EP1145266A1

EP1145266A1 - Pressure switch device

Info

Publication number: EP1145266A1
Application number: EP00906206A
Authority: EP
Inventors: Simon Jon May; Robert John Black
Original assignee: Ranco Controls Ltd
Current assignee: Ranco Controls Ltd
Priority date: 1999-01-21
Filing date: 2000-01-18
Publication date: 2001-10-17
Also published as: WO2000044017A1; GB9901327D0; AU2797300A

Abstract

A pressure switch device (1) is disclosed in which an electrical switch (3) is operated when a fluid pressure (P) in a working pressure chamber (34) acting on an operating diaphragm (9) exceeds a limit value. A pivotally mounted T-shaped switch actuator (4) has its main limb extending generally along the longitudinal axis of the switch device when no fluid pressure is received in the working pressure chamber. A compression spring (5), when adjusted, acts against the indicator (4), thereby changing the resistance force acting against the fluid pressure (P) and therefore providing adjustment for the pressure limit value at which the pressure switch device responds. The switch (3) and the compression spring (5), are arranged on opposite sides of the T main limb, thereby providing a compact arrangement for the switch device. The main limb of the T-actuator includes a cantilevered actuating arm (40) which can be laterally deflected by a setting screw (52), for factory adjusting the position of an actuating part or operating tongue (22) of the switch (3) when no fluid pressure is admitted to the working pressure chamber (34).

Description

PRESSURE SWITCH DEVICE

This invention relates to a pressure switch device, or pressostat, which is designed to operate in response to an operating pressure of a gas or liquid m a working pressure chamber of the pressure switch exceeding a limit pressure value. Such a switch may be used in a refrigerant circuit, for example, for safety reasons, such as for shutting off the electrical circuitry of the refrigeration unit and/or providing a visual or aural warning.

Such a device is known from US-A-4 821 626, belonging to Ranco Incorporated of Delaware. In this device, an operating pressure chamber is defined by a housing and an operating diaphragm (membrane or the like) . The diaphragm includes a pressure surface acting on an actuator, which is arranged to operate an electrical switch of the pressure switch αevice, there being a spring arrangement counteracting the fluid pressure m the working pressure chamber. When the amount of this pressure exceeds a limit pressure value, the displacement of the pressure surface of the operating diaphragm caused by distortion of the diaphragm under pressure causes the actuator to operate the electrical switch.

The spring arrangement can comprise a single tension spring which is arranged coaxially with the principal axis of the pressure switch device inside a passageway forming an axial extension of the working pressure chamber and which acts between the operating diapnragm and a screw adjusted end fixing, whose adjustment screwhead is seated in, and accessible through, a pressure inlet end portion of the passageway. As disclosed n US-A-4 821 626 the spring arrangement additionally comprise a second spring comprising a spider or disc spring, which is arranged outside the working pressure chamber and is arranged for counteracting the pressure in that pressure chamber. There is also a protective or safety diaphragm (membrane or the like) having a pressure surface for acting on the actuator. The pressure surfaces of the two diaphragms are interconnected via an intermediate member, which is firmly connected with one of the pressure surfaces. With this arrangement, if a fracture of the operating diaphragm takes place, the pressure switch device will switch-off at a substantially lower limit pressure limit value than it would do normally.

The pressure switch device according to US-A-4 821 626 is of simple construction and effective in operation. However, it does suffer from the following drawbacks.

Firstly, the tension spring, operating diaphragm, intermediate member, safety diaphragm, spider spring, actuator, and electrical switch are arranged in axial succession within the pressure switch device. Therefore, it is difficult to make the overall axial dimension of the pressure switch device small. Furthermore, there exists unutilized space between the actuator and the surrounding housing of the pressure switch device, whereas there exists a need for this switch device to be as compact in construction as possible.

Secondly, the positioning of the tension spring does not lead to a compact construction because it contributes to the overall length of the pressure switch device. An associated secondary problem is that the screwhead of the adjustment screw for setting the tension of the tension spring is accessible only through the inlet end of the passageway leading to the working pressure chamber. This means that although the screw setting can be set by the manufacturer, it cannot be adjusted by the customer or user without disconnecting the pressure switch device from the refrigeration circuit. It would be desirable for the screw to be accessible to the operator without the need for disconnection of the pressure switch device.

