected
ratio of flows across the differential mixing device remains
constant. While double pressure balance staging is illustrated as
preferred for increased accuracy in pressure and flow rate control,
it is understood that the invention contemplates that a single
pressure balance stage could be employed.
Each of the pressure balance stages 34,36 are substantially
identical in construction and are shown as defined by pairs of flat
blocks of a suitable material such as a synthetic polymer. The
blocks are bored and relieved to define the various cavities and
gas passageways. First pressure balance stage 34 is defined by a
pair of abutting blocks 39,41 and second pressure balance stage 36
is defined by a pair of abutting blocks 42,43. The four blocks are
secured together in the illustrated assembled relationship by
suitable means such as bolts, not shown.
First pressure balance stage 34 comprises a pair of frusto-conical
flow control valve elements 44,46 mounted for conjoint movement on
common valve spool 47 which is axially positioned within bore 48.
The valves 44,46 regulate the flow from respective chambers 52,53
through tapered valve seats 49,51 communicating with bore 48.
Chamber 52 is in communication with one gas circuit, e.g., the
source of pressurized oxygen, through passageway 54 leading to
fitting 33 and chamber 53 is in communication with the other of the
gas circuits, e.g., the source of pressurized air, through
passageway 56 leading to fitting 32.
Pressure responsive valve operating means comprising a flexible
diaphragm 57 is provided to operate control valves 44,46. The two
blocks 39,41 are relieved at their interface to define a flat
cylindrical cavity 58 communicating at its inner periphery with
bore 48. Diaphragm 57 comprises an inner rigid disk 59 secured to
valve spool 47 and bonded to a flexible annulus 61 of a suitable
elastomeric material mounted in fluid sealing engagement at its
outer periphery between blocks 39 and 41. Diaphragm 57 divides
cavity 58 into two pressure chambers each communicating with the
downstream flow of gases from a respective valve 44,46. Deflection
of the diaphragm responsive to an imbalance in pressure in the two
chambers of cavity 58 urges valve spool 47 in an axial direction to
open one of the control valves while simultaneously closing the
other valve. Whichever gas circuit is at a higher pressure will
produce a resultant force on the diaphragm for urging it in a
direction tending to close the valve associated with the high
pressure side and to conjointly and reciprocally open the other
valve associated with the low pressure gas circuit a proportional
amount. The reduced gas flow into the high pressure side of the
diaphragm and the concomitant increased flow into the low pressure
side reaches a null point at which the two gas pressures downstream
of their respective valves are balanced. The pressure balanced
gases next flow in series to the second pressure balance stage 36
for further refinement in a balancing of the two pressures. The
gas, such as oxygen, flowing past valve 44 enters passageway 62,
while the gas, such as air, flowing past valve 46 enters passageway
63.
Second pressure balance stage 36 is similar in construction and
operation to that of first stage 34 and is defined by the two
blocks 42,43 bored and relieved to provide a chamber 64 in
communication with passageway 62 and a chamber 66 in communication
with passageway 63. Chamber 66 is sealed from opposing chamber 53
of the first stage by a suitable sealing disk 67. Suitable gas
sealing means, such as the O-ring type seals 68, are mounted in
grooves provided at the block interfaces around each of the
passages 56,62,63 to preclude escape of the pressurized gases.
Second stage 36 further includes a pair of frusto-conical flow
control valve elements 69,71 mounted for conjoint movement on
common valve spool 72 which is axially positioned within bore 73.
Valve element 69 moves relative to tapered valve seat 74 to control
gas flow from chamber 66 and valve element 71 moves relative to
tapered valve seat 76 to control gas flow from chamber 64. The
valve operating means for these valves comprises a flexible
diaphragm 77 mounted within cavity 78. The diaphragm includes an
inner rigid disk 79 secured to valve spool 72 and bonded to a
flexible annulus 81 mounted in fluid sealing engagement at its
outer periphery between blocks 42,43 to divide cavity 78 into two
chambers communicating the pressure of gases downstream from valves
69,71 to act against the diaphragm. An imbalance in gas pressure on
either side of the diaphragm produces a resultant force urging the
valve spool in a direction tending to close the valve on the high
pressure side while conjointly opening the valve on the low
pressure side until a null point is reached at which the downstream
pressures of the two gases are balanced. The gas flowing past valve
69 is directed through outlet passageway 82 while the gas flowing
past valve 71 is directed through outlet passageway or delivery
flowpath 83, and these two passageways in turn communicate with
differential gas-mixing device 37.
In the pressure balancing system of the invention final delivery
pressure is always that of the inlet value of the lowest pressure
gas. For example, if the oxygen inlet pressure is 50 psi and the
air inlet 80 psi, final delivery pressure would be 50 psi. An
important feature is that if either gas source fails, the other gas
flow is automatically shut down. This is of paramount importance
when a respiratory or anesthetic gas other than air is being mixed
with oxygen.
In an embodiment of the invention utilizing an oxygen inlet
pressure of 50 psi and an air inlet pressure of 70 psi, it has been
found that the use of a single stage for pressure balancing results
in an error of 25 cm H.sub.2 O. Greater accuracy in pressure
balancing was obtained utilizing the illustrated two-stage system
and with the same 20 psi inlet differential where the error was
reduced to 4 cm H.sub.2 O, or an accuracy of 99.6 percent. The 4 cm
H.sub.2 O balancing error results in a .+-. 2 percent error in the
percent of oxygen delivered.
