US3330925A

US3330925A - Snap-acting pressure switch

Info

Publication number: US3330925A
Application number: US477206A
Authority: US
Inventors: Thomas A Andrew; Robert S C Silver; Edward E Johnson; Kauker Joseph
Original assignee: Servonic Instruments Inc
Current assignee: Servonic Instruments Inc
Priority date: 1965-08-04
Filing date: 1965-08-04
Publication date: 1967-07-11
Anticipated expiration: 1984-07-11

Description

T. A. ANDREW ETAL 3,330,925

SNAP-ACTING PRESSURE SWITCH Filed Aug. 4, 1965 5 Sheets-Sheet l july l1', W67 T. A. ANDREW ETAL 3,330,925

SNAP-ACTING PRESSURE SWITCH Filed Aug. 4, 1965 5 Sheets-Sheet E' w di N Z a a )Tempera/gra July N, i967 T. A. ANDREW ETAL SNAP-ACTING PRESSURE SWITCH Filed Aug. 4, i965 3 Sheets-Sheet 5 United States Patent O Filed Aug. 4, 1965, Ser. No. 477,206 6 Claims. (Cl. 200-83) The present invention relates generally to electric switches, and it relates more particularly to snap-action electric switches which respond to changes in liuid pressure to be actuated from one operating condition to another.

The improved pressure-responsive switch of the present invention, as indicated above, is of the snap-action type. The switch is constructed so that only when the uid pressure introduced to the switch exceeds a particular threshold, will the switch snap from a first electrical condition to a second electrical condition. Conversely, only when the fluid pressure drops below a particular threshold will the switch snap back to its first electrical condition.

The improved snap-action pressure-responsive switch of the present invention includes a sealed double-diaphram aneroid snap capsule element as the actuating component of the assembly. The snap capsule is so constructed that it utilizes the snap (or over-center) action of one of the two diaphragms forming the unit. This snap acting diaphragm operates in principle similar to that of a Belleville washer, while the other diaphragm acts as a yieldable support, the two forming a sealed chamber and both providing motion that results in a maximum overcenter displacement. This combination of yieldable supporting diaphragm and a Belleville disk to form an aneroid capsule givesrmuch greater over-center travel than could be obtained with a single Belleville disk. Basically, this enables larger contact gap in the final pressure switch, giving it the capacity to switch higher currents in circuits having high inductance.

An important object of the present invention is to provide an improved pressure-responsive snap-acting electric switch of the type mentioned above, and which is rugged and durable, and yet relatively inexpensive.

Another object of the invention is to provide such an improved pressure-responsive snap-acting switch which can be accurately and precisely set to respond to predetermined maximum and minimum pressures so as to be actuated from one electrical condition to the other.

Yet another object of the invention is to provide such an improved pressure-responsive electric switch which is not subject to mechanical or electrical failures, and which is capable of performing efficiently throughout a long and trouble-free operational life.

A feature of the improved snap-acting pressure-responsive electric switch of the present invention is the provision therein of a sealed double-diaphragm snap-acting capsule as the actuating element, as mentioned above.

The provision of such a snap-acting capsule permits the switch of the invention to incorporate the desired snapacting diaphragm action without the concomitant production of excessively high stresses at the outer periphery of the diaphragm. Such high stresses are encountered, for example, in prior art switches using a single snap-acting diaphragm.

The high stresses, so encountered in the prior art switches, have a tendency to produce permanent deformation in the metal at the rim of the diaphragm, with resulting loss in reliability and change of performance characteristics in the prior art switches.

When a single Belleville disk is used in the prior art units, the sealing effectiveness is compromised in that a ICC certain amount of the thickness of the disk must be used for the sealing diaphragm. To obtain accurate performance in such prior art units, the thickness of the sealing diaphragm must be held at a minimum, which creates a tendency for leakage due to material porosity and corrosion during the operational life of the unit.

In the improved unit of the invention, a single sheet of material is used to obtain the over-center or snap action, so that thickness may be maximized to provide enhanced sealing effectiveness and corrosion resistance.

