Gas mixing apparatus for respirator

A gas mixing valve has first and second inlet chambers linked to supplies of a first and second gas, and a mixing chamber having a blended gas outlet. The inlet chambers are connected to the mixing chamber via first and second control valves, each valve having a valve seat and valve member with opposing Surfaces defining a flow control orifice. Each valve seat and valve member are relatively movable between a closed position in which no flow occurs and a maximum opening position to define a series of orifices of progressively increasing area corresponding to the same geometrical progression. The position of each valve member can be controlled to provide a desired mixing ratio. Each valve seat is provided in a respective piston, and the pistons are tied together to move in response to variations in pressure drop across each piston, in order to vary the orifice size to compensate for changes in flow rate without changing the mixing ratio.

BACKGROUND OF THE INVENTION 
The present invention relates generally to gas mixing valves and methods 
for mixing together two or more gases in selected proportions, and is 
particularly concerned with a gas mixing valve or apparatus for use with a 
medical respirator in order to mix air and oxygen in a desired ratio. 
Gas mixing valves are used to mix two or more different gases in desired 
proportions and to provide a desired output gas mixture. Such valves are 
used in medical respirators to mix air and oxygen to provide a suitable 
breathing mixture to a patient. Normally, a pair of poppet valve members 
are used to adjust the size of two orifices for controlling the 
proportions of the two different gases to be mixed. The size of the 
orifices is adjusted according to the desired mixture. One problem with 
such an arrangement is that mixing accuracy may be reduced as a result of 
reduction in the gas flow rates. Pressure drop across each valve will be 
proportional to the flow rate, and will increase as flow rate increases 
and decrease as the flow rate decreases. Generally, gas mixing valves have 
relatively good mixing accuracy at high flow rates, but significant errors 
in mixing accuracy can arise at low flow rates. 
In the past, this problem has been dealt with by designing special mixing 
valves for low flow rate applications, which have smaller scale valves. 
This limits the range of flow rates over which any one mixing valve can 
operate effectively. Another solution is described in U.S. Pat. No. 
4,072,148 of Munson et al., in which the mixing valve is provided with two 
stages. One, smaller valve stage is operated at all times, regardless of 
flow rate. The other, larger valve stage is operated only at high flow 
rates. Another alternative mixing valve arrangement is described in U.S. 
Pat. No. 4,085,766 of Weigl et al. In this apparatus, a piston is slidable 
in response to change in a reference gas pressure in order to adjust the 
size of two gas orifices in a sleeve surrounding the piston. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a new and improved gas 
mixing valve assembly in which relatively high mixing accuracy is provided 
regardless of gas flow rate, so that the mixing valve may be used for both 
high and low flow rates. 
According to the present invention, a gas mixing valve assembly is provided 
which comprises an outer housing having a first inlet for a first gas, a 
second inlet for a second gas, and a blended gas outlet, the housing 
having a first gas chamber connected to the first inlet, a first piston 
slidably mounted in the first gas chamber, and having a first valve seat 
having a first orifice, a second gas chamber connected to the second 
inlet, a second piston slidably mounted in the second gas chamber and 
secured to the first piston so that the two pistons move together as a 
unit, the second piston having a second valve seat having a second 
orifice, a mixing chamber connected to the first and second chambers via 
the first and second piston orifices, respectively, a first valve member 
movably mounted in the housing for movement from a seated position against 
said first valve seat to close said first orifice away from said seated 
position to define an adjustable orifice of progressively increasing flow 
area as the valve member moves away from the valve seat, a second valve 
member movably mounted in the housing for movement from a seated position 
against said second valve seat to close said second orifice away from said 
valve seat to define an adjustable orifice of progressively increasing 
flow area as the valve member moves away from the seat, and a control 
member for controlling the positions of the first and second valve members 
to define a preselected ratio between the flow areas of the first and 
second orifices corresponding to a desired gas mixing ratio, and the 
opposing surfaces of each valve member and valve seat being of 
predetermined shape to define a series of progressive flow areas which 
correspond to a geometrical progression, the pistons being movable 
together in response to variations in pressure drop across the two valves 
to compensate for the pressure drop while maintaining substantially the 
same ratio between the first and second flow areas. 
