Pressure vessel system, pressure responsive device for the system, and method for forming a diaphragm for the device

A pressure system has a pressure vessel holding gas under pressure and has a pressure responsive device arranged to be responsive to loss of gas from the vessel to provide a warning signal or the like corresponding to the gas loss. The device has two springs opposing movement of a device diaphragm in response to the gas pressure in the vessel, one spring being arranged to oppose diaphragm movement resulting from change in the gas pressure due to change in gas temperature from a selected temperature in accordance with the gas law, and the other spring being arranged to permit diaphragm movement to change position of a control to provide a signal corresponding to a gas loss when change in the gas pressure in the vessel decreases to a selected level at the selected gas temperature. The cooperation of the two springs permits the device to respond to pressure changes in the vessel which are due to change in gas temperature as well as to any loss of gas which may occur and provides the desired warning signal when loss of gas is sensed at any system temperature within a temperature range likely to be encountered.

BACKGROUND OF THE INVENTION 
The field of the invention is that of pressure responsive devices, and the 
invention relates more particularly to a temperature compensated device 
responsive to change in pressure in a pressure system indicative of a loss 
of gas from the system at any temperature within a selected temperature 
range. 
Conventional pressure responsive devices arrange a diaphragm to move in 
response to an applied fluid pressure to actuate a control to provide a 
signal or perform a related function corresponding to the level of the 
applied pressure. In a typical device, movement of the diaphragm is 
normally opposed by a spring which calibrates the device to provide the 
desired control signal when the applied fluid pressure is at a selected 
level. Sometimes the diaphragm itself or a member of the control is 
provided with selected resilience which cooperates with the spring to 
actuate the control when the applied pressure is at the actuating level. 
Frequently, however, it is difficult or impossible to calibrate such 
pressure responsive devices to provide the desired control signal at a 
precisely predetermined pressure level, and this is particularly true 
where portions of the applied fluid pressure are attributable to different 
causes or are subject to different calibration requirements. For example, 
where a pressure responsive device is intended to sense a loss of gas from 
a pressure system but where the gas pressure in the system varies with 
temperature in accordance with the ideal gas law for example, it has not 
previously been possible to detect the loss of gas from the system with 
any suitable degree of accuracy. Similarly, where a pressure responsive 
device is intended to withstand very high pressure levels while also being 
responsive to small changes in applied pressure, it has been difficult to 
provide a suitably compact device which meets both high pressure and 
accurate calibration or pressure sensing requirements. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the invention to provide a novel and improved pressure 
responsive device; to provide such a device which is adapted to be easily 
and reliably calibrated; to provide such a device which is 
temperature-compensated; to provide such a device responsive to gas 
pressure which is adapted to precisely determine when a loss of gas occurs 
in a closed pressure system while the system is subjected to widely 
varying temperature conditions; to provide a compact pressure responsive 
device which is adapted to be subjected to very high levels of applied 
pressure while being precisely calibrated to sense small changes in the 
applied pressure; to provide a novel and improved pressure system; to 
provide such a gas pressure system which is adapted to precisely determine 
when a loss of gas occurs in the system while the system is subjected to 
widely varying temperature conditions; and to provide a novel and improved 
method for forming a metal diaphragm for use in such a pressure device and 
system. 
Briefly described, the novel and improved pressure system of the invention 
comprises a pressure vessel holding a fluid such as a gas under pressure 
and includes a novel and improved pressure responsive device arranged to 
be responsive to changes in the pressure within the vessel. In accordance 
with the invention, the pressure-responsive device comprises a diaphragm 
which is mounted on a support to move in response to change in a gas or 
other fluid pressure applied to the diaphragm. Preferably the diaphragm 
and the support are formed of metal and are welded together to form an 
hermetic seal capable of withstanding high pressure forces. A control is 
also arranged on the support so that a control member such as a switch arm 
is movable between first and second control positions in response to 
selected diaphragm movement. The pressure-responsive device also includes 
first and second springs which normally oppose movement of the diaphragm 
in response to an applied fluid pressure but which cooperate in permitting 
sufficient diaphragm movement to move the control member to a control 
position against its bias under predetermined pressure conditions. 
