US3573778A

US3573778A - Cantileverlike device in fluid-mechanical alarm

Info

Publication number: US3573778A
Application number: US772595A
Authority: US
Inventors: Philip H Sanford
Original assignee: Foxboro Co
Current assignee: Schneider Electric Systems USA Inc
Priority date: 1968-11-01
Filing date: 1968-11-01
Publication date: 1971-04-06
Anticipated expiration: 1988-04-06
Also published as: FR2022391A1; GB1279289A; JPS508133B1; FI54202B; DE1954947A1; NL6916454A; NL145926B; FI54202C; FR2022391B1; DE1954947B2; DE1954947C3

Abstract

In fluid operated instrumentation of process and/or energy control, a flexibly supported cantileverlike diaphragm unit as a high sensitivity device which lends itself to miniaturization and integrated fluid systems; further, in such instrumentation, wherein mechanical movement devices exemplified by diaphragms, force-bars, and baffles for fluid nozzles are combined with fluid circuitry, and wherein design is in the nature of miniaturization and integrated fluid systems, a dynamic system plate in which different portions are flexible and separate movement devices, as exemplified above, and which may include cantileverlike structures whereby the plate includes movement devices of a system or subsystem, as a dynamic integrated system plate; and further, in such instrumentation, an alarm system operable through change in the restriction of a nozzle upon the application of a signal to a nozzle-baffle assembly, one example of such an alarm system including a simple cantilever, or a dynamic plate with a cantilever and a force arm as parts of the plate. Passive elements, such as pneumatic restrictors, may also be embodied in the plate.

Description

United States Patent 72] Inventor Philip H. Sanford Walpole, Mass. [2!] Appl. No 772,595 [22] Filed Nov. 1, 1968 [45] Patented Apr. 6, 1971 [73 Assignee The Foxboro Company Foxboro, Mass.

[54] CANTILEVER-LIKE DEVICE IN FLUID- MECHANICAL ALARM 6 Claims, 25 Drawing Figs.

[52] US. Cl 340/240, 200/81 [51] Int. Cl I-I0lr 11/26 [50] Field of Search 340/240; 200/8 1 .2, 81.3

[56] References Cited UNITED STATES PATENTS 3,374,323 3/1968 Peek 340/240X Primary Examiner-Ralph D. Blakeslee Attorney-Lawrence H. Poeton ABSTRACT: In fluid operated instrumentation of process and/or energy control, a flexibly supported cantileverlike diaphragm unit as a high sensitivity device which lends itself to miniaturization and integrated fluid systems; further, in such instrumentation, wherein mechanical movement devices ex emplified by diaphragms, force-bars, and baffles for fluid nozzles are combined with fluid circuitry, and wherein design is in the nature of miniaturization and integrated fluid systems, a dynamic system plate in which different portions are flexible and separate movement devices, as exemplified above, and which may include cantileverlike structures whereby the plate includes movement devices of a system or subsystem, as a dynamic integrated system plate; and further, in such instrumentation, an alarm system operable through change in the restriction of a nozzle upon the application of a signal to a nozzle-baffle assembly, one example of such an alarm system including a simple cantilever, or a dynamic plate with a cantilever and a force arm as parts of the plate. Passive elements,

' such as pneumatic restrictors, may also be embodied in the plate.

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PHILIP H. SANF'ORI) CANTILEVER-LIKE DEVICE IN FLUID-MECHANICAL ALARM This invention relates to process and/or energy control instrumentation of the fluid operated type having fluid systems which operate in part by applying forces to movement parts, in a fluid mechanical combination. Such movement parts are various in their nature and use and may be made quite small, for example, to lend themselves to ultimate miniaturization in terms of a size allowing several such devices to be integrated with a total size of an ordinary postage stamp or less.

Such movement parts, especially when there are several in a system, ordinarily require considerable space, and often substantial fluid force and/or volume.

The modern need in such instrumentation is for miniaturization and circuit integration. Fluidics, like electronics, has been developed in this direction with various advantageous forms of circuit boards for fluid passages in miniaturized and integrated designs and structures.