Thirdly, in order to get the pressure switch device to operate when the operating pressure in the working pressure chamber exceeds a predetermined pressure value, an end portion of the actuator acting against the electrical switch has to be ground away. This involves a trial-and-error process in which a small amount of material is ground away from a slightly overlength actuator, the pressure switch device is then tested to determine its operating pressure, and this process repeated until the switch operates when the specified pressure is reached. Such an adjustment process is time-consuming and costly to implement. If, inadvertently, too much material is ground away, the actuator has to be discarded and the whole process repeated on a fresh one.

With a view to solving the first problem, the present invention provides a pressure switch device comprising a casing defining a longitudinal axis of the pressure switch device, an operating diaphragm disposed within the casing transverse to said longitudinal axis and defining therewith a working pressure chamber for receiving a fluid pressure, an electrical switch disposed within the casing at a location laterally offset from said longitudinal axis, a spring whose action counteracts the fluid pressure in said working pressure chamber, and an actuator disposed within the casing and arranged to convert displacement of the operating diaphragm in the longitudinal direction into displacement laterally of said longitudinal axis, whereby the switch is caused co operate when a limit pressure valve is exceeded in said working pressure chamber.

It will be appreciated that the positioning of the switch at a location laterally offset from the longitudinal axis of the casing occupies space there that would otherwise be unutilised. In addition, the overall length dimension of the pressure switch device does not have to take account of the corresponding dimension of the switch, as is the case with the device according to US-A-4 , 821 , 626. Furthermore, the internal space alongside the switch m the region of the longitudinal axis can be utilised by the actuator. As a result, the pressure switch device can be made very compact.

In a preferred embodiment, the actuator is T-shaped and is positioned with its two arms extending generally transversely of said longitudinal axis and its main limb extending generally therealong when no fluid pressure is received in the working pressure chamber, the actuator being pivotally mounted at one end of one of its arms, the spring acting against the other arm and its main limb being arranged for acting against an operating part of said switch. Such a T- shaped actuator is a simple and effective component for carrying out its required functionality. In addition, its configuration enhances the compact construction because the switch can be arranged alongside the limb of the T.

Preferably, the spring is a coiled compression spring extending alongside the ma n limb of the T-shaped actuator on the opposite side of the main limb to said switch. As in the case of the switch, the coiled compression spring thereby occupies a space that otherwise would not be filled. Preferably, means such as a setting screw are provided for setting the magnitude of the spring force provided by said spring.

In a preferred arrangement, the main limb of the T-shaped actuator comprises two spaced-apart bar-like members connected by a bridge and a cantilevered actuating arm positioned between the bar-like members and extending from the junction between the main limb and the two arms, the cantilevered arm pressing against an actuating part of the switch, there being a setting screw m said bridge which can be set to deflect the cantilevered arm for setting the position of the actuating part when no fluid pressure is received in the working pressure chamber. This provides an especially simple arrangement for factory-setting the switch actuation point. The head of the setting screw can be arranged to be accessible from outside said casing through an aperture in an end wall thereof, and a cover is fitted on said casing over said aperture to prevent future adjustment by the user following factory setting.

Having regard to the second problem discussed above, the present invention also provides a pressure switch device comprising a casing defining a longitudinal axis of the pressure switch device, an operating diaphragm disposed within tne casing transverse to the said longitudinal axis and defining therewith a working pressure chamber for receiving a fluid pressure, a spring whose action counteracts the fluid pressure in said working pressure chamber, said spring being arranged at a location within the casing laterally offset from said longitudinal axis, an electrical switch disposed within the casing and arranged to be operated wnen a limit pressure value is exceeded m said working pressure chamber, and an actuator for applying the spring action to the diaphragm.

In view of the location of the spring, the pressure switch device can have a more compact construction with reduced overall length as compared with the device disclosed in US-A- 4,821, 626.