Preferably the first and second springs are selected to oppose movement of 
the diaphragm in response to respective portions of the applied fluid 
pressure and are individually calibrated to cooperate in determining the 
applied pressure at which the control member moves between its control 
positions under different conditions. 
In one preferred embodiment of the invention, the pressure-responsive 
device is arranged in a pressure system so that the device diaphragm is 
exposed to a gas pressure in a pressure vessel in the system, and the 
device is temperature-compensated to provide a warning signal or the like 
at any temperature within a selected range when any loss of gas from the 
vessel permits the mass of gas in the vessel to fall below a selected 
minimum. In that device, the first spring comprises a dished bimetal 
spring which moves from an original to an inverted dished configuration, 
preferably with a snap-like or overcenter action with a nonlinear spring 
rate as shown in U.S. Pat. No. 4,861,953 for example, when a selected 
level of force is applied to one side of the dished spring and which 
returns to its original dished configuration with corresponding overcenter 
action when applied force falls below the selected level. The dished 
spring means is arranged so the diaphragm bears against said one dished 
spring side and so that the dished spring normally opposes diaphragm 
movement in response to an applied pressure. The bimetal characteristics 
of the first spring are selected so that the dished spring is adapted to 
move to its inverted dished configuration with little or no applied force 
when the spring is at a selected low temperature such as -40.degree. C. 
but so that relatively greater force is required to move the dished spring 
to its inverted dished configuration as the temperature of the dished 
spring increases up to a second temperature such as 107.degree. C. 
Preferably the increase in force required to move the first spring to its 
inverted dished configuration corresponds to or is the same as the 
increase in gas pressure which occurs in the pressure vessel due to 
increase in temperature of the gas from -40.degree. C. to 107.degree. C or 
other temperature range which maybe selected. The second spring preferably 
comprises a coil spring which is arranged to bear against an opposite side 
of the dished first spring with a force corresponding to or the same as 
the force which would balance a gas pressure such as 2000 psi. applied to 
the diaphragm at the selected low temperature of -40.degree. C. when a 
desired minimum mass of gas is present in the pressure vessel. The 
pressure vessel in the system is then provided with a predetermined mass 
of gas somewhat greater than the desired minimum mass of gas. In that 
arrangement, the pressure established by the initial mass of gas is 
sufficient to move the dished bimetal spring to its inverted dished 
configuration and to move the control member to its second control 
position. The gas pressure in the vessel is sufficient to retain the 
control member in its second position even though the temperature of the 
gas and of the dished bimetal spring varies throughout the temperature 
range from -40.degree. C. to 107.degree. C. However, if there is any loss 
of gas from the pressure vessel such that the mass of gas falls below the 
desired minimum level, the dished bimetal spring returns to its original 
dished configuration with overcenter action allowing the control member to 
return to its first control position in response to the control member 
bias to provide a warning indicating the loss of gas below the minimum 
level. This is true when the temperature of the gas and of the dished 
spring is at any temperature within the described temperature range. 
Typically, for example, the pressure-responsive device is adapted to 
provide the loss-of-gas warning with substantially equal accuracy when the 
gas pressure in the vessel is at 2000 psi. at the selected low temperature 
of -40.degree. C. or when the gas pressure is at 3500 to 4200 psi. or the 
like at the opposite end of the noted temperature range. 
In one preferred embodiment of the pressure responsive device of the 
invention, the diaphragm is mounted between two metal washers by welding 
peripheral portions of the washers and diaphragm together. A piston has 
one end of relatively small diameter disposed against the diaphragm 
through a bore in one of the washers and has an annular portion of a 
second, relatively much larger diameter at its opposite end engaging said 
one side of the dished spring to transfer force and movement from the 
diaphragm to the dished spring. A force-transmitting member disposed at an 
opposite side of the dished spring has an annular portion, preferably of 
said second diameter, disposed against the opposite dished spring side. 