This invention provides a concept in aid of this effort in that it provides miniaturization and integration techniques and structure, particularly for the moving parts of a fluid-mechanical system, while providing in small space the gain and stroke factors of previous larger and separate devices.

An example of this invention is a thin, flexible metal plate with an integral cantilever structure formed therein. One use of this cantilever is as a high sensitivity, long stroke diaphragm unit, taking up but little space, and formed by covering the flexible metal plate with a sheet which may be formed of elastic, such as rubber, so that the cantilever is a flexibly supported arm mounted on and essentially surrounded by rubber. A stiffener for the cantilever may be used where desired. Thus, pressure on one side of this combination will produce a long bending movement of the cantilever followed by the surrounding rubber. Very sensitive short stroke devices can similarly be formed.

A cantilever structure in a thin, flexible metal plate can be used to produce a force-strip, cantilever mounted somewhere along its length in a sensitive, light weight and small version of a force-bar structure in the much larger and heavier control unit devices of the prior art. This structure can also produce a very useful, high sensitivity, long stroke diaphragm unit, and also may be stiffened as desired.

Various movement devices may be built into the thin metal plate, for example, by etching away metal around the desired forms or devices. Metal diaphragm areas of usual and unusual diaphragm shapes and forms may be integrally formed in the thin metal plate. For example, a capsule with a portion of the flexible plate as a movable end, can take the place of a bellows. Spring rates, gain and stroke considerations may be built in or provided by local area treatment, as desired.

This invention provides a dynamic integrated plate in which moving parts of a particular system, or subsystem, may be incorporated in a single thin metal plate, a considerable advance for miniaturization and integrated structure in instrumentation control systems of fluid-mechanical combinations. This plate structure lends itself to the use of a sandwich unit in which the plate is sandwiched with a fluid circuit board, with the passages needed to service the mechanics of the plate located within the circuit board.

This invention further provides an alarm system wherein a nozzle-baffle assembly is operated to vary the nozzle-baffle restriction in response to an alarm signal, usually a fluid pressure applied to the baffle. In a small, compact assembly of such an alarm system, one structure includes a dynamic movement plate wherein the baffle is a cantilever structure as described hereinbefore. Other movement structures may be used in such an alarm assembly flexible plate, such as a forcearm, and a flexible-end capsule, to the end that an alarm system may be provided, in the nature of a miniaturized, integrated dynamic plate device wherein moving parts are incorporated in a thin flexible metal plate. Passive elements such as pneumatic restrictors may also be embodied in the plate.

Other objects and advantages of this invention will be in part apparent and in part pointed out hereinafter and in the accompanying drawings, wherein:

FIGS. 1 through 6 are various views of a simple cantilever diaphragm unit according to this invention;

FIGS. 7 and 8 are top view and central section views respectively, of a cantilever unit according to this invention, with housing backing formation in support of the molded or elastic body in the cantilever diaphragm unit;

FIGS. 9 and 10 are, respectively, plan and edge views of a dynamic plate including, by way of example, simple cantilevers and a force-arm cantilever, by way of illustrating the integrated dynamic plate concept of this invention, in the nature of miniaturization of fluid systems in process and/or energy instrumentation; passive pneumatic restrictors are also shown;

FIG. 11 is a schematic illustration of a nozzle-baffle alarm system concept according to this invention;

FIG. 12 is an illustration of an alarm system in further explanation of the concept of FIG. 11;

FIG. 13 shows physical locations on the dynamic plate of the elements shown schematically in FIG. 12;

FIG. 14 is a vertical lengthwise section through an alarm structure embodying the system of FIGS. 11 and 12;

FIGS. 15, 16, 17 and 18 are, respectively, top, side, sideview section, and right end views of the structure of FIG. 14, in terms of the position of FIG. 15;

FIG. 19 is an illustration of a reed switch operational system according to the alarm structure of this invention with a specific showing of diaphragm structure;

FIG. 19A is a lengthwise section through the structure of FIG. 19;

FIG. 20 illustrates an alternative form of the device of FIGS. 11 and 12;

FIG. 21 illustrates an alternative system for alarm on rateof-process change;

FIG. 22 is an alternative to the system of FIG. 20, as an alarm acknowledge device;

FIG. 23 is an alternative to FIG. 12, utilizing an electrical snap-switch; and

FIG. 24 is a further variant of the structure of FIG. 12.