Thus, each valve is designed to have the same geometrical progression in 
flow area from the maximum orifice opening down to the minimum opening. 
The shape of the valve member or the valve seat, or both, is designed such 
that the desired geometrical progression is achieved. This is done by 
first determining the maximum orifice opening or flow area and the length 
of valve member travel away from the closed position to reach the maximum 
orifice opening. The valve movement is then divided into increments, with 
each increment of valve movement corresponding to a term of the 
progression. An orifice opening area is calculated for each increment of 
valve opening according to the progression ratio, and the valve member or 
valve seat is shaped in order to achieve the calculated areas for each 
step of valve movement in the progression. In this way, the ratio between 
the two orifices will be maintained regardless of the piston position. In 
other words, if each valve has an orifice area which increases by the same 
amount for each increment of valve or valve seat movement, regardless of 
the initial position of the valve or valve seat, once the mixing ratio has 
been set, movement of both valve seats by the same distance or number of 
increments will not change the mixing ratio. 
In one embodiment of the invention, each orifice is cylindrical or circular 
and each valve member comprises a generally cylindrical poppet having a 
radius substantially equal to that of the orifice, with opposing 
wedge-shaped, curved cut-out surfaces on opposite sides of the poppet 
which define a pair of semi-cylindrical openings of gradually increasing 
area between respective cut-out surfaces and the opposing surface of the 
opening. This shape is particularly convenient for obtaining a geometrical 
progressive orifice area, since a large change in radius will produce a 
relatively small change in orifice area, making the cut-out surfaces 
relatively easy to machine with sufficient accuracy. 
In alternative embodiments, the poppet valve may have a wedge-shaped 
cut-out defining a single semi-cylindrical opening. Alternatively, the 
poppet valve may be cylindrical while the wedge-shaped cut-outs are 
provided in opposing regions of the opening in the piston. Alternatively, 
the poppet valve or valve seat may be of conical shape to define the 
desired progression, or the poppet valve or valve seat may have an 
appropriately shaped cut-out or slot to define a triangular orifice of 
gradually increasing size as the valve and valve seat move apart. 
In this way, a mixing valve can be provided which provides accurate mixing 
at both high and low flow rates. If the flow rate decreases, the pistons 
will move towards the valve members, reducing the size of each orifice 
while maintaining the same ratio between the two orifice flow areas. Thus, 
the passageway or orifice area for each of the two gases is increased or 
decreased automatically with changes in flow rate, with the percentage 
increase or decrease being the same in both valves so that the mixing 
proportions are maintained. This enables the blender or mixing apparatus 
to act as both a high flow and low flow blender while maintaining blending 
accuracy. Although the gas mixing apparatus or valve of this invention is 
primarily for use in a respirator, it will be understood that it may 
alternatively be used in other applications where controlled mixing of 
gases is required.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 of the drawings schematically illustrates a typical gas blending 
system or respirator comprising a source of air 10 and a source of oxygen 
12 each connected via a filter 14,15, respectively, to a cross-over valve 
circuit 16. The air and oxygen outputs of circuit 16 are connected to a 
balance module 18, and the outputs of module 18 are connected to a mixing 
valve 20 which mixes the gases according to the selected proportions and 
provides a blended gas output 22. The blender mixes air and oxygen to 
provide a pressurized gas source that ranges from 21% to 100% in oxygen 
concentration. Filters 14 and 15 are typically 5 micron filters. The 
filtered gases pass through the cross-over valve/alarm circuit which is 
designed to cause an alarm if inlet pressure of either gas drops below a 
predetermined level. Balance module 18 is designed to equalize the 
operating pressure of the gas sources before entering the mixing valve 20. 