Another feature of the improved pressure-responsive switch of the present invention is the adjustability of the assembly, whereby operating thresholds, dead bands, and other switch characteristics, can be precisely preset to any desired values.

Other objects, features and advantages of the invention will become apparent from a consideration of the following description, when the description is taken in conjunction with the accompanying drawings, in which:

FIGURE l is a side sectional view of a pressureresponsive electric switch constructed in accordance with one embodiment of the invention;

FIGURE 2 is a side sectional view of a modified construction o-f the assembly of FIGURE l;

FIGURE 3 is a side sectional view of a second modification; and

FIGURES 4, 5 and 6 are curves which are useful in explaining the operation and characteristics of switches constructed to incorporate the concepts of the invention.

The pressure-responsive snap-acting switch of the present invention may, for example, have a generally cylindrical configuration and the switch may be enclosed by a switch housing lti.

The switch housing 1t) has a first end casing 12 welded,v

or otherwise attached to its right hand end, by means, for example, of a weld 14. The casing 12 supports, for exam-- ple, the electric terminal assembly of the switch.

The housing 10 also has a second casing 16 welded, or otherwise attached to its right hand end in FIGURE l by means, for example, of a weld 18. The casing 16 constitutes, for example, a fitting for the switch, the iluid pressure being introduced through the right hand end of the tting.

A sealed double-diaphragm snap-acting cartridge 20 ismounted on the inner face of the casing 16 by means, for example, of a weld 22. The snap-acting cartridge 20 includes a first diaphragm 2a and a second diaphragm 26, the two diaphragms being sealed to one another, for example, by means of 'a peripheral weld 28.

A plunger assembly 30 is slidably mounted in the casing 16, and the assembly includes a tubular member 32 which extends into the interior of the snap-acting capsule 20 and bears against the central portion of the inner face of the diaphragm 26.

The plunger assembly 30, together with the tubular member 32 are biased against the diaphragm 26 by means, for example, of a back-up spring 34. The back-up spring 34 is held in place by a cup-shaped nut 36 which is threaded into the casing 16. The nut 36 may be adjusted, so .as to adjust the force of the back-up spring 38 as it bears against the plunger assembly 30.

The tiuid pressure is introduced through the casing 16, and through the spring 34 and through the tubular member 32 into the interior of the sealed capsule 20.

When the fluid pressure exceeds a particular threshold, the diaphragm 26 of the sealed capsule 20, which is in the form of a Belleville spring, snaps from the position shown in FIGURE l, to a second position displaced to the left in FIGURE 1.

It will be appreciated that the switch housing 10 forms `a chamber for the capsule 20. A header assembly 50 is threaded into the left hand end of the housing 10. The

Q header assembly supports a pair of fixed contacts 52 and 54, each of which has a bent-over end portion at its right hand end, as illustrated. The xed contacts are supported in the header by bushings 56 and 58 of any suitable insulating material.

A cup-shaped plunger 60 is slidably supported at the center of the header assembly 50, this member 60 being composed, for example, of stainless steel. A movable contact 62 of the assembly is supported on the plunger 60, and is insulated therefrom by means of a suitable insulating member 64 composed, for example, of sapphire, or other jewel, or other suitable insulating material.

An operating pin 66 is slidable in the cup-shaped plunger 60 and in the member 64. The pin 66 has a shoulder which bears against the member 64 when the pin 66 is displaced to the left in FIGURE l, so as to move theV movable contact 62 out of bridging engagement with the xed contacts 52 and 54.

It will be observed that the movable contact 62 has a central boss portion 62a which normally extends between the extremities of the fixed contacts 52 and 54. This boss on the movable contact forms a protective skirt, and serves to prevent arcing between the extremities of the xed contacts.

Previously, it had been found that splattering, which takes place when the circuit is first broken, had a tendency to bridge over the insulation formed by the sapphire member 64. The portion 62a of the movable contact 62 prevents splattering from reaching a location where it can bridge over the member 64. This boss has the effect of raising the breakdown voltage of the assembly, and especially between the xed contacts 52, 58 and the operating pin 66 which is in contact with the remainder of the assembly.