The second, coil spring has one end bearing against the force-transmitting 
member to press the force-transmitting member against the dished spring to 
cooperate in opposing movement of the diaphragm in response to an applied 
fluid pressure. A motion transfer pin has one end engaging the dished 
spring and is slidable in the force-transmitting member to move the 
control member to its second control position when the dished spring moves 
to its inverted dished configuration. The support preferably comprises a 
metal sleeve welded to one of the washers mounting the diaphragm and 
extends around the control and around the first and second springs. Rings 
are welded to the sleeve at selected locations to bear against a 
peripheral part of the dished spring and against an opposite end of the 
coil spring respectively to permit easy and reliable construction and 
calibration of the device. 
In another preferred embodiment of the pressure responsive device of the 
invention, the second spring comprises a dished monometal spring which is 
disposed in nested relation with the first dished bimetal spring. In that 
device, the two springs cooperate in providing a very compact device 
capable of withstanding very high pressure forces. 
In one preferred embodiment of the invention, the diaphragm mounted between 
the two metal washers is selectively deformed by applying a selected fluid 
overpressure to the diaphragm through a bore in one washer, thereby to 
form a depressed central portion of the diaphragm which extends into a 
bore in the other washer and which has a flat part of selected area 
bearing against the small diameter end of the piston in that other washer 
bore. In that arrangement, movement of the diaphragm in response to change 
in applied gas pressure transfers force more uniformly to the dished first 
spring because the flat area of engagement between the diaphragm and 
piston remains relatively constant during such diaphragm movement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, 10 in FIGS. 1-3 indicates the novel and improved 
pressure system of the invention which is shown to include a pressure 
vessel 12 of any conventional type adapted to hold a fluid 14 such as a 
gas or liquid under pressure within the vessel and to include a pressure 
responsive device 16 which is arranged to be responsive to pressure in the 
vessel to give a warning or indication such as a control signal or the 
like representative of the pressure in the vessel under different 
conditions. In a typical embodiment, for example, the pressure system is 
adapted to inflate an automotive safety air bag or the like of known type, 
and the fluid in the pressure vessel comprises an inert gas to be used to 
inflate or supplement inflation of the safety air bag. In that system, the 
pressure responsive device 16 of the present invention is arranged to 
monitor the gas level in the pressure vessel to detect any loss of gas 
from the vessel which may occur over a long service life and to give a 
warning or control signal or the like when gas loss from the vessel has 
been sufficient to require refilling or replacement of the pressure vessel 
in the system. In such an automotive application it will be understood 
that the temperature of the gas in the vessel is likely to vary over a 
wide temperature range from -40.degree. C. to 107.degree. . for example 
and that, in accordance with the ideal gas law (or with the ideal gas law 
modified for compressibility factors for higher pressures), the gas 
pressure level within the vessel will vary substantially with the 
temperature change even though no gas loss occurs. Typically, for example, 
where the gas pressure level is intended to be maintained at least above 
2000 psi. at a selected low temperature such as -40.degree. C.--that 
pressure level representing a specific desired minimum mass of the gas at 
that temperature--the pressure level in the vessel will increase up to the 
level of 3500 to 4200 psi., for example, during gas temperature change up 
to 107.degree. C. depending on the characteristics of the specific gas 
used in the vessel. In that regard, the ideal gas law is stated as 
follows: 
EQU PV=mRT 
where P is gas pressure, V is vessel volume, m is mass of the gas, R is the 
universal gas constant, and T is the gas temperature. Accordingly, the 
pressure responsive device 16 of the invention is adapted to provide the 
desired signal when loss of gas is such that the pressure level in the 
vessel falls below a pressure in the range from 2000 to 4200 psi. or the 
like depending on the gas temperature at the time when the mass of gas in 
the vessel falls below the desired minimum level. 