The cross-over valve and balance module will both be of a conventional 
nature and are therefore not described in any more detail. 
FIG. 2 of the drawings illustrates a mixing valve according to a first 
embodiment of the present invention for use in the mixing system or gas 
blender of FIG. 1. The mixing valve is designed to control the proportions 
of the two gases provided at blended gas output 22. The mixing valve 
basically comprises a housing 24 having an inlet 26 for air or a first 
gas, a second inlet 28 for oxygen or a second gas, and blended gas output 
22. A control knob 30 is provided for controlling the proportions of air 
and oxygen in the blended gas output provided to a patient. The housing 
has a first chamber 32 connected to first inlet 26, a second chamber 34 
connected to second inlet 28, and a mixing chamber 36 connected to outlet 
22. 
A first piston 38 is slidably mounted in chamber 32, and has a central 
opening 40 connecting chamber 32 to mixing chamber 36. A poppet valve 
member 42 is slidably mounted in chamber 32 in alignment with opening 40, 
and forms a variable flow area orifice 44 between the surface of valve 
member 42 and the peripheral rim 45 of opening 40. Similarly, a second 
piston 46 is slidably mounted in chamber 34, and has a central opening 48 
communicating with mixing chamber 36. A second poppet valve member 50 is 
slidably mounted in chamber 34 in alignment with opening 48 and forms a 
variable flow area orifice 52 between the surface of valve member 50 and 
the peripheral rim 54 of opening 48, as best illustrated in FIGS. 4 and 5. 
Each of the poppet valve members 42 and 50 are of identical shape, and are 
preferably generally cylindrical valve rods of diameter equivalent to the 
diameters of openings 42 and 50, each having two diametrically-opposed, 
upwardly curved and inwardly tapering flow control surfaces 55, 56, 
respectively adjacent their upper end which provide precise control of the 
size of the respective orifices 44,52, as will be explained in more detail 
below. Each flow control surface is of generally parabolic shape and 
defines a geometrical progression with the rim of the respective piston 
opening as the piston and poppet move relative to one another. A reduced 
thickness, upper end portion 57,58 of each valve member 42,50 is provided 
for calibration adjustment purposes. 
The position of each valve member relative to the opening in the respective 
piston will control the flow area, and thus the proportions of the two 
gases flowing from the respective chambers into mixing chamber. The two 
pistons 38,46 are tied together via diaphragm or plate 60 forming the 
lower end wall of chamber 36, as best illustrated in FIGS. 2 and 3. Plate 
60 is biassed towards the lower end of chamber 36 by spring 61. A guide 
rod 62 extends downwardly from the center of plate 60 into guide bore 63 
between the two chambers 32,34. With this arrangement, the two valve seats 
formed by the openings in pistons 38 and 46 are linked together and will 
move in response to variations in the pressure drop across the two pistons 
as a result of changes in the gas flow rate. 
The positions of the two poppet valve members are controlled by rotating 
the control knob 30, which is linked to cam shaft 64 which is rotatably 
supported in a lower chamber 65 of the housing. Each poppet valve or rod 
has a lower end 66,67, respectively, which projects into the lower chamber 
65 and is biassed against a respective one of the cam members 68,69 
mounted on cam shaft 64 beneath the respective chambers 32 and 34. The 
respective poppet valves are biassed against the respective cam members in 
any appropriate manner, for example as described below in connection with 
FIG. 23. 