The arcing between the contacts can also be inhibited by introducing a gas into the interior of the housing 10, that is, into the chamber containing the capsule 20. This gas may, for example, take the form of nitrogen having an absolute pressure in a range, for example, of from 2 p.s.i. to atmospheric.

Any tendency for the aforesaid arcing to take place can be inhibited, of course, by maintaining a high vacuum within the assembly. However, such a high vacuum is diicult to achieve and maintain, on a practical and economical basis. For that reason, it has been found a simple expedient to fill the switch chamber with nitrogen of an absolute pressure of from 6-8 p.s.i. for arc quenching purposes.

The nitrogen also has the advantage of being temperature compensating. The elastic characteristics of the capsule undergoes slight changes with variations in environmental temperature conditions. This has a tendency to create operational errors in the mechanism. However, it has been found that the thermo-elastic changes of the capsule under such varying temperature conditions can be negated by the corresponding effect the temperature changes has on the nitrogen filling the switch chamber.

For example, and as Vshown in FIGURE 4, assuming that the chamber is evacuated, the curve of pressure versus diaphragm displacement would follow the curve A yat a relatively low temperature, and would follow the curve B at a relatively high temperature. The introduction of gas, such as nitrogen, into the chamber, tends to maintain the two curves A and'B relatively close to one another, so that the operating characteristics of the switch are relatively independent of ambient temperature.

The right hand end, of the operating pin 66 is positioned adjacent the diaphragm 26 of the `capsule 20. When the uid pressure introduced in to the casing 16 exceeds a certain threshold, the diaphragm 26rsnaps to the left in FIGURE 1, thereby moving the pin 66 and causing it to move the movable Contact 62 out of engagement with the Vends of the xed contacts 52 and 54.

,A ,small gap is provided at the aforesaid shoulder of the pin 66, so as to permit a slight movement of the diaphragm 26 beforel it encounters a load. This serves to overcome initial inertia effects in the assembly. This gap, for example, may be of the order of .001 inch.

A spring 70 bears against the inner surface of the cupshaped plunger 60 thereby biasing the plunger to the right in FIGURE l. The spring 70 is the contact loading spring, and it determines the physical resistance to be exerted by the movable contact 62 against movement thereof by the diaphragm 26.

The force exerted by the contact loading spring 70 is determined by a nut 72 which is threaded into the header assembly 50. A lock -nut 74 is also provided to hold the nut '72 -in place.

In addition, a threaded shaft 76 extends into the center of the header assembly, and the right hand end of the shaft 76 is displaced a predetermined amount from the left hand end of the operating pin 66.`

The shaft 76 provides an adjustable stop, determining the extent to which the fluid pressure in the capcule 20 will bias the diaphragm 26 to the left in FIGURE l, and also determining the extent of opening of the movable contact 62. Y

A pair Iof terminal pins 80 and 82 are mounted in the casing 12 in appropriate insulating bushings 84 and `86. These pins extend into the respective fixed contacts 52 and 54, `so that -an electrical connection may be established to the Switch.

As mentioned above, one of the principal advantages of the double diaphragm snap-acting capsule assembly 20 is that the stresses at the peripheral edge of the snapacting diaphragm may be decreased to a large extent, as compared with the single diaphragm construction. The stresses around the periphery of the single diaphragm type of unit, where the diaphragm is mounted in the casing, for example, have a tendency to exceed the yield point of the material of the diaphragm, so that degradation of the switch rapidly occurs.

The construction shown in FIGURE l, wherein the capsule 20 includes a rst Idiaphragm 24 welded to thc casing 16, and includes a second diaphragm 26 welded to the diaphragm 24 at its peripheral edge, is such that no unduly high stresses are set up at the periphery of the snap-acting diaphragm 26, when it is snapped. from one condition to the other.

In pressure-responsive switches of the type under consideration, the diaphragm displacement generally follows a curve, such as shown in FIGURE 5. Starting at point A, the unrestrained diaphragm snaps to its second position C, and a restrained diaphragm snaps to `a point somewhere between A a-nd C, this latter point being determined by the position at which the end of pin 76 is contacted by operating pin `66. In actual switch assemblies, the stop is positioned at a point considerably closer to A than to C.