In a preferred embodiment of the pressure-responsive device 16, a somewhat 
resilient metal diaphragm 18 formed of stainless steel or the like is 
disposed so that one side 18.1 of the diaphragm is exposed to gas pressure 
in the vessel as indicated by the arrow 20 in FIGS. 1-2. Preferably, for 
example, a first strong and rigid washer 22 of a metal such as cold rolled 
or stainless steel or the like is disposed at an opposite side 18.3 of the 
diaphragm, the first washer preferably having the same outer diameter as 
the diaphragm and having a bore 22.1 of selected cross-section or 
diameter, preferably at a central location in the washer. A second strong 
and rigid washer 24 preferably of the same or similar metal and preferably 
of the same diameter as the diaphragm is disposed at that outer side 18.1 
of the diaphragm. The second washer also has a bore 24.1 aligned with the 
bore in the first washer. The peripheral portions of the diaphragm and the 
two washers are then welded together around their peripheries as indicated 
at 25 to be hermetically sealed together in a common structure 26. A 
diaphragm structure of this type is shown in the commonly assigned U.S. 
Pat. No. 4,616,114. Preferably the second washer bore 24.1 is of 
relatively larger cross-section or diameter than the first washer bore. 
In the preferred embodiment of the invention as shown in FIGS. 1-3, a metal 
sleeve 34, of cold rolled or stainless steel or the like and preferably of 
relatively smaller outer diameter than the diaphragm structure 26, is 
secured to the first washer 22, preferably by welding as indicated at 27, 
to cooperate with the diaphragm structure to form a general device support 
35. The sleeve is aligned, preferably coaxially, with the washer bores 
22.1, 24.1. The sleeve is preferably thin-walled to permit easy welding 
thereto as described below and to provide the sleeve with a substantial 
interior volume. 
In a preferred embodiment of the invention, the diaphragm structure 26 as 
above-described is initially subjected to a selected fluid overpressure as 
indicated by the arrow 28 in FIG. 6 while a back-up element such as the 
piston 32 having a flat end 32.1 as described below is firmly held in 
position in the first washer bore 22.1. The overpressure is applied so 
that a portion 18.2 of the diaphragm, preferably at the center of the 
diaphragm, is depressed from the diaphragm to extend a predetermined 
distance into the first washer bore 22.1 to provide a flat lower bearing 
surface part 18.2a connected to the remainder of the diaphragm by a part 
18.2b of somewhat U- or J-shaped cross-section as shown in FIG. 6. The 
overpressure is then removed. With that diaphragm deformation, the flat 
bearing surface part 18.2a of the diaphragm is adapted to move along the 
axis of the first washer 22.1 by unrolling or extension of the U- or 
J-shaped part of the diaphragm in response to fluid pressure subsequently 
applied to the diaphragm without substantial change in the surface area of 
the bearing part 18.2a for a purpose to be described below. Preferably the 
resilience of the diaphragm is selected t be insignificant (relative to 
the springs 36 and 50 hereinafter discussed) in opposing movement of the 
diaphragm in response to an applied fluid pressure. 
In the preferred embodiment of the device 16, a piston 32 of a strong, 
rigid metal material or the like (as previously used in forming the 
diaphragm) is disposed within the sleeve to bear against the diaphragm 18. 