CANTILEVER DIAPHRAGM Diaphragm devices are in common use in fluid instrumentation. They are devices which are fully peripherally supported, capable of having only relatively short stroke and low sensitivity, and they take up substantial space. This invention, in the nature of miniaturization and system integration, provides a diaphragm unit which may be of any shape, supported in a way to produce a diaphragm with long stroke, high sensitivity capability in small space. It is based ideally on a cantilever support to achieve the most sensitivity and highest stroke but variations on this mounting may be used.

Accordingly, as illustrated, this invention provides a new and useful concept, based on cantilever diaphragm units. They are useful in any diaphragm application, particularly in miniaturization and integrated systems. The cantilever support concept may use a single cantilever support or in special cases, several cantileverlike supports. They can be used as force-balance or motion diaphragms in a variety of miniaturized applications in fluid-mechanical systems. The cantilever forms may be achieved by etching, stamping, or other suitable process from materials such as thin beryllium copper, stainless steel, or other suitable materials, one example being glass. These devices have the advantages of metal diaphragms combined with the advantages of simple elastic dividing walls.

A simple cantilever is formed by removing material of a horseshoe form. The surrounding material is fixed, leaving the tongue free to move. The device may be backed up by a sheet of rubber cemented on the pressure side to form a pressure barrier.

in FIG. 1, a cantilever plate 10 is provided with a cutout cantilever arm 11, which may be pivotable about its base, on

line 12. In FIG. 2, the edge view shows the cantilever arm 11 in the plane of the plate 10, and in FIG. 3, the cantilever arm II as bent out at an angle from the plate 10, illustrating its long stroke capability. In FIG. 4 the simple showing is of the backup rubber sheet 13 molded with the deflected shape of FIG. 3.

In FIGS. 5 and 6 respectively, the rubber sheet I3 is shown as sandwiched with the plate 10, with the rubber sheet 13 and the cantilever arm 11 extended, and in simple alignment.

In FIGS. 7 and 8, a simple cantilever diaphragm unit is shown, wherein a fluid signal input passage 14 leads to a pres sure chamber 15, on the rubber side of the diaphragm unit. Pressure in the chamber 15 raises the cantilever arm 11. Further, the cantilever arm is provided with a stiffener plate 16, and an upper housing 17 is provided with a backup wall 18, to support the rubber sheet 13 as it is deflected by the movement of the cantilever arm, to leave a minimum area of the sheet free and open to the actuating pressure. The contour of the backup wall 18 is designed to maximize flexibility and stroke. The pressure thus increases as the stroke lengthens, to a point of maximum support of the rubber.

The rubber sheet 13 may be an elastic, stretchable body, or it may be a slack body of rubber or other suitably extendablc material, for example, various fabrics may be used. Their purpose is to move with the cantilever and to provide fluid seal condition around the cantilever, to whatever controlled degree is desirable for the particular application.

DYNAMIC SYSTEM PLATE AS INTEGRATED MOVEMENT PLATE The necessary trend of modem instrumentation has been toward miniaturization while maintaining and improving system integrity, dependability and repeatability, in the face of increased requirements as to performance and complexity. The pneumatic field has developed useful circuit boards to this end and for such purposes.

This invention presents a new and useful step in this area by providing a dynamic system integrated plate for the mechanical movement devices associated in and with fluid instrument systems. Pure fluid devices, with no moving parts, have their important place in this art, but more often it is found that a practical combination of pure fluid devices and some moving parts is most desirable.