The :pressure in the diaphragm is assumed to be the actuating pressure, and its magnitude at all times will be determined by the magnitude of the pressure source, with the effects of volume change within the diaphragm having a very small, if any, effect.

Theline passing through the point B and the line passing through the points A and C define an area known as the dead band of the switch.

When the pressure applied to the unrestrained diaphragm is decreased from C, it will snap back to the position marked on FIGURE 5 as D. A restrained diaphragm, restrained from instance at point E, would snap from A to E on increasing pressure. With decreasing pressure, when the pressure magnitude has reached the level indicated at F, the diaphragm will snap -back to position G, the pressure magnitude being determined to `a large degree by positioning of the stop or pin 76.

The dead band of the switch defines, therefore, the difference in pressure between the pressure required to actuate the switch from la first condition to a second condition, and the different, lower pressure, required before the switch will return to its first position.

The free or initial dead band of the capsule 20 may, for example, be made to be around 8 p.s.i. This is the dead band, for example, which would exist if there were no external forces on the capsule 20 from either the right or the left in FIGURE 1. In any event, the initial dead band is made to be from two to six times as large as the final desired dead band, under usual circumstances.

The tinal dead band is arrived -at during assembly by selecting a spring 34 that has ythe required spring rate and by adjustment of pin 76 which determines the amount of over-center travel of the capsule. The increasing pressure switch point is established by adjusting the nut 36 which controls the force with which the spring 34 causes the tubular member 32 to bear against diaphragm 26.

In a constructed embodiment of the invention, for example, the tinal desired dead band was of the order of 4 p.s.i., so that the initial dead band was designed to be anywhere from e-ight to 24 p.s.i. Then the final dead band was provided by the suitable adjustments described above.

The design of the constructed embodiment was such that the diaphragm 26 would normally snap over at an applied presure of the order of 170-175 p.s.i. The snapover point was then lowered by increasing, for example, the assisting force exerted by the spring 34, this being achieved by adjustment of the nut 36.

It is evident, therefore, that suitable adjustment of nut 36 supporting spring 34 can precisely set the snapover point or increasing pressure switch point of the switch, and that the spring rate of spring '34 and proper adjustment of stop pin 76 will determine the dead band. The function of spring 70 is largely parasitic in nature, and basically is used for maintaining minimum contact pressure.

It is to be noted lthat the st-op shaft 7'6 can also be adjusted to determine the dead band. That is, this shaft provides the amount of displacement possible by the diaphragm 26, and thereby determines the extent to which the iiuid pressure must be decreased, in order to permit the diaphragm t-o return to its original state.

The position of the entire switch contact assembly with respect to the capsule 20 may be adjusted, by initially threading the header 50 into the housing 10 so that it has a desired placement therein. Then, the header can be sealed to the housing by means, for example, of spot welds, such as the weld 51. The setting of the header assembly places the operating point of the capsule 20 at the desired point on the curve of FIGURE 5.

In adjusting the assembly of FIGURE l, the nut 72 is first adjusted in the header assembly 50, after the header assembly has initially been adjusted land sealed by the welds S1. The adjustment of the nut 72 determines the compression to be applied to the movable contact 62 by the spring 70, thereby adjusting the contact pressure of the switch.

An initial adjustment of the shaft 76 is made to give somewhat greater travel to the operating pin 66 than is ultimately desired. The adjustin-g nut 36 adjusts the compression force of the spring 34 to give the proper snapover point of the diaphragm 26.

The header assembly 50 is then threaded into the -housing until the pin 66 has a very slight clearance from the diaphragm 26, to give the capsule a chance to move and over-come inertia before it has to do any work. Then, 4a final adjustment of the nut 36 is made precisely to tix the snap-over pressure, and a final adjustment of the stop shaft 76 is made to tix the dead band span of the switch. T-he shaft 76 is then locked by t-he nut 74.