As the cam shaft rotates, the two cam members also rotate to push up or 
lower the respective poppet valves and thus change the respective flow 
areas. The arrangement is such that the proportions of air and oxygen may 
be varied between 21% oxygen (i.e. no oxygen flow added to the air), and 
100% oxygen, 0% air. Cam member 68 therefore moves poppet valve member 42 
between a closed position in which the opening 40 is closed and a maximum 
opening position corresponding to 100% air, while cam member 69 moves 
poppet valve member or rod 50 between a maximum opening position 
corresponding to 100% oxygen and a closed position corresponding to no 
added oxygen. The cams are arranged such that opening 40 is closed when 
opening 48 is at a maximum, and opening 48 is closed when opening 40 is at 
a maximum and the size of each opening decreases as the other increases. 
Thus, the cam members can be fixed in position to provide desired 
proportions of air and oxygen at the outlet. The ratio between the flow 
areas defined by orifices 44 and 52 will correspond to the desired 
blending ratio. 
For low flow rates of gas through the blender, the piston diaphragm will be 
biassed by spring into the lowermost position illustrated in FIG. 2. 
However, as the gas flow rate increases, the pistons will be pushed up and 
the orifice sizes will increase. The control surfaces 55 and 56 on each 
poppet valve are shaped such that the ratio between the orifice sizes, 
once set by control knob 30, will remain the same regardless of the 
position of pistons 38 and 46. Thus, the same mixing proportions are 
maintained regardless of flow rate, and the same mixing valve can be used 
for low flow rates and high flow rates while maintaining the desired 
blending accuracy. 
As discussed above, each of the orifice control surfaces 55,56 on the 
respective poppet valve is precisely shaped to form a generally parabolic 
surface in an axial direction of gradually tapering width in a 
circumferential direction, as illustrated in FIGS. 4 and 6. The surface 
dimensions are designed such that the flow area for each incremental 
movement of the piston plate will correspond to a successive term of a 
geometrical progression. A geometrical progression is a series of terms in 
which each term is derived by multiplying the preceding term by a constant 
multiplier called the ratio of the progression. Assuming that the 
progression has a first term a corresponding to a minimum valve orifice 
and a last term 1 corresponding to a maximum valve orifice, the number of 
terms in the progression is n and the progression ratio is r, the general 
formula for the progression is: 
EQU 1ar.sup.(n-1) (1) 
In one particular example, the total poppet travel, or piston travel 
relative to the poppets, was 0.075". In order to provide a progression of 
60 terms with a first term a of 0.00006047 sq. in. and a last term 1 of 
0.0006047 sq. in., a progression ratio of 1.0397984 is used in equation 
(1) above. Using this relationship, an orifice area for each increment of 
poppet travel, i.e. each 0.0125" of travel, can be calculated. Once all 
the values are calculated, a suitable shape for the poppet valve can be 
devised in order to produce the desired progression. 
One suitable orifice shape which will produce a geometrical progression is 
a circular segment, as illustrated in FIG. 5. This orifice shape has the 
advantage that a large change of radius produces a relatively small change 
in segment area. This makes it easier to machine the poppet surfaces with 
sufficient accuracy to produce the desired geometric progression. In the 
embodiment illustrated in FIGS. 4-6, each flow orifice corresponds to the 
total area of the two circular segments on each side of the poppet, as 
illustrated in FIG. 5. Referring to FIG. 22, the area A of each circular 
segment between the poppet surface and rim of the opening may be derived 
from the following equation: 
EQU A=1/2[r.sub.1 l.sub.1 -c(r.sub.1 -h)] (2) 
where r.sub.1 is the radius of the opening, l.sub.1 is the arc length, c is 
the chord length, and h is the height of the segment. This area is not 
exactly correct for determining the flow area because the flow area must 
be measured normal to the flow path, as illustrated in FIG. 4, but it is a 
relatively good approximation. Using equations (1) and (2), successive 
circular segment orifice areas conforming to a geometric progression may 
be calculated, and the surface dimensions for producing the series of 
orifice areas can then be calculated. In this manner, poppets may be 
machined with control surfaces according to the calculated dimensions. At 
this point, each poppet valve is experimentally tested to determine flow 
rate at a series of successive positions. Since flow rate will be 
proportional to orifice area, any deviation of flow rate from the 
progression is noted, and the surface may be adjusted to provide the 
correct flow. 