This invention provides a dynamic system plate in which moving parts of such a system are located in a thin metal sheet, with each element established in a different portion of such a sheet in suitable form and flexibility for its particular purpose. As an example, such elements can be in cantilever form, in various cantilever shapes or combinations. Thus, any moving part of an instrument system may be built into such a dynamic plate in one form or another, with whatever associated elements, such as stiffeners, as may be desired. Many high sensitivity pneumatic components may be combined on a single small sheet of spring material such as thin stainless steel for such an integrated pneumatic component, In addition it is possible to include in this integration, springs, force bars, and any other mechanical elements which may lend themselves to this concept of design. Restrictor openings may also be provided through such a sheet.

An example of application of a dynamic system integrated movement plate is provided hereinafter with respect to the alarm system presented as an embodiment of this invention.

In FIGS. 9 and 10, to illustrate the dynamic system, such a movement system plate is shown at 19, with rubber backing sheet 20. This plate contains a group of simple cantilever diaphragm units in two different sizes at 21 and 22, with the smaller cantilever having pivot area cutouts as at 23, to reduce the spring rateof those particular elements, and a single forcearm cantilever at 24. This system plate is applicable to the alarm system exemplified later herein as an embodiment of this invention. Many other such plates may readily be formed for other desired applications, in great complexity and variety, in the nature of concentrating all the moving parts of an in strument or a subsystem of an instrument in a single, thin sheet. Various shapes and functions may thus be so incorporatedv While this concept springs from the cantilever construction, other movement constructions can similarly be incorporated as needed for particular applications. For example, various spring formations may be locally incorporated in different areas ofthc integrated sheet.

ALARM ACTUATOR SYSTEM AND STRUCTURE WITH CANTILEVER EXAMPLE Alarm systems are vital and necessary adjuncts to instrumentation systems, Especially in modern high volume processes and expensive energy systems the cost of uncontrolled or wrongly controlled conditions or parameters can be very large. Instant and reliable alarm systems are essential.

Since they are an adjunct to an operations system, their size and compatibility with the operations systems are matters of real concem.

In this invention, simple nozzle-baffle alarm systems are presented, and where miniaturization and system integration needs are involved, the dynamic system plate of this invention fills the needs, and can be based on the cantilever diaphragm concept of this invention. All of the switch diaphragms, inverter diaphragms and the like in the actuator systems are incorporated in the integrated system plate of FIG. 9, in its application to the structure of FIG. I4 through 18.

The alarm actuator of this invention provides an accurate, adaptable, pneumatically actuated alarm unit for use with controllers, recorders, indicators and auxiliary stations. Integrally actuated 24-volt AC alarm lights with or without an external signal for rclay'operation can be provided. The basic actuator, with minor changes, is capable of providing the following alarm functions (not shown):

Absolute Alarmthe alarm is a function of an absolute value in the measurement span of the instrument. Whenever the measurement reaches either an adjustable high level or a separately adjustable low level, lights may be activated and a signal for external 24-volt AC relay actuation may be provided.

Deviation Alarm-The alarm is a function of difference between controller set point and measurement. The deviation alarm actuator provides two independent adjustments which permit the operation of alarm lights at a predetermined deviation of the measurement above and below the control point, or above or below the set point. A common signal is provided to actuate a common external relay if desired. More than one such combination may be used for different applications.

In order to avoid switch flutter" where the measurement contains random noise variations, the unit may be equipped with adjustable dead bands.

The operating principle of the actuator is illustrated in FIG. 11. The measurement pressure is introduced into a bellows 40 which drives a flapper bar 41. The flapper bar also rests upon two air nozzles designated high alarm and low alarm. Above the bar at the high alarm nozzle is a spring 42 having adjustable compression, and above the bar at the low nozzle is a second spring 43 having adjustable compression. This constitutes a force-balance system whereby an increase in measurement may overcome the compression spring, causing the flapper bar to pivot on the low nozzle, venting the high nozzle to the atmosphere. Conversely, a decrease in measurement, with the aid of the tension spring, will cause the flapper bar to pivot on the high nozzle, to vent to the low nozzle. Thus, the pressure in the bellows vs the adjustment of the springs determines the point at which each nozzle will open.