Preferably the piston has one end 32.1 of a diameter just a little smaller 
than the diameter of the first washer bore 22.1 which extends part way 
into the bore 22.1 to be engaged by the bearing part 18.2a of the central 
depressed portion of the diaphragm. The piston also has an annular portion 
32.2 of selected relatively larger diameter disposed at an opposite end of 
the piston. A first, dished spring 36, preferably formed of a bimetal 
material (only one layer of which is shown for clarity of illustration) is 
also disposed in the sleeve so that the annular portion 32.2 of the piston 
engages one side 36.1 of the dished spring, preferably near but somewhat 
spaced from the periphery 36.2 of the dished spring. Preferably a 
force-transmitting member 38 is also disposed in the sleeve so that an 
annular portion 38.1 at one end of the force-transmitting member, 
preferably of the same diameter as the annular portion 32.2 of the piston, 
engages an opposite side 36.3 of the dished spring. Preferably the 
force-transmitting member has a central bore 38.2 slidably receiving a 
motion transfer pin 40 of an electrically insulating, ceramic material or 
the like in the bore 38.2 so that one end 40.1 of the pin bears against 
the dished spring side 36.3. A first thin metal ring 42 of cold rolled or 
stainless steel or the like is also disposed in the sleeve. The outer 
diameters of the piston, dished spring, force-transmitting member and ring 
are preferably selected relative to each other and to the inner diameter 
of the sleeve 34 so that the piston and dished spring are axially movable 
within the sleeve as guided by the sleeve so that the ring fits snugly in 
the sleeve and is adapted to engage the periphery of the dished spring to 
limit such axial movement of the piston and dished spring, and so that the 
force-transmitting member is axially movable within the ring 42. 
The dished bimetal spring 36 is of a conventional type which is adapted to 
move from an original dished configuration as shown in FIG. 2 to an 
inverted dished configuration as shown in FIG. 1, preferably with a 
snap-like or overcenter action with a nonlinear spring rate as shown in 
U.S. Pat. No. 4,861,953, when sufficient force is applied to one side 36.1 
of the spring. Preferably the dished bimetal spring is also adapted to 
return to its original dished configuration with snap action when 
corresponding overcenter action applied force is reduced below a selected 
level. The force required to move the dished bimetal spring to its 
inverted dished configuration varies with the temperature of the dished 
spring in the manner conventional with such dished bimetal springs. 
Preferably the bimetal characteristics of the dished spring 36 are 
selected so that very little or substantially no force is required to move 
the dished spring between its original and inverted dished configurations 
when the dished spring is at a selected low temperature such as 
-40.degree. C. for example, but so that relatively much greater force is 
required to move the dished spring to between its original and inverted 
dished configuration as the dished spring temperature is increased up to 
107.degree. C. for example. Preferably the dished spring is selected so 
that the increase in force required to move the dished spring to its 
inverted dished configuration at 107.degree. C. is substantially the same 
as the increase in force applied to the dished spring via the diaphragm 18 
and piston 32 due to increase in the temperature of the gas in the 
pressure vessel 12 up to the temperature of 107.degree. C. Typically 
dished spring characteristics are selected so that the change in force 
required to move the dished spring to its inverted dished configuration is 
linear with respect to the change in pressure applied to the diaphragm as 
the gas temperature increases to 107.degree. C. e.g. 
In assembly and partially calibrating the device 16 as thus far described, 
a predetermined calibrating pressure is applied to the diaphragm 18 as 
indicated by the arrow 44 in FIG. 1. Preferably, for example, the 
calibrating pressure is selected so that, at the calibrating temperature, 
the calibrating pressure equals or is proportional to that portion of the 
pressure which would be applied to the diaphragm by the intended minimum 
mass of gas in the pressure vessel 12 due to increase in the gas 
temperature from -40.degree. C. up to the calibrating temperature. A force 
is then applied to the ring 42 as indicated by the arrows 46 (using a tool 
not shown) to press the ring 42, the dished spring, and the piston toward 
the diaphragm 18 until reaction force from the diaphragm moves the dished 
spring to its inverted dished configuration. The ring 42 is then secured 
in its position in sleeve 24, preferably by resistance welding or the like 
as indicated 48. 