Preferably said actuator is pivotally mounted within the casing and said spring is a coiled compression spring extending in said longitudinal direction and acting against the actuator. The use of a pivotally mounted actuator is an effective way to apply to the counteracting force of the spring. Additionally, the orientation of the compression spring contributes to the compactness.

Conveniently, a screw adjustment means accessible from, outside an end wall of the casing is provided for adjusting the spring force of the compression spring for setting said limit pressure value, so that the adjustment can be made not only by the manufacturer but also by the user. The head of a setting screw of the screw adjustment means can be made to be accessible through aligned openings respectively in said end wall and in a cover fitted on the casing over said opening.

With a view to addressing the third problem, the invention provides a pressure switch device comprising a casing defining a longitudinal axis of the pressure switch device, an operating diaphragm disposed within the casing transverse to said longitudinal axis and defining therewith a working pressure chamber for receiving a fluid pressure, an electrical switch disposed within the casing, a spring whose action counteracts the fluid pressure in said working pressure chamber, and an actuator disposed within the casing and arranged to cause displacement of the operating diaphragm in the longitudinal direction to operate the switch, whereby the switch is caused to operate when a limit pressure value is exceeded in said working pressure chamber, said actuator being moveable under the action of the operating diaphragm and having a cantilevered actuating arm thereon, arranged for acting against an actuating part of the electrical switch, and setting means for acting against said actuating arm to produce a deflection thereof, whereby to set the position of the actuating part when no fluid pressure is received in the working pressure chamber, said setting means being accessible from the casing exterior. It is much easier to set the limit pressure value using this arrangement than the trial-and-error grinding process disclosed m US-A-4 , 821 , 626.

Preferably, the actuator is T-shaped and is positioned with its two arms extending generally transversely of said longitudinal axis and its main limb extending generally therealong when no fluid pressure is received m the working pressure chamber, said actuator being pivotally mounted at one end of one of its arms, the spring acting against the other arm and its main limb including said actuating arm, the latter extending from the junction between the main limb and the two arms, said setting means being a setting screw in said mam limb which can be set to press against an inclined surface on the actuating arm.

Suitably, the axis of the setting screw is aligned with said longitudinal axis and the head of the screw is accessible from outside said casing through an aperture m an end wall thereof, a cover being fitted on said casing over said aperture .

In one arrangement, said electrical switch has a bistable snap-action contact assembly having a first stable position and, when said limit pressure value is exceeded m said working pressure chamber, a second position, and as the fluid pressure in the working pressure chamber falls back below said pressure value, said contact assembly snaps back into its first stable position. This arrangement is suitable where automatic resetting is required when the received fluid pressure value reduces again.

In another arrangement, said electrical switch has a bistable snap-action contact assembly having a first stable position and, when said limit pressure value is exceeded m said working pressure chamber, it undergoes a one-off operation into a second stable position, and wherein a manual reset button on the casing is arranged, when depressed, for causing the bistable contact assembly to switch back to its first stable position, said manual reset button being a one-piece elastomeric component and having a plunger for pressing against said operating part of said switch. This arrangement is suitable for use where the switch is not permitted to be reset without user intervention, for safety reasons.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which :

Figure 1 is a longitudinal sectional view through one pressure switch device, forming a first embodiment in accordance with the invention;

Figure 2 is an exploded perspective view;

Figure 3 is a perspective view seen generally from the opposite side, with the body of the pressure switch device shown partially cut away;

Figure 4 is a perspective view similar to that of Figure 3 but seen from a lower viewing point;

Figure 5 is a perspective view, on an enlarged scale, of a bistable contact assembly with operating tongue;

Figure 6 is a perspective view, on an enlarged scale, of a switch actuator; and

Figure 7 is a longitudinal sectional view corresponding to chat of Figure 1, showing a second embodiment. Referring to the Figures in general and Figures 1 to 4 in particular, a pressure switch device 1 is shown having a generally cylindrical body 2, conveniently a plastic moulding inside which is mounted a two-way electrical switch 3, a switch actuator 4 and a compression spring 5, and an end assembly 6 fitted onto the cylindrical body 2 and comprising an end fitting 7, a metal retaining or clamping ring 27, a metal end housing 8, an operating diaphragm 9, a spider or disc spring 10 and a rigid end plate 11. The body 2 and end housing 8 together form a mam casing of the pressure switch device .