For example, suppose the springs are adjusted so that with 9 p.s.i. in the bellows, both nozzles are covered, the compression spring could be adjusted so that the high nozzle would vent at psi. and the tension spring could be set so that the low nozzle would vent 8 p.s.i. By using the vented nozzle to initiate an alarm signal, duplex alarm action is provided. By adjusting one of the springs so that the nozzle beneath it does not vent within the 3 l 5 psi. span of measurement pressure, single high or low alarm signals can be accomplished.

FIG. 12 illustrates how the venting of the nozzles maybe used to produce an alarm signal and provide a means for adjusting dead band (also known as lockup). It also indicates the difference between the deviation and absolute alarm units. As drawn, it is a schematic for a duplex deviation alarm. The force at the bellows end of the flapper is due to any difference between the pressures in the measurement and set bellows. Once set to function at specific values above and below the set-point, alarm actuation will occur whenever the measurement and set-point pressures deviate by the preset values.

By removing the set bellows, the unit becomes an absolute alarm. The only force at the bellows end of the flapper bar is now due to the pressure in the measuring bellows. The high and low alarms, once set, will now be representative of absolute points in the span of measurement and remain fixed.

In FIG. 12, trace a high alarm signal through the circuit. The high nozzle 44 is connected to the diaphragm chamber of a spring-adjusted air switch 45 and receives its air supply through

restrictors

46 and 47. The high alarm nozzle 44 being open, the spring 48 of the air switch will lift the diaphragm 49 and its attached magnetic shunt 50. The magnet now closes the reed switch 51, actuating the alarm light 52. The air switch spring 48 may be made adjustable. No air is consumed except during alarm conditions.

At the same time the alarm signal is actuated, a connection between

restrictors

46 and 47 permits pressure from the upper diaphragm chamber of a pneumatic signal inverter 54 to bleed off through restrictor 47 and the high alarm nozzle 44. An air supply 55 to the lower inverter chamber 56 lifts the diaphragm 57 from the inverter nozzle 58 permitting air to flow from chamber 56 through restrictor 59 to the positive feedback capsule diaphragm 60. The line to this capsule is vented through an adjustable restrictor 61. Pressure, and thus force in the positive feedback capsule 60 is a function of the difference in resistance between restrictors 59 and 6]. Force applied by the feedback capsule 60 opposes the effect of the compression spring 62 and must be overcome by pressure drop in the measurement bellows before the high alarm nozzle 44 can close. This provides a dead band adjustable from a minimum of the order of l percent; of measured span to a maximum of the order of 8 percent. The positive feedback capsule is located to produce the same dead band on either alarm point.

A single output actuator differs from FIG. 12 in that a connection 64 is made between the high and

low nozzles

44 and 63, and one of the inverter and pressure switch circuits is omitted, providing only one output signal. Since the springs are adjustable over 100 percent of span, this signal may be made to represent high alarm only, low alarm only, or common high and low alarm using the same alann device.

This unit provides optional basic alarm functions plus combinations: high accuracy; repeatability of the order of gpen cent; high and low alarm adjustable over I00 percent measured span; deviation alarm adjustable 1100 percent: adjusta ble dead band lockup. The feature that no air is consumed except during alarm conditions is apparent since both nozzles are ordinarily closed by the baffle arm 65. Since the drop in air pressure in the nozzle circuits actuate the alarm, the device is fail-safe in the event of leakage or air failure.

Actual structure of the system of FIG. 12, utilizing the principles and operation of FIG. 11, based on the cantilever diaphragm unit and dynamic system plate concepts hereinbefore described, as in FIGS. 13 through 19A, is shown with like reference numbers applied to like elements as from FIG. 12. Note in FIG. 13, pneumatic restrictor openings indicated as R.

In the remaining FIGS. through 24, various alternative alann actuator systems are set forth schematically. Actual structure for these systems is similar to that of FIGS. 14

through I9A, and similarly lends itself to the use of cantilever diaphragm units and integrated system plates as described hereinbefore.