Thus, the opposing control surfaces 55 and 56 on opposite sides of each 
poppet each define flow areas of circular segment shape with the rim of 
the respective piston opening, such that the total flow area defined by 
each poppet increases with incremental separation between the poppet and 
piston according to the same geometrical progression. Once the control 
knob has been set to define a predetermined blending ratio with the 
pistons in the lowermost position, the ratio between the area of the two 
orifices will be the same as the selected blending ratio. If the pistons 
move away from the poppets, the same blending ratio will be maintained 
since the proportional increase in size of each orifice will be the same, 
as controlled by the geometrical progression ratio. Thus, for example, 
assume that the flow area of orifice 44 is A.sub.1 and the flow area of 
orifice 52 is A.sub.2, and the progression ratio is r, then the mixing 
ratio will be A.sub.2 /A.sub.1. If the piston moves by one increment from 
a first position, then the new area of each opening will be equal to the 
area multiplied by the progression ratio r. In other words, A'.sub.1 
=rA.sub.1 and A'.sub.2 =rA.sub.2. The ratio between the two flow areas at 
the new position is then: 
##EQU1## 
Thus it can be seen that the same blending ratio can be maintained 
accurately as the pistons move to compensate for changes in flow rate. 
Other shapes of control surfaces may be provided on the two poppet valves 
or in the piston openings in order to produce a flow area corresponding to 
a geometrical progression. FIGS. 7-20 illustrate some other examples of 
valve configurations for providing a geometrical progression. FIGS. 7 and 
8 illustrate a first alternative valve configuration in which poppet valve 
member 70 has a single tapered wedge surface 71 on one side, rather than 
two opposing wedge surfaces 55,56 as in the first embodiment. In this 
case, each orifice comprises a single circular segment orifice 72 formed 
between the rim of the opening in piston 73 and the opposing surface 71 of 
the valve member, rather than two circular segments. Again, a series of 
opening areas may be calculated to provide the dimensions for appropriate 
machining of control surface 71. Each poppet valve will be provided with 
an identical control surface. 
FIGS. 9 and 10 illustrate another alternative poppet valve 74 which has a 
conical control surface 75 forming an annular flow orifice 76 of varying 
area between the rim of a circular opening in piston 73 and the surface of 
the valve member. Again, the dimensions of control surface 75 for 
producing an orifice area which varies according to a geometrical 
progression may be suitably calculated. 
FIGS. 11 and 12 illustrate another alternative poppet valve 77 which has an 
indentation or depression 78 of triangular cross-section and gradually 
increasing depth along curved inner end 79. This forms a triangular area 
orifice 80 between depression 78 and the rim of the opening in piston 73. 
The dimensions of this orifice required to form an area which increases 
incrementally according to a desired geometric progression may be readily 
calculated. 
In the previous embodiments, the poppet valve was provided with a control 
surface for forming a flow orifice having the desired variation in area to 
produce a geometrical progression. However, the opening or through bore in 
each piston which forms the valve seat may alternatively be provided with 
an appropriately shaped and dimensioned control surface. FIGS. 13 and 14 
illustrate one such arrangement, in which piston 81 has a through bore 82 
into which cylindrical poppet valve 83 extends. One side of through bore 
82 is provided with a generally wedge-like surface 84 corresponding 
substantially in shape and curvature to the surface 71 provided on the 
poppet valve in FIGS. 7 and 8. This provides a crescent-shaped orifice 85, 
and surface 84 can be shaped and dimensioned such that orifice 85 varies 
according to a desired geometrical progression. 
FIGS. 15 and 16 illustrate a modified valve seat or piston through bore 86 
which has opposing, wedge-like control surfaces 87 for controlling flow 
area. In this arrangement, two crescent-shaped orifices 88 are provided on 
opposite sides of poppet valve 83. 