With particular reference to Figures 2 and 5, the electrical switch 3 comprises a first electrical contact arm 12, onto an angle portion 12a of which is secured a bistable contact assembly 13 having an electrical contact 20, a terminal portion 12b of the contact arm 12 extending through, and fitting tightly within, a slot 14 in the end wall 15 of the cylindrical body 2. The free end of the terminal portion 12b projecting from the end wall 15 serves for connection to an external lead (not shown) .

Similarly, electrical switch 3 further comprises second and third electrical contact arms 16, 17 having respective fixed electrical contacts 18, 19, on respective angle portions 16a, 17a, the contact arms being mounted inside the cylindrical body 2 with terminal portions 16b, 17b respectively projecting through slots 58, 59 in end wall 15.

As best shown in Figure 5, the bistable contact assembly comprises a piece of resilient stamped metal 21, which is substantially U-shaped, with an operating tongue 22 extending from the base of the U between the arms 26 of the U and with a mounting plate projecting 23 from the base of the U in the opposite direction to the operating tongue. The mounting plate 23 is secured to the angle portion 12a of the electrical contact arm 12 in any suitable manner, such as by rivets 24. The free ends of the arms of the U have inwardly projecting portions 26a which are drawn together, held by a connecting bridge 61 and secured, for example by means of a rivet serving as electrical contact 20, to cause the arms 26 of the U to flex out of the plane of the operating tongue 22 and mounting plate 23. This position of the arms 26 represents one stable position of the bistable contact assembly in which, as shown in Figure 1, the contact 20 is in electrical contact with contact 19 of electrical contact arm 17. In this way the external electrical lead connected to terminal portion 12b is in electrical connection with the lead connection to electrical terminal portion 17b, the resilience of the arms 26 maintaining contact pressure between the electrical contacts 19, 20. In the other stable position of the bistable contact assembly 13, the arms are flexed out of the plane of the operating tongue 22 and mounting plate 23, in the opposite sense to that corresponding the first stable position. In this other stable position, the contact 20 of the bistable contact assembly 13 is held pressed against the contact 18 of electrical contact arm 16, so that the external lead connected to terminal portion 12b is in electrical connection with terminal portion 16b. An adjustment screw 51 in the cylindrical wall of body 2 pressing against the electrical contact arm 17 can be set to adjust the position of the fixed electrical contact 19.

With reference to Figure 1, the retaining ring 27 has an annular end face 27a and a rim 27b to enable the end assembly 6 briefly described above to be secured to the cylindrical body 2 by deforming the edge portion of the rim 27b inwardly, so that its inwardly deformed portion 27c engages behind three lugs 28 arranged at 120° positions about the adjacent end of the cylindrical body 2. The fluid pressure P to be monitored is admitted through a central passageway 29 in the end fitting 7, thus fitting being internally screwthreaded at

30 to receive an extended screwthreaded connection of a conduit for carrying the pressure fluid. The operating diaphragm 9 is seated against, and secured to, the inner end face of end casing 8 and has an actuating pm 31 projecting axially against switch actuator 4. Figures 1 and 2 show a modified arrangement (described below) m which actuating p

31 projects from a safety diaphragm 49, against which the operating diaphragm 9 acts. The diaphragm 9 and end housing 8 together define a working pressure chamber 34 open to receive the fluid pressure P. The spider spring 10, which is m the form of an outer annular portion 54 with a plurality of generally radially inwardly projecting legs 35 (Figure 2) , is sandwiched between end plate 11 and a collar 36 at the base of actuating pm 31, the pm projecting through the center of the spider spring and an opening 37 m the center of end plate 11. The legs 35 are inclined slightly towards the collar 36 so as to apply a force on the operating diaphragm 9 acting m the opposite direction to the fluid pressure P. By cutting off one or more of the legs 35, the spider spring pressure can be adjusted.