FIG. 20 illustrates a high-low alarm with avariable switch differential or dead band. In this version the typical nozzle restrictor is replaced by a variable pressure divider 66. When one of the nozzles is open by an alarm condition a portion of the drop across the restrictor appears as a drop in the smaller bellows 67 opposite the nozzles. This results in a variable snap action or dead band which may be adjusted as a function of the setting of the variable pneumatic potentiometer.

In some instances it would be desirable to monitor the measurement signal and produce an alarm when the process changes at an excessive rate. Shown in FIG. 21 is a deviation type of alarm modified by piping the measurement pressure through an adjustable restrictor 69 to the bellows normally used for the set point pressure. This will produce an alarm signal whenever the measurement changes faster than a predetermined rate.

FIG. 22 illustrates a high-low similar to that shown in FIG. 20. In this system however, the small bellows used for dead band is replaced by a bellows 70 large enough to generate a snap action which is capable of percent dead band resulting in a system which must be acknowledged after each alarm. Acknowledgement is accomplished by repressurizing the snap action bellows 70. This may be accomplished by mechanically depressing a reset bellows 71 connected to the bellows 70.

A further modification is illustrated in FIG. 23. This is a simple device using a microswitch 72 to obtain an alarm directly. Dead band is fixed at the value provided by the switch.

FIG. 24 illustrates another way of accomplishing similar alarm systems, like FIG. II except the measurement signal is applied to the baffle arm between the nozzles.

This invention therefore provides new and useful movement means in fluid-mechanical instrumentation in the nature of miniaturization and integrated fluid systems.

As many embodiments may be made in the above invention, and as changes may be made in the embodiment set forth above without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth and in the accompanying drawings is to be interpreted as illustrative only and not in a limiting sense.

Iclaim:

I. A fluid operated alarm system comprising; a battle arm, a pair of nozzles disposed along said baffle arm, each such nozzle being arranged to apply a fluid jet to said baffle arm, resilient means biasing said baffle arm restrictively against said nozzles, and means for applying a signal force to said baffle arm in one direction to significantly close one of said nozzles and significantly open the other of said nozzles, and in another direction to significantly open said one of said nozzles and significantly close said other of said nozzles, with the consequence of alarm operating changes in pressure in each of said nozzles.

2. An alarm system according to claim I wherein said nozzles are both on the same side of said baffle, and said resilient biasing means comprises individual and adjustable springs, one disposed oppositely of said baffle at the location of each of said nozzles, said means for applying a signal force to said baffle arm comprising bellows means adjacent one end of said arm.

3. An alarm system according to claim I wherein a fluid supply line is provided to at least one of said nozzles, a pressure divider restrictor is located in said supply line, a deadband bellows is applied to said baffle arm, and a fluid connection is provided between a tap connection of said pressure divider and said dead-band bellows.

4. An alarm system according to claim 3 wherein said deadband bellows is relatively large and capable of 100 percent dead-band snap action, and a reset bellows is connected to said fluid connection from said pressure divider as an alarm acknowledge device.

5. An alarm system according to claim 1 wherein said means for applying a signal force to said baffle arm comprises a measurement bellows on one side of said baffle arm, a set-point bellows on the other side of said baffle arm, a fluid rate connection between said measurement and set-point bellows.

6, An air operated alarm system for use in industrial instrumentation as a device for responding to an air signal as a measurement representation of variation of a value of a parameter beyond a predetermined norm, said system comprising:

A support base, a baffle arm mounted on said base in cantilever fashion, a high alarm nozzle mounted on said base to direct an air jet to the under side of said baffle arm adjacent one end of said arm, a positive feedback capsule diaphragm mounted on said base for force engagement with the under side of said baffle arm at an intermediate point along the length of said arm, a low alarm noule mounted on said base to direct an airjet to the under side of said baffle arm at a point beyond said capsule diaphragm from said high alarm nozzle, a set pressure bellows mounted on said base and in force engagement with the under side of the other end of said baffle arm;

support top plate, a measurement bellows mounted dependingly from said top plate and in force engagement with the top of said baffle arm and in opposition to said set bellows, and a spring adjustably mounted effectively on said top plate and against the top of said baffle arm in opposition to said low alarm nozzle and another spring in like mounting in opposition to said high alarm nozzle;