In the embodiment of FIGS. 17 and 18, piston 89 has a through bore 90 of 
conically tapering shape, equivalent to the conically shaped poppet of 
FIGS. 9 and 10. Poppet valve 83 is identical to the previous embodiment 
and defines with the piston through bore an annular flow orifice 91 with 
an area which will vary due to the conically tapering surface of bore 90. 
Again, the shape and dimensions of the conically tapering surface will be 
designed according to the desired geometrical progression. 
FIGS. 19 and 20 illustrate another alternative in which the poppet valve 83 
is identical to the previous three embodiments and a piston 92 is provided 
with a through bore 93 having a tapering, V-shaped recess 94 of gradually 
increasing depth, corresponding to the control surface provided in FIGS. 
11 and 12 on the poppet valve. The V-shaped recess 94 forms a triangular 
shape flow orifice 95 with poppet valve 83, and the taper and dimensions 
of the V-shaped recess are designed such that the area will vary according 
to the desired progression. 
These are just some examples of possible control surface shapes for 
producing a flow orifice of area which varies according to a geometrical 
progression. Any suitable surface for providing an area which varies in 
this way may be provided either on the poppet valve or on the piston 
through bore. In each case the curvature of the tapering surface is 
preferably of parabolic shape, since this produces a flow area which 
changes by a relatively large amount over a relatively short distance of 
movement of the poppet valve or piston. 
FIG. 21 illustrates a mixing valve assembly according to another embodiment 
of the invention. In this embodiment, valve housing 110 has an air inlet 
112, an oxygen inlet 114, and a blended gas outlet 116. The air inlet 112 
is connected to an air chamber 118 and the oxygen inlet is connected to 
oxygen chamber 120. Control valves 122,124 respectively, control the 
connection of the air and oxygen chambers to a mixing chamber 126 which in 
turn is connected to the blended gas outlet 116. The two inlet chambers 
are separated from the mixing chamber by dividing wall 128, and are 
separated from each other by baffle 130. A lower wall 132 separates the 
two inlet chambers from cam chamber 134. 
Dividing wall 128 has a pair of openings 135,136 above the respective inlet 
chambers 118 and 120, in which the respective control valves are mounted. 
Control valve 122 includes a first piston 138 mounted in opening 135 via 
rolling diaphragm 139, while control valve 124 includes a second piston 
140 mounted in opening 136 via rolling diaphragm 142. Piston 138 is 
secured to a first valve seat member 144 extending upwardly into mixing 
chamber 126, while piston 140 is secured to a second valve seat member 145 
extending upwardly into the mixing chamber. The two valve seat members are 
tied together at their upper ends by means of tie plate 146. Tie plate 146 
has a central sleeve 148 slidably mounted over centering rod 150 which 
projects upwardly from lower wall 132 through baffle 130 and into the 
mixing chamber. Biassing spring 152 biases the tie plate and attached seat 
members and pistons in a downwards direction. 
Piston 138 and valve seat member 144 have aligned through bores 153,154, 
respectively, and a first poppet valve member 155 projects upwardly 
through bore 153 and into bore 154. Popper valve member 155 comprises a 
cylindrical rod having a shaped, flow control surface 156. The rod 
diameter is less than the diameter of bore 153. Bore 154 has a step in 
diameter forming a reduced diameter portion 158 of diameter close to the 
diameter of valve member 155. The rim of the step forms a flow control 
orifice 159 with the opposing portion of flow control surface 156 of the 
valve member. 
Similarly, piston 140 and valve seat member 145 have aligned through bores 
160,161, respectively, and a second poppet valve member 162 projects 
upwardly through bore 160 and into bore 161. Poppet valve member 162 
comprises a cylindrical rod having a shaped, flow control surface 164 
identical to flow control surface 156 of the first poppet valve member. 