The actuator 4 is T-shaped and is formed with a pivot pm 32 at the end of one arm 46 of the T. This pivot pm is pivotally mounted m an actuator support 33 formed on an internal integral part of the cylindrical body 2, and held captive m support 33 by end plate 11, which is held against the annular end surface provided by the cylindrical wall of body 2.

As best shown in Figures 1 and 4, the compression spring 5, which is held at one end in an adjustable fixing 44, applies a spring force, via an end piece 45 at its other end, against the other arm 47 of the T, so as to apply a force against the operating diaphragm 9 in the opposite direction to the gas pressure P. A setting screw 48 serves for adjustment of the spring force of compression spring 5 and therefore, the said limit pressure value. Preferably the setting screwhead is accessible not only to the manufacturer but also to the customer, through aligned openings 43, 62, respectively in the end wall 15 of the cylindrical body 2 and in an end cover 60 located in a channel in the end wall 15. The opening 63 can be covered by a removable seal 64 which prevents ingress of dirt but can be removed to provide access when adjustment of the screw adjustment means is required.

The main limb of the T comprises a pair of spaced apart barlike portions 38, connected by a bridge position 39. Extending from the junction with the arms of the T between the bar-like portions is a cantilevered actuating arm 40, which is formed with an inclined surface 40a at its end and terminates at a short distance from the bridge portion 39 (see Figure 1) . A protruberance 41 on the actuating arm 40 is set, as described below, to press against a bump or similar deformation 42 formed in the operating tongue 22, when there is no fluid pressure in the working pressure chamber .

Optionally (see Figures 1 and 2), a safety diaphragm 49 is secured at its outer periphery between end housing 8 and spider spring 10 and is held at a given spacing from operating diaphragm 9 by an intermediate element 50, fixed to one or the other diaphragm, suitably to the operating diaphragm 9. The actuating pin 31 would then be mounted on the safety diaphragm 49. The function of the safety diaphragm will be explained below.

Operation is as follows. Initially when no fluid pressure is received in working pressure chamber 34, the two-way switch 3 is in a normal operating position shown in Figure 1 with electrical connection established between contacts 20,19. The fluid pressure P acting in the working pressure chamber 34 will cause the operating diaphragm 9 to deflect m the axial direction against the actions of the compression and spider springs 5,10 by an amount dependent on the magnitude of the fluid pressure. The resulting axial movement of actuating pm 31 causes the actuator 4 to pivot about the pivot axis of pm 32, thereby applying protruberance 41 against bump 42 and resiliently deflecting operating tongue 22. As the tongue passes between and beyond the two arms 26 of the bistable contact assembly 13, the until-present stable position of the contact assembly 13 becomes unstable and the contact assembly 13 snaps back, bringing contact 20 into electrical connection with contact 18.

In the present arrangement, the pressure switch device functions with one-off operation, m that when the received pressure P falls back to zero, allowing the operating tongue 22 to return to the position shown m Figure 1, the latter is unable to cause the contact assembly 13 to become unstable again, and return to its first stable position m which contact 20 is again in contact with contact 19. Therefore, for resetting the pressure switch device, a manual reset button 55, formed as a one-piece elastomeric component, is mounted the cylindrical wall of the cylindrical body 2. On depressing the manual reset button, a slideably mounted plunger 50 acts against operating tongue 22, causing the contact assembly 13 to snap back to provide electrical connection between contacts 19,20.

It will be seen from Figure 1 that compression spring 5 is mounted laterally of the mam limb of the T-shaped actuator 4, thereoy utilising available space inside the cylindrical body 2 instead of increasing the axial length of the pressure switch device, as occurs as a result of the positioning of the tension spring of the device according to US-A-4821626. In addition, the compression force of spring 5 can easily be set by the user by inserting a screwdriver through opening 43 into the head of setting screw 48.