A second spring 50 of the coil spring type is also disposed in the sleeve 
so that one end 50.1 of the coil spring bears against the 
force-transmitting member opposite the annular portion 38.1 on the 
force-transmitting member to further press the annular portion 38.1 
against the side 36.3 of the dished spring. A second metal ring 52 is then 
disposed in the sleeve to bear against the opposite end 50.2 of the coil 
spring. In assembling and further calibrating the device 16 as thus far 
described, another predetermined calibrating pressure is applied to the 
diaphragm 18 as indicated by the arrow 54 in FIG. 1. Preferably, for 
example, this calibrating pressure is selected so that, at the calibrating 
temperature, the calibrating pressure 54 equals or is proportional to the 
pressure which would be applied by the intended minimum mass of gas in the 
pressure vessel 12 at the calibrating temperature. A force is then applied 
to the ring 52 as indicated by the arrows 56 (using a tool not shown) to 
press the ring 52, the coil spring 50, the force-transmitter 38, the 
dished spring 36 and the piston 32 toward the diaphragm 18 until reaction 
force from the diaphragm snaps the dished spring to its inverted dished 
configuration. The ring 52 is then secured in its position in the sleeve, 
preferably by resistance welding or the like as indicated at 58. 
A control 60 having a control member 62 which is movable between first and 
second control positions is also mounted on the device support so that the 
control member is moved between its control positions in response to 
movement of the device diaphragm. Preferably, for example, the control 
comprises an electrical switch wherein a first movable control or switch 
member comprises a movable contact arm 62 as shown in FIGS. 1-3. 
Preferably the movable contact arm is resilient to be inherently biased to 
move to a first or open circuit position spaced from a second control or 
switch element such as a complementary contact 64 as shown in FIG. 2 but 
is adapted to be moved against its inherent bias to a second or closed 
circuit position engaging the complementary contact as shown in FIG. 1. 
Alternately if desired, the control comprises a valve or other 
conventional motion-responsive component within the scope of the 
invention. If desired, the control 60 is attached to the support by use of 
a third metal ring (not shown) which is also welded to the sleeve 34, the 
third ring being secured to the sleeve by welding or the like to position 
the control member 62 to be held in its second control position engaging 
the complementary contact 64 with selected contact engagement force when 
the dished spring is in its inverted dished configuration. Preferably, 
however, the control 60 is adjustably mounted on the second ring 52 to be 
axially movable within the sleeve to position the control member 62 in its 
second position with desired contact pressure when the dished spring is in 
its inverted dished configuration. 
In a preferred embodiment of the device 16, the control 60 comprises a body 
66 of a ceramic or organic material of electrically insulating properties 
having a relatively large cross-section portion 66.1 threadedly attached 
as at 66.2 to the second metal ring 52. A pair of electrically conductive 
terminals 68, 70 are mounted on the body extending through the body 
portion 66.1 so that first terminal ends 68.1,70.1 are accessible from 
outside the device 16 while opposite terminal ends 68.2,70.2 are disposed 
within the sleeve 34 adjacent to respective opposite sides of an integral, 
relatively smaller cross-section portion 66.3 of the body. Preferably the 
body has a recess 66.4 in one side of the small body portion. The second 
or complementary control or switch contact element 64 comprises a thin 
strip of resilient electrically conductive metal having a bight 64.1 
intermediate its ends. One end 64.2 of the complementary contact is 
secured in electrically conductive relation to on of the terminals 68 
preferably by welding and the bight 64.1 is snugly accommodated in the 
body recess 66.4 to dispose an opposite end 64.3 of the complementary 
contact upstanding from one end 66.5 of the body. In that arrangement, the 
complementary contact is stiffly supported against twisting by the bight 
but is easily flexed by a force applied along the axis of the upstanding 
end of the complementary contact to provided a desired limit to contact 
pressure between the movable contact arm 62 and the complementary contact 
64. Preferably the movable contact member 62 is also formed of a strip of 
resilient electrically conductive metal having one end 62.1 of U-shape 
fitted around the smaller body portion 66.3 and having an opposite arm 
portion 62.2 extending generally at a right angle to the first end to 
overly the body end 66.5 and be normally biased by its inherent resilience 
away from the complementary contact 64 but to be movable into electrical 
engagement with the complementary contact. Preferably any force applied to 
the diaphragm 18 by the resilience of the contact member 62 is selected to 
be insignificant relative to the forces applied by the spring 36 and by 
the coil spring 50. Preferably the legs of U-shape of the movable contact 