The embodiment of the pressure switch shown in FIG- URE 2 is generally similar to the embodiment shown in FIGURE 1. However, in the latter embodiment, the backup spring 34, design-ated 34a in FIGURE 2, has been transferred out of the pressurized fiuid area, and has been placed on the other side of the capsule 20.

This latter embodiment has an advantage in that it removes the back-up spring from contact with the pres- 6 surized fluid, in that the pressurized tiuid often has a tendency to contaminate the spring material, when it comes in contact with the spring.

In the switch of FIGURE 2, like components which are the same as those of FIGURE l havel been designated by the same numbers. As before, the capsule 20 is disposed in the chamber formed by the housing 10 and the end casing. The right hand end casing is now ldesignated 16a, and it may have the same configuration `as the casing 16 shown in FIGURE 1. However, the spring 34 is removed, so that the casing 16a forms merely an orifice into the interior of the capsule 20, as shown.

In the latter embodiment, the loading spring, designated 70a bears directly yagainst an insulating disc 100 which, in turn, lbears against the movable contact 62. The movable contact is mounted on the operating pin 66a by the sapphire insulating Amember 64.

The operating pin 66a is integral with a plunger 102 which receives the Iback-up spring 34a. The back-up spring 34a may be adjusted by a nut 104 which is threaded into the header lassembly 50a and which is threaded on the adjustable pin 76. A lock nut 106 is also provided.

In the illustrated embodiment, the operating pin 66a is in its extreme position to the left in FIGURE 2, a-nd it bears against the right hand end of the stop shaft 76. As in the previous embodiment, the shaft 76 is adjustable to establish the amount of deection of the capsule 20 and the resulting dead band of the switch.

In the embodiment of FIGURE 2, the spring 34a is on the opposite side of the capsule 20, and its adjustment oppose the actu-ation of the diaphragm 26 to the left in FIGURE 2, whereas it assists the deliection to the left in FIGURE l. In FIGURE 2, the spring 34a loads the diaphragm directly with the spring 70a, the contact loading spring, serving the same function as the spring 70 in FIG- URE l. In a like manner, the spring 34a serves a similar Afunction as the spring 34 in FIGURE 1. The basic difieren-ce is that with increasing spring force, spring 34a tends to raise lthe diaphragm snap-over pressure, whereas increasing force on the diaphragm by spring 34 tends to -decrease increasing pressure switch point.

The embodiment of FIGURE 3 is generally similar to that of FIGURE 2. However, in the latter embodiment, the capsule 20 has been reversed, and it is mounted on the housing 10, rather than on the casing 16.

Since the housing and casing each have a somewhat different configuration in the embodiment of FIGURE 3, the housing is designated 10a, and the casing is designated 16a.

It will be observed that the operating pin 66a extends into the capsule 20 to be engaged by the inner surface of the diaphragm 26. The pressurized uid is introduced through the orifice formed by the end casing 16a, and into Va chamber 200 which is sealed by the capsule 20.

In the modification shown in FIGURE 3, the pressur-' ized fluid is applied to the outside of the capsule 20, yrather than the inside. This has an Iadvantage in that the pressurized iiuid does not reach the inner weld 22a of the capsule. Under some conditions, the pressurized iiuid has an errosive effect on the weld, so that it may have a tendency to fail.

The movable contact of the latter embodiment is designated 62b, and it has ya somewhat different configuration from the movable contact of the previous embodiment. The movable contact is formed as an annular ring composed, for example, of silver about lan insulating armature 202, the armature being supported on the operating pin 66a by means, for example, of a nut 204.

The contact loading spring 70a bears against the member 202 which, for example, may -be composed of sapphire. The annular movable contact 62b engages the fixed contacts 52 and S4, when the switch is in its closed condition.

The embodiment of the invention shown in FIGURE 3 may be modified to include the spring assembly 30, 34,

36 of the embodiment of FIGURE l in the pressure inlet orifice. This inclusion permits linear spring loading of the diaphragm from the pressure side of the unit.

With such a modification, the advantage is gained of being able to adjust the increasing pressure switch point of the capsule either to a higher magnitude by being able to apply increased force from the spring 34a, or to a lower magnitude by increasing the spring force on the diaphragm by compressing spring 34 from the pressure side.