As can also be seen in Figure 1, the two-way switch 3 is located within cylindrical body 2 in laterally offset position at the opposite side of the actuator main limb to compression spring 5. Again, this results in a compact construction for the pressure switch device and minimises its axial length.

It will be appreciated that it is critical that the deformation 42 be in a precise predetermined position when the electrical switch 3 is in its operating state shown in Figure 1, in which no fluid pressure is admitted to the working pressure chamber. However, owing to slight variations in component dimensions resulting from manufacturing tolerances, the actual position of the deformation 42 can differ slightly from the predetermined position. For adjusting this position of the deformation 42, a setting screw 52 mounted in the bridge portion and acting against inclined surface 40a can be turned to deflect the end of the actuating arm 40 towards deformation 42 on operating tongue 22. This adjustment is intended to be a factory setting only. Access to the head of screw 52 for the manufacturer is provided by an aperture 53 in the cylindrical body end wall. This aperture is closed by end cover 60 in the end wall 15, preventing the user from changing the setting of screw 52.

A second embodiment is shown in Figure 7, having automatic reset. The structure differs from that of the first embodiment only in two respects. Firstly, no reset button and associated plunger are provided. Secondly, the contact surface of contact 18 is positioned with a lesser spacing from contact 19 than in the first embodiment. When the pressure limit value of the pressure switch device is exceeded, the switch 3 operates in precisely the same manner as in the first embodiment, causing contact assembly 13 to snap back and move contact 20 out of electrical connection with contact 19 and into connection with contact 18. However, as the fluid pressure P declines allowing the operating diaphragm 9 to return, the resulting return axial movement of the actuating pin 31 causes the actuator 4 to pivot about pin 32, thereby allowing protuberance 41 in contact with bump 42 to move back, whereby tongue 22 returns automatically. Again, as the tongue passes between arms 26 of the contact assembly 13, the assembly 13 becomes unstable again and accordingly snaps back, bringing the contact 20 back into electrical connection with contact 19.

In the event of the operating diaphragm 9 rupturing, the fluid pressure P acts on the safety diaphragm 7 which, because it has a larger pressure area than operating diaphragm 9, is deflected to a greater extent for any given applied fluid pressure than diaphragm 9 would have been. Consequently, the switch 3 switches at a lower applied pressure when the diaphragm has ruptured. In this way, safe operation is ensured.

In the disclosed embodiment, electrical switch 3 is a two-way switch. Alternatively, it can be an on-off switch which is normally in its on state or off state and which changes to its off state or on state when the applied fluid pressure exceeds a predetermined value.

It is preferred, though not essential, that the pressure switch device have both a compression spring and a spider or disc spring. Suitably, the respective contributions to the total counteracting force produced by these two springs is approximately equal. However, it is possible instead to use only a single spring, such as the compression spring 5, and to dispense with the spider spring.

Claims

CLAIMS :

1. A pressure switch device comprising a casing defining a longitudinal axis of the pressure switch device, an operating diaphragm disposed within the casing transverse to said longitudinal axis and defining therewith a working pressure chamber for receiving a fluid pressure, an electrical switch disposed within the casing at a location laterally offset from said longitudinal axis, a spring whose action counteracts the fluid pressure in said working pressure chamber, and an actuator disposed within the casing and arranged to convert displacement of the operating diaphragm m the longitudinal direction into displacement laterally of said longitudinal axis, whereby the switch is caused to operate when a limit pressure value is exceeded said working pressure chamber.

2. A pressure switch device according to claim 1, wherein the actuator is T-shaped and is positioned with its two arms extending generally transversely of said longitudinal axis and its mam limb extending generally therealong when no fluid pressure is received in the working pressure chamber, the actuator being pivotally mounted at one end of one of its arms, the spring acting against the other arm and its ma limb being arranged for acting against an operating part of said switch.

3. A pressure switch device according to claim 2, wherein said spring is a coiled compression spring extending alongside the ma limb of the T-shaped actuator on tne opposite side of the mam limb to said switch.

4. A pressure switch device according to claim 2 or 3, wherein means are provided for setting the magnitude of the spring force provided by said spring.