member end 62 are secured to the body portion 66.3 by any conventional 
means such as a rivet 72. Preferably an electrical resistance element 74 
of selected value is secured in electrically connected relation to the 
movable member 62 and to the other terminal 70 to be in series therewith 
in closely spaced relation to the point of engagement between the movable 
and complementary contacts 62, 64. Preferably the movable contact member 
has a dimple 62.3 in the arm end to be engaged by an opposite end 40.2 of 
the motion transfer pin 40 and preferably the arm has stiffening flange 
62.4 and rib 62.5 portions. In assembling and completing calibration of 
the device 16, the control member 60 is rotated in its threaded engagement 
with the metal ring 52 to axially advance the control member in the sleeve 
so that, with the dished spring 36 in its inverted dished configuration, 
the movable contact arm is engaged with the pin 40 and moved into 
engagement with the complementary contact 64 sufficient to flex the bight 
64.1 to provide the desired level of contact engagement between contacts 
62 and 64. The construction of the control 60 as thus described permits 
this rotation to be accomplished without twisting of the contact members. 
If desired, the threaded engagement of the control with the ring 52 is 
then fixed by staking or by application of an epoxy or the like to the 
threads 66.2 as will be understood. Preferably the control body has a 
shield part 66.6 protecting the terminals 68, 70 and preferably has a 
notch 66.7 in the shield part for detachably retaining a connector (not 
shown) attached to the terminals. 
Preferably the pressure responsive device 16 includes the components as 
above described. The device is then adapted for mounting in different 
applications by addition of a mount or other housing 76 adapted for the 
specific applications. Preferably, for example, a metal housing member 76 
having the general configuration of a tube has one end 76.1 welded or 
otherwise secured in hermetically sealed relation to the periphery of the 
diaphragm structure 26 as indicated at 77 and has a flange 76.2 at its 
opposite end welded or otherwise secured in hermetically sealed relation 
as indicated at 79 to the pressure vessel 12. If desired, the flange 76.2 
includes a port 76.3 to receive a valve or the like (not shown) for 
filling the pressure vessel with gas or other fluid as desired. 
In that arrangement of the device 16 in the pressure system 10, a selected 
mass of gas relatively greater than the desired minimum mass of gas is 
provided in the pressure vessel 12. Accordingly, the pressure of that gas 
against the device diaphragm 18 is sufficient to move the dished spring 36 
to its inverted dished configuration to electrically engage the movable 
control member 62 with the complimentary control member 64 with selected 
contact engagement pressure to provide an electrical signal across the 
device terminal 68,70 as shown in FIG. 1. The engagement of the annular 
portions 32.2 and 38.1 of the piston and force-transmitting member with 
the dished spring 36 near the periphery of the dished spring provides the 
diaphragm 18 with substantial support even against very high fluid 
pressures in the vessel 12 so that there is no tendency for the high 
pressures to cause excessive bending of the control members 62 and 64. The 
presence of the resistance element 74 in series with the movable control 
member 62 close to its point of engagement with the complementary control 
member 64 permits the signal across the device terminals to be sensed to 
provide a signal value representing proper closing of the control contacts 
and to distinguish that proper closing from any possible short circuiting 
which may occur across the device terminals. The control 60 is retained in 
its second control position by the applied gas pressure even though the 
gas pressure in the vessel 12 varies over a wide range due to change in 
the gas temperature over a temperature range from -40.degree. C. to 
107.degree. C. so long as the minimum desired mass of gas is retained in 
the pressure vessel. However, if sufficient gas is lost from the pressure 
vessel by leakage or the like over a long service life or by catastrophic 
failure of the pressure vessel or of an hermetic seal so that the minimum 
desired mass of gas is no longer present in the vessel, the dished spring 
moves with overcenter action back to its original dished configuration as 
shown in FIG. 2 to permit the control members 62 and 64 to separate 
response to the bias of the control member 62, thereby to interrupt the 
electrical signal provided by the device 16 to give warning of the gas 
loss. That switch-opening movement of the dished spring 36 is adapted to 
occur at selected different gas pressure levels in the vessel 12 as 
indicated by a curve a in FIG. 4 depending on the gas temperature at the 
time loss of gas below the minimum desired mass level occurs. That is, as 
shown in FIG. 4, the device 16 is adapted to indicate the gas loss at 2000 
psi. pressure when the gas is at a temperature of -40.degree. C. or at a 
pressure of 4200 psi. when the gas is at a temperature of 107.degree. C. 