Two main advantages result from the latter configuration. The first is the ability to use capsules having a greater spread of increasing pressure snap magnitudes, thereby increasing producibility. This is because the increasing pressure switch point can be increased or decreased by adjustment.

In the single spring load diaphragm coniiguration, on `the other hand, there is only the choice of one or the other.

The second advantage stems from the fact that a third variable is available in the basic spring system. This permits better spring rate adjustment so as to obtain better dead band control and adjustment. This allows greater utilization of snap capsules having high initial dead bands and results in greater producibility.

It has been established that certain advantages would stem from having a stop on the pressure side that would Y limit diaphragm travel. This would make possible presetting a diaphragm in the unpressurized condition to 4a position on the curve shown in FIGURES between A and B. In this arrangement, having the diaphragm stop above and below or on the pressure side and on the reference side would permit successful operation in the event extremely narrow dead bands must be provided. In this case, final switch performance would be as shown in FIGURE 6.

The stop on the pressure side will limit movement to a position shown at a in FIGURE 6 with the stop adjusted on the reference side limiting capsule movement in the snapped-over condition at b. Increasing pressure actuation would occur at the pressure magnitude present -at a, snapping from a to b. Decreasing pressure would snap the diaphragm back from d to c when the curve is reached at d. Dead bandwould be the difference of the pressure magnitude present at d and that present at a.

In other words, operation is completely on the negative slope side of the snap capsule, as shown on the initial capsule pressure vs. defiection curve. It is obvious that in this latter arrangement `capsule movement is considerably restrained from that existent in other configurations. Advantages of this type of configuration would be greater useful life due to increased fatigue resistance, and greater stability of performance.

In other words, to summarize, this latter type of switch would provide greater accuracies and better dead band control, particularly if extremely narrow dead bands are required, maintaining these properties over an increased useful life, both as to total cycles as well as the time element.

The invention provides, therefore, an improved pressure-responsive switch which is durable in its construction and reliable in its performance, and which has the capabilities of operating over long periods of time without failure.

While particular embodiments of the invention have been shown and described, modifications may be made. It is intended in the claims to cover all modifications which come within the scope of the invention.

What is claimed is:

1. A snap-acting pressure responsive switching assembly which is actuable to a first condition in response to a first pressure and which is actuable to a second condition in response to a second pressure, said switching assembly including: a housing enclosing a chamber and defining an orifice; a disc-shaped yieldable supporting diaphragm mounted in said chamber, said diaphragm having an aperture therein and being affixed to said housing at the periph ery of said orifice and of said aperture with said aperture aligned with said orifice; a disc-shaped snap-acting diaphragm mounted in said chamber in facing relationship with said supporting diaphragm and sealed thereto at the peripheral edge of said supporting diaphragm and snapacting diaphragm; a pair of elongated fixed contacts mounted in said housing in spaced insulated relationship and. extending into said chamber, said fixed contacts having end portions extending essentially parallel to the plane of said snap-acting diaphragm and spaced therefrom; a plunger member slidably mounted in said housing for reciprocal motion essentially perpendicular to the plane of said snap-acting diaphragm; a movable contact mounted on said plunger'member for selective bridging engagement with said end portions of said fixed contacts; an operating pin mounted on said plunger member and extending perpendicular to said snap-acting diaphragm to a position adjacent the surface thereof so as to move said movable contact in one direction relative to said end portions of said fixed contacts when said snap-acting diaphragm snaps to one operating condition; and resilient `means mounted in said housing for moving said movable contact in the opposite direction relative to said end portions of said fixed contacts when said snap-acting diaphragm snaps to a second operating condition.

2. The combination defined in claim 1 and which includes adjustable means mounted in said housing for controlling the force exerted by saidresilient means.

3. The combination defined in claim 1 in which said movable contact has a central portion which normally extends between the extremities of said end portions of said fixed contacts to prevent arcin-g ltherebetween.

4. The combination defined in claim 1 and which includes a pin adjustably mounted in said housing in axial alignment with Said plunger member vto provide a stop y therefor and thereby tolimit the extent of travel of said snap-acting diaphragm in one of the operating conditions thereof.