an air supply connection to said high alarm nozzle through one pair of rcstrictors in series, an air supply connection to said low alarm nozzle through another pair of restrictors in series, a signal output connection from each of said air supply connections from a point between said restrictors and their respective nozzles;

a signal inverter for said high alarm nozzle, comprising a housing, a diaphragm dividing said housing into two chambers, an inverter nozzle into one of said chambers and restrictable by said diaphragm, a spring in the other of said chambers biasing said diaphragm restrictably against said inverter nozzle, an air supply connection to said nozzle chamber, an air connection between said spring chamber and said high alarm air supply at a point between the restrictors thereof,

a like signal inverter for said low alarm nozzle, a common air connection to said capsule diaphragm, an air connection from each of said inverter nozzles through individual restrictors to said common connection, and a variable restrictor bleed to atmosphere from said common con nection. I

Claims

1. A fluid operated alarm system comprising; a baffle arm, a pair of nozzles disposed along said baffle arm, each such nozzle being arranged to apply a fluid jet to said baffle arm, resilient means biasing said baffle arm restrictively against said nozzles, and means for applying a signal force to said baffle arm in one direction to significantly close one of said nozzles and significantly open the other of said nozzles, and in another direction to significantly open said one of said nozzles and significantly close said other of said nozzles, with the consequence of alarm operating changes in pressure in each of said nozzles.

2. An alarm system according to claim 1 wherein said nozzles are both on the same side of said baffle, and said resilient biasing means comprises individual and adjustable springs, one disposed oppositely of said baffle at the location of each of said nozzles, said means for applying a signal force to said baffle arm comprising bellows means adjacent one end of said arm.

3. An alarm system according to claim 1 wherein a fluid supply line is provided to at least one of said nozzles, a pressure divider restrictor is located in said supply line, a dead-band bellows is applied to said baffle arm, and a fluid connection is provided between a tap connection of said pressure divider and said dead-band bellows.

4. An alarm system according to claim 3 wherein said dead-band bellows is relatively large and capable of 100 percent dead-band snap action, and a reset bellows is connected to said fluid connection from said pressure divider as an alarm acknowledge device.

6. An air operated alarm system for use in industrial instrumentation as a device for responding to an air signal as a measurement representation of variation of a value of a parameter beyond a predetermined norm, said system comprising: A support base, a baffle arm mounted on said base in cantilever fashion, a high alarm nozzle mounted on said base to direct an air jet to the under side of said baffle arm adjacent one end of said arm, a positive feedback capsule diaphragm mounted on said base for force engagement with the under side of said baffle arm at an intermediate point along the length of said arm, a low alarm nozzle mounted on said base to direct an air jet to the under side of said baffle arm at a point beyond said capsule diaphragm from said high alarm nozzle, a set pressure bellows mounted on said base and in force engagement with the under side of the other end of said baffle arm; a support top plate, a measurement bellows mounted dependingly from said top plate and in force engagement with the top of said baffle arm and in opposition to said set bellows, and a spring adjustably mounted effectively on said top plate and against the top of said baffle arm in opposition to said low alarm nozzle and another spring in like mounting in opposition to said high alarm nozzle; an air supply connection to said high alarm nozzle through one pair of restrictors in series, an air supply connection to said low alarm nozzle through another pair of restrictors in series, a signal output connection from each of said air supply connections from a point between said restrictors and tHeir respective nozzles; a signal inverter for said high alarm nozzle, comprising a housing, a diaphragm dividing said housing into two chambers, an inverter nozzle into one of said chambers and restrictable by said diaphragm, a spring in the other of said chambers biasing said diaphragm restrictably against said inverter nozzle, an air supply connection to said nozzle chamber, an air connection between said spring chamber and said high alarm air supply at a point between the restrictors thereof; a like signal inverter for said low alarm nozzle, a common air connection to said capsule diaphragm, an air connection from each of said inverter nozzles through individual restrictors to said common connection, and a variable restrictor bleed to atmosphere from said common connection.