The rod diameter is less than the diameter of bore 160. Bore 161 has a 
step in diameter forming a reduced diameter portion 165 of diameter close 
to the diameter of valve member 162. The rim of the step forms a flow 
control orifice 166 with the opposing portion of flow control surface 164 
of the valve member. Each valve seat member has a transverse through bore 
167,168, respectively, extending across the diameter of the seat member 
and intersecting the axial through bore 154,161, respectively. Bores 
167,168 connect the flow control orifice 159,166 to the mixing chamber. 
The upper end of each poppet valve member extends into the upper end 
portion of the valve seat member through bore for centering purposes. 
As in the first embodiment, a control knob 170 on the front of the housing 
is linked to a cam shaft 172 extending through cam chamber 134 and 
rotatably mounted in the opposite end wall of the chamber to knob 170. A 
pair of eccentric cam members 174,175 are mounted on the cam shaft 172 in 
alignment with the respective poppet valve members, and are positioned to 
provide the desired range of mixing proportions from 100% oxygen down to 
21% oxygen (100% air). The lower end of each poppet valve rod is secured 
in a respective plunger 176,177 which extends slidably through an opening 
178,179, respectively, in lower wall 132. Each plunger 176,177 is 
threadably secured to a respective yoke or sleeve member 180,182, 
encircling the respective cam members 174,175. Each cam member is 
rotatable in the respective yoke, so that rotation of the cam shaft will 
act to pull the respective plungers and attached poppet valve members up 
and down. Springs 184,185 bias the yokes 180,182 against the cam surfaces 
at all times. 
The control surfaces 156,164 in this embodiment are preferably identical to 
the control surface 71 of FIGS. 7 and 8 above, and are of generally 
parabolic shape in an axial direction, with a gradually tapering width to 
define a wedge-like, parabolically curved surface. This will form a 
circular segment flow orifice of area dependent on the position of the 
poppet valve relative to the orifice 159,166, respectively. Control knob 
170 is rotated to provide a desired blending ratio of air to oxygen. As 
one gas orifice is reduced in size a set amount, the other orifice is 
increased by the same amount. The full counter-clockwise position of the 
control knob corresponds to 21% oxygen, in other words the oxygen 
passageway is completely shut off at this position and air flow only is 
permitted to the blender outlet. If the control knob is turned to the 
fully clockwise position, corresponding to 100% oxygen, the air orifice 
will be completely closed and oxygen only will flow to the blender outlet. 
Between these two extremes, various settings are provided which correspond 
to various proportions of air and oxygen in the mixture. 
During operation of the mixing valve, pistons 138,140 will move together in 
response to change in flow rate. Since the incremental movement of each 
piston and orifice will be the same, the preset ratio between the flow 
areas of the two orifices will remain the same regardless of any piston 
movement, due to the fact that each flow control surface is designed to 
correspond to a geometrical progression, as explained above in connection 
with FIGS. 1-6. 
The spring 152 will create a pressure drop across the control valves. At 
low flow rates, the pistons will be biassed downwardly by the spring into 
the lowermost position which is illustrated in FIG. 21. If successive 
areas of the flow control orifice at incremental positions correspond to a 
geometrical progression with a ratio R, the desired orifice area ratio is 
R1, and the area of the first flow control orifice is A, then the area of 
the second flow control orifice at the lowermost position of the orifice 
will be controlled by the cam position to be R1xA. Assuming that the flow 
rate is increased such that the pistons move up by one increment, then the 
new area of the first orifice will be RxA and the area of the second 
orifice will be RxR1xA. Thus, the orifice area ratio will remain constant. 
The same area ratio will be maintained substantially constantly regardless 
of piston movement. 
The position of the pistons will be dependent on the pressure drop. If the 
flow is low, the pressure applied due to the spring force of spring 152 
will be greater than the pressure drop, and the pistons will be held down. 