5. The combination defined in claim 1 and which includes a tubular member slidably mounted in said orifice and extending through said aperture into engagement with said snap-acting diaphragm, spring means positioned in said orifice and engaging said tubular member to bias said tubular member against the surface of said snap-acting diphragm, and adjustable means positioned in said orifice and engaging said spring means to control the force exerted by said tubularl member on said snap-acting diaphragm.

6. The combination defined in claim 1 in which said operating pin extends through said aperture in said supporting diaphragm into position with its end adjacent the inner surface of said snap-acting diaphragm.

References Cited UNITED STATES PATENTS 1,684,530 9/1928 Bast I 200-83 X 1,790,564 l/l931 Williams 200-83 l2,615,104 10/1952 Hosford 20G-83 2,632,119 3/1953 Schneider 20G-83 X BERNARD A. GILHEANY, Primary Examiner. H. B. GII/SON, G. MAIER, Assistant Examiners.

Claims

1. A SNAP-ACTING PRESSURE RESPONSIVE SWITCHING ASSEMBLY WHICH IS ACTUABLE TO A FIRST CONDITION IN RESPONSE TO A FIRST PRESSURE AND WHICH IT ACTUABLE TO A SECOND CONDITION IN RESPONSE TO A SECOND PRESSURE, SAID SWITCHING ASSEMBLY INCLUDING: A HOUSING ENCLOSING A CHAMBER AND DEFINING AN ORIFICE; A DISC-SHAPED YIELDABLE SUPPORTING DIAPHRAGM MOUNTED IN SAID CHAMBER, SAID DIAPHRAGM HAVING AN APERTURE THEREIN AND BEING AFFIXED TO SAID HOUSING AT THE PERIPHERY OF SAID ORIFICE AND OF SAID APERTURE WITH SAID APERTURE ALIGNED WITH SAID ORIFICE; A DISC-SHAPED SNAP-ACTING DIAPHRAGM MOUNTED IN SAID CHAMBER IN FACING RELATIONSHIP WITH SAID SUPPORTING DIAPHRAGM AND SEALED THERETO AT THE PERIPHERAL EDGE OF SAID SUPPORTING DIAPHRAGM AND SNAPACTING DIAPHRAGM; A PAIR OF ELONGATED FIXED CONTACTS MOUNTED IN SAID HOUSING IN SPACED INSULATED RELATIONSHIP AND EXTENDING INTO SAID CHAMBER, SAID FIXED CONTACTS HAVING END PORTIONS EXTENDING ESSENTIALLY PARALLEL TO THE PLANE OF SAID SNAP-ACTING DIAPHRAGM AND SPACED THEREFROM; A PLUNGER MEMBER SLIDABLY MOUNTED IN SAID HOUSING FOR RECIPROCAL MOTION ESSENTIALLY PERPENDICULAR TO THE PLANE OF SAID SNAP-ACTING DIAPHRAGM; A MOVABLE CONTACT MOUNTED ON SID PLUNGER MEMBER FOR SELECTIVE BRIDGING ENGAGEMENT WITH SAID END PORTIONS OF SAID FIXED CONTACTS; AN OPERATING PIN MOUNTED ON SAID PLUNGER MEMBER AND EXTENDING PERPENDICULAR TO SAID SNAP-ACTING DIAPHRAGM TO A POSITION ADJACENT THE SURFACE THEREOF SO AS TO MOVE SAID MOVABLE CONTACT IN ONE DIRECTION RELATIVE TO SAID END PORTIONS OF SAID FIXED CONTACTS WHEN SAID SNAP-ACTING DIAPHRAGM SNAPS TO ONE OPERATING CONDITION; AND RESILIENT MEANS MOUNTED IN SAID HOUSING FOR MOVING SAID MOVABLE CONTACT IN THE OPPOSITE DIRECTION RELATIVE TO SAID END PORTIONS OF SAID FIXED CONTACTS WHEN SAID SNAP-ACTING DIAPHRAGM SNAPS TO A SECOND OPERATING CONDITION.