Alternately, by selection of a different gas or gas volume, by selection 
of springs 36 and 50 of different properties, by change of the relative 
diameters of the annual portions of the piston and force-transmitting 
member, or by change in the calibration of the device 16 or the like, the 
performance characteristics of the device 16 are adapted to be varied as 
desired as indicated by curves b and c in FIG. 4. The particular 
arrangement of the deformed central portion 18 of the diaphragm is such 
that the area of engagement between the piston 32 and the flat bearing 
surface 18.2a of the diaphragm does not change substantially during 
movement of the diaphragm in response to an applied pressure so that the 
piston ratio of the device does not change substantially during the 
diaphragm movement. 
It should be understood that various modifications of the pressure system 
and pressure responsive device are possible within the scope of the 
present invention. For example, in an alternate embodiment of the 
invention, the dished spring 36 is formed of a monometal material and the 
characteristics of the spring are selected so that the dished spring 
opposes a major portion of an applied fluid pressure. The second spring 50 
is then selected to oppose a relatively much smaller part of the applied 
fluid pressure. In that arrangement, the dished spring is adapted to 
support the diaphragm 18 against a very high applied fluid pressure in a 
very compact manner while the second spring is easily mounted to provide 
close and precise calibration of the device to be actuated in response to 
small change in the applied fluid pressure. 
It should also be understood that the pressure responsive device shown in a 
normally-open circuit structure is also adapted to be modified in 
conventional manner to provide a normally-closed circuit structure. 
In another preferred embodiment of the pressure responsive device of the 
invention as indicated at 78 in FIG. 5, wherein corresponding reference 
numerals indicate corresponding device components, the second spring 50a 
also comprises a dished spring which is movable from an original dished 
configuration to an inverted dished configuration with overcenter action 
when a selected force is applied to one side 50a.1 of the second spring. 
The second spring is also adapted to return to its original dished 
configuration with overcenter action when the force applied to the dished 
spring side 50a.1 falls below the selected force level. The dished second 
spring is mounted in the device in nested relation to the first dished 
spring 36a so that the peripheries of both of the dished springs are in 
effect supported by the first metal ring 42a. In that arrangement, the 
first dished spring 36a is adapted to be formed either of a bimetal or a 
monometal material as desired to perform functions of the first spring 36 
as specified in either of the device embodiments previously discussed, and 
the second dished spring is selected to perform the function of the coil 
spring 50 previously discussed. That is, when the pressure applied to the 
diaphragm is sufficient to cause movement of the two dished springs to 
their inverted dished configurations with overcenter action, a motion 
transfer member 40a moves in response to the dished spring movement to 
move a device control member between control positions. The use of two 
dished springs permits the device 78 to be of a very compact structure 
while meeting all of the performance requirements of the previously 
described embodiments of the invention. The device 78 is calibrated by 
selection of the properties of the dished springs relative to each other 
or by other means conventionally employed for calibrating dished springs 
of the type described. 
It should be understood that although particular embodiments of the system, 
device and method of the invention have been described by way of 
illustrating the invention, the invention includes all modifications and 
equivalents of the described embodiments falling within the scope of the 
appended claims.