The differential gas mixing device 37 comprises a housing 84
mounted below the pair of blocks 42,43 by suitable bolts, not
shown, and with the housing formed with a pair of passages 86,87
communicating with respective passages 82,83 from the outlets of
second pressure balance stage 36. Suitable sealing means such as
O-ring type seals 90 are mounted in grooves formed in the interface
between housing 84 and the underside of blocks 42,43 for precluding
gas escape from the passages.
Differential gas mixing device 37 further comprises a backlash-free
metering valve for establishing a pre-determined ratio of the flow
rates for the two gases being mixed or blended. Housing 84 is
formed with a bore 88 receiving a valve spool 89. Valve spool 89 is
formed with opposed frusto-conical valving ends 92,93 having
identical tapers and diameters and adapted to move in relation to
respective orifices 94,95 formed with identical diameters in
bushings 96,97 threadably received in bore 88. An operating shaft
98 mounted at one end of the valve spool is provided with external
threads 99 engaging internal threads formed in bushing 96. A shaft
100 at the other end of the valve spool is of identical diameter to
that of shaft 98 and extends through orifice 95 for support on a
bearing 101. The distal end of shaft 98 projects from the housing
for attachment by a set screw with an oxygen percent control knob
101. Suitable compression spring means 102 is provided between the
inner cavity of knob 101 and a retainer nut 103 on bushing 96 to
urge the valve spool shaft outwardly in an axial direction to
eliminate thread backlash. Suitable calibrations are provided on
the control knob for indicating the desired range of oxygen mixing,
which is from 21- 100 percent in the illustrated embodiment. Manual
angular adjustment of the control knob displaces valve spool 89 so
that the flow areas of orifices 94,95 are reciprocally opened and
closed by the translatory movement of tapered ends 92,93. The gases
moving past the two orifices enter the central segment of bore 88
serving as a central mixing chamber. This mixing chamber delivers
the mixed gas into bore 104 and outlet fitting 38. Since the gas
pressures delivered to the differential gas mixing device are
balanced to a high degree of accuracy, and since the pressures
downstream of the orifices are equal in the mixing chamber, the
pressure drop across the orifices will be equal for both gases. As
a result, the preselected ratio of gas flows for mixing will be
maintained at a constant value regardless of differences in inlet
pressures and flow rates. The invention thus will provide automatic
compensation for any changes in inlet pressures.
In the diagram of FIG. 6 the use and operation of the invention is
illustrated in schematic form. The gas flow paths from inlet
passageways 54,56 of the two gas circuits, e.g. oxygen and air, are
illustrated in flowing from first pressure balance stage 34 into
second pressure balance stage 36 for delivery at a precise pressure
balance to differential gas mixing device 37. Gas mixing device 37
is illustrated in modified form with valve element 106 sliding
within bore 107 on a gas seal 108. Gas is metered outwardly through
tapered valve ends 109,111 into orifices 112,113 for delivery
through branch passageways 114,116 connecting with outlet bore 117
serving as the mixing chamber.
Various functions of the apparatus may be modified as required by
certain design considerations. In the pressure balance stages the
maximum gas flow is determined by the size of the openings for the
control valves 44,46 and 69,71. The higher the balancing gas
pressures, then the smaller the diaphragm and valve area for a
given flow and area of accuracy. Also, the lower the durometric
values designed for the flexible annulus of the diaphragms, then
the greater the response to flow and pressure variations with a
concomitant increase in accuracy for a given diaphragmatic
area.
The modular design of differential gas mixing device 37 provides
flexibility for interchanging differential gas mixing devices of
varied valve geometry with the pressure balance stages. The flow
limits are controlled by the geometry of valving element 89 in the
differential gas mixer. Thus, one turn calibration for a given
scale is controlled by reducing the distance between the orifices
to contract the calibrated scale, and by increasing this distance
to expand the scale. Also, linearization of the calibrated mixing
knob 101 can be obtained with two gases of different viscosity and
density by suitable design of the geometry of valving element 89.
Thus, the valve element diameter in the orifice would be increased
to flow a gas of lesser density, and decreased to flow a gas of
higher density. Increments of calibration can also be expanded or
contracted by changing thread pitch for valve operating shaft 98. A
fine thread pitch provides slow travel to spread the calibrated
scale and a coarse thread pitch provides faster travel to contract
the scale. The radius of control knob 101 can also be increased to
expand a calibrated scale. The interchangability of mixing devices
37 having varied design features allows a common pressure balance
mechanism to deliver a number of respiratory and anesthetic gases
to selected mixing devices calibrated to mix specific gases.
It is apparent from the foregoing that we have provided a new and
improved method and apparatus for mixing gases which has many
advantageous features. It will be understood that various changes
in the details, material, steps, and arrangement of parts, which
have been described and illustrated in order to explain the nature
of the invention, may be made by those skilled in the art within
the principal and scope of the invention as expressed in the
appended claims.
* * * * *