If the flow rate increases, the pistons will rise up until the pressure 
drop across the pistons times the effective area of each piston equals the 
spring force, unrolling the rolling diaphragms as they move upwards. 
Table 1 gives one specific example of a set of orifice areas corresponding 
to a geometric progression having a progression ratio of 1.0397984, for an 
oxygen percentage in the range from 28% to 93%, and with flow control 
surfaces as described above in connection with FIGS. 1-6. In Table 1, 
poppet travel is in inches and poppet area in square inches. 
TABLE 1 
______________________________________ 
AIR AIR O.sub.2 O.sub.2 % 
POPPET POPPET POPPET POPPET OXY- 
TRAVEL AREA TRAVEL AREA RATIO GEN 
______________________________________ 
.0750000 
.0006047 .0012500 .0000605 
10.00051 
28 
.0737500 
.0005816 .0025000 .0000628 
9.260906 
29 
.0725000 
.0005593 .00375 .0000654 
8.554653 
29 
.0712500 
.0005379 .0050000 .0000680 
7.912325 
30 
.0700000 
.0005173 .0062500 .0000707 
7.318228 
30 
.0687500 
.0004975 .0075000 .0000735 
6.768736 
31 
.0675000 
.0004785 .0087500 .0000764 
6.260505 
32 
.0662500 
.0004602 .0100000 .0000795 
5.790433 
33 
.0650000 
.0004426 .0112500 .0000826 
5.355656 
33 
.0637500 
.0004256 .0125000 .0000859 
4.953528 
34 
.0625000 
.0004093 .0137500 .0000893 
4.581591 
35 
.0612500 
.0003937 .0150000 .0000929 
4.237580 
36 
.0600000 
.0003786 .0162500 .0000966 
3.919400 
37 
.0587500 
.0003641 .0175000 .0001004 
3.625113 
38 
.0575000 
.0003502 .0187500 .0001044 
3.352921 
39 
.0562500 
.0003368 .0200000 .0001086 
3.101166 
40 
.0550000 
.0003239 .0212500 .0001129 
2.868314 
41 
.0537500 
.0003115 .0225000 .0001174 
2.652947 
43 
.0525000 
.0002996 .0237500 .0001221 
2.453749 
44 
.0512500 
.0002881 .0250000 .0001269 
2.269509 
45 
.0500000 
.0002771 .0262500 .0001320 
2.099102 
46 
.0487500 
.0002665 .0275000 .0001372 
1.941490 
48 
.0475000 
.0002563 .0287500 .0001427 
1.795713 
49 
.0462500 
.0002465 .0300000 .0001484 
1.660882 
51 
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1.536174 
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1.420830 
54 
.0425000 
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1.314147 
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1.215474 
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.0400000 
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1.124210 
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1.039798 
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.0062500 
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.00375 .0000654 .0725000 .0005593 
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.0025000 
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.0999949 
93 
______________________________________ 
With the above arrangement, compensation for increasing and decreasing gas 
flow rate can be made automatically without changing the gas mixing 
proportions. The geometrical progression of the air and oxygen control 
orifices allows a single blender to act as a combined low and high flow 
rate blender. In the past, separate high and low flow blenders have been 
required, with different orifice sizes. For low flow rate, a high pressure 
drop is needed for proper operation. A small orifice setting will provide 
an adequate pressure drop for proper operation of a mixing valve at low 
flow rates. However, the same orifice setting will not provide a 
sufficient pressure drop at higher flow rates. Thus, a separate mixing 
valve with higher orifice settings is typically needed for high flow rate 
applications. This invention permits the same mixing valve to be used for 
all flow rates, and automatically compensates for changing flow rate by 
increasing the orifice sizes while maintaining the same area ratio as 
required for the selected mixing ratio. 
Although some preferred embodiments of the invention have been described 
above by way of example only, it will be understood by those skilled in 
the field that modifications may be made to the disclosed embodiments 
without departing from the scope of the invention, which is defined by the 
appended claims.