5. A pressure switch device according to claim 2, 3 or 4, wherein said mam limb of the T-shaped actuator comprises two spaced-apart bar-like members connected by a bridge and a cantilevered actuating arm positioned between the bar-like members and extending from the junction between the mam limb and the two arms, the cantilevered arm pressing against an actuating part of the switch, there being a setting screw said bridge which can be set to deflect the cantilevered arm for setting the position of the actuating part when no fluid pressure is received m the working pressure chamber.

6. A pressure switch device according to claim 5, wherein the head of said setting screw is accessible from outside said casing through an aperture m an end wall thereof, and a cover is fitted on said casing over said aperture.

7. A pressure switch device comprising a casing defining a longitudinal axis of the pressure switch device, an operating diaphragm disposed withm the casing transverse to the said longitudinal axis and defining therewith a working pressure chamber for receiving a fluid pressure, a spring whose action counteracts the fluid pressure in said working pressure chamber, said spring being arranged at a location withm the casing laterally offset from said longitudinal axis, an electrical switch disposed withm the casing and arranged to be operated when a limit pressure value is exceeded in said working pressure chamber, and an actuator for applying the spring action to the diaphragm.

8. A pressure switch device according to claim 7, wherein said actuator is pivotally mounted withm the casing and said spring is a coiled compression spring extending in said longitudinal direction and acting against the actuator.

9. A pressure switch device according to claim 8, wherein a screw adjustment means accessible from outside an end wall of 18

the casing is provided for adjusting the spring force of the compression spring for setting said limit pressure value.

10. A pressure switch device according to claim 8, wherein the head of a setting screw of the screw adjustment means is accessible through aligned openings respectively in said end wall and in a cover fitted on the casing over said opening.

11. A pressure switch device comprising a casing defining a longitudinal axis of the pressure switch device, an operating diaphragm disposed within the casing transverse to said longitudinal axis and defining therewith a working pressure chamber for receiving a fluid pressure, an electrical switch disposed within the casing, a spring whose action counteracts the fluid pressure in said working pressure chamber, and an actuator disposed within the casing and arranged to cause displacement of the operating diaphragm in the longitudinal direction to operate the switch, whereby the switch is caused to operate when a limit pressure value is exceeded in said working pressure chamber, said actuator being moveable under the action of the operating diaphragm and having a cantilevered actuating arm thereon, arranged for acting against an actuating part of the electrical switch, and setting means for acting against said actuating arm to produce a deflection thereof, whereby to set the position of the actuating part when no fluid pressure is received in the working pressure chamber, said setting means being accessible from the casing exterior.

12. A pressure switch device according to claim 11, wherein said actuator is T-shaped and is positioned with its two arms extending generally transversely of said longitudinal axis and its main limb extending generally therealong when no fluid pressure is received in the working pressure chamber, said actuator being pivotally mounted at one end of one of its arms, the spring acting against the other arm and its main limb including said actuating arm, the latter extending from the junction between the main limb and the two arms, said setting means being a setting screw in said main limb which can be set to press against an inclined surface on the actuating arm.

13. A pressure switch device according to claim 12, wherein the axis of the setting screw is aligned with said longitudinal axis and the head of the screw is accessible from outside said casing through an aperture in an end wall thereof, a removable cover being fitted on said casing over said aperture.

14. A pressure switch according to any one of claims 2 to 5, wherein said electrical switch has a bistable snap-action contact assembly having a first stable position and, when said limit pressure value is exceeded in said working pressure chamber, a second position, and wherein, as the fluid pressure in the working pressure chamber falls back below said pressure value, said contact assembly snaps back into its first stable position.

15. A pressure switch according to any one of claims 1 to 13, wherein said electrical switch has a bistable snap-action contact assembly having a first stable position and, when said limit pressure value is exceeded in said working pressure chamber, it undergoes a one-off operation into a second stable position, and wherein a manual reset button on the casing is arranged, when depressed, for causing the bistable contact assembly to switch back to its first stable position, said manual reset button being a one-piece elastomeric component and having a plunger for pressing against said operating part of said switch.