The invention relates to a pressure switch device, that is, a switch device which is operable in response to a predetermined pressure difference.

Such pressure switch devices conventionally comprise a diaphragm separating two chambers communicating with respective fluid pressures. Movement of the diaphragm in response to a predetermined difference between the two pressures effects switch operation. Typically such pressure switch devices function to open one or more switches when a pressure falls below a predetermined level, as a safety measure.

The present invention is concerned with a pressure switch device of this kind which affords improved operating facilities and characteristics.

The invention thus provides a pressure switch device of the kind described including "hold-on" means acting to oppose diaphragm movement until a predetermined pressure condition is reached. Such "hold-on" means provide a "snap" action of the diaphragm, so that the pressure forces acting on the diaphragm at the time of switch operation can be lower than would be possible in the absence of the magnet means. The "hold on" means is advantageously selectively adjustable, so as to permit adjustment of pressure difference at which the diaphragm moves. The "hold-on" means conveniently operates magnetically, and can thus be constituted by selectively movable magnet means, conveniently in the form of one or more permanent magnets engaged by the diaphragm when in its switch operating position. A return spring can be provided to assist return of the diaphragm from this operating position.

The invention also provides a pressure switch device of the kind described in which means is provided to hold the diaphragm in its inoperative position until such time as it is desired that the diaphragm may be movable to the operating

position if the predetermined pressure differential exists. Otherwise expressed, the invention provides a pressure switch device of the kind described including means by which pressure responsive switch operation can be inhibited until the device is selectively put into operation. The diaphragm may thus be held in the inoperative position by a releasable mechanical latch but preferably magnet means ("hold-off" magnet means) is employed, conveniently in the form of a plurality of permanent magnets engageable around the periphery of the diaphragm, together with a reset device, conveniently located on the same side of the diaphragm as the magnet means, and operable electrically, mechanically, pneumatically or hydraulically to free the diaphragm from the magnet means so as to be capable of responding to the pressure difference.

The invention also provides a pressure switch device of the kind described including means limiting the diaphragm movement in the switch operating direction, to avoid excess load on the mechanism of the switch means. The stop means can conveniently be constituted by a portion of the mounting means for the switch means.

The invention can thus be constituted as a pressure switch device in which a diaphragm divides the interior of a casing into a switch chamber containing one or more electric, hydraulic or pneumatic switches positioned so as to be switched on and/or off by movement of the diaphragm under a pressure difference across it, and a reset chamber containing magnet means normally holding the diaphragm in an "off" or inoperative position, together with means selectively operable to separate the diaphragm from the magnet means.

The invention is described below, by way of example, with reference to the single figure of the accompanying drawing, which is a sectional side view of a pressure switch device in accordance with the invention.

The illustrated pressure switch device comprises a casing 1 having a tubular side wall 2 of any appropriate cross-section extending between opposed end walls 4 and 5 . The side wall 2 is in two parts connected together so as to clamp between them the outer edge of a diaphragm member 9 of a diaphragm assembly 10 dividing the casing interior into a reset chamber 11 and a switch chamber 12 . Apertures 15 and 16 located in the side wall 2 permit communication of the reset and switch chambers with sources of fluids pressures Pi and P2 respectively.

The diaphragm assembly 10 comprises a reset plate 17 in the reset chamber 11 and a pressure plate 18 in the switch chamber 12 , both plates being of magnetic material . The resilient diaphragm member 9 is received between them so as to have an annular portion between their outer edges and the casing side wall 2. The diaphragm member 9 and the reset and pressure plates 17 , 18 are clamped together by suitable fastening means such as the illustrated fasteners 19 , by which a seal is effected.

Within the switch chamber 12 , switch means , here comprising two switches 20 and 21 are received. The switches shown are electrical microswitches each operated by a plunger projecting towards the pressure plate 18 . The switch means could instead comprise any number or combination of hydraulic and/or pneumatic and/or electric switches of any kind . The switches 20 and 2 1 are mounted by a generally U-shaped bracket 25 having a base secured, by any suitable means ( not shown ) , centrally to the end wall 5 and parallel side limbs 26 extending axially , to the outer s ides of which the switches are secured by means 27 ( shown schematically) , so as to extend axially of the casing with the operating plungers adjacent the pressure plate 18. The bracket could be tubular so as to accommodate more of the switches 20 , 21. A cable 29 extends outwardly of the casing 1 from each switch through an

aperture in the end wall 5, the cable being sealed through the wall by any suitable seal means, for example, the cable seal 30 shown in the drawing for the upper cable only.

Along the axis of the casing 1, a screw-threaded stem 31 extends through a tapped hole in the end wall 5. At its end within the switch chamber 12, the stem 31 mounts "hold-on" magnet means in the form of a permanent magnet 32 at the region of the axially inner ends of the switches 20 and 21, so as to be engageable by the pressure plate 18. The end of the stem 31 outside the casing is provided with a head 34 by which the stem can be turned so as to adjust the axial position of the magnet 32. The magnet 32 can be locked in a selected position by means of a locknut 35 on the stem engageable against the outer face of the end wall. Optionally, a diaphragm assembly return spring can be provided, for example, as shown, in the form of a helical compression spring 36 around the magnet 32 and the stem 31 engages the pressure plate 18 at one end and the base of the switch mounting bracket 25 at the other.

Within the reset casing chamber 11, "hold-off" magnet means engageable by the reset plate 17 is provided, here shown in the form of two magnets 40,41 mounted by posts 42 on the end wall 4 so as to present pole faces to radially outer regions of the reset plate 17.

Between the magnets 40,41, the end wall 4 supports a reset device 45 selectively operable to act on the diaphragm assembly 10, by means of a plunger portion 46 which can be advanced along the axis. The reset device 45 can be of any kind appropriate to the context in which the pressure switch is used, thus it can operate mechanically, pneumatically or hydraulically or electrically.

In the position of the pressure switch device shown, the diaphragm assembly 10 is held so that the pressure plate 18 is spaced from the plungers of the switches 20,21 by

engagement of the reset plate 17 with the magnets 40,41.

Actuation of the reset device 45 causes the plunger portion 46 to move the diaphragm assembly 10 towards the magnet 32for long enough for the assembly to separate itself from the magnets 40,41. If the pressure PI within the reset chamber 11 sufficiently exceeds the pressure P2 within the switch chamber 12, the diaphragm assembly 10, freed from the magnets 40,41, will move further to the right to operate the switches 20,21 by engagement of the switch plungers by the pressure plate 18. This movement is resisted by the biassing of the switch plungers themselves and by the return spring 36. The reset device can be arranged to act additionally as a manual override, that is, to hold the diaphragm assembly 10 against the magnet 32 and switches 20,21 regardless of the pressure difference experienced by the assembly.

If, when the reset device 45 is actuated, there is insufficient pressure difference to effect the diaphragm movement needed to operate the switches 20,21, the diaphragm assembly will return to the illustrated position directly, under the influence of the return spring 36 and the magnets 40,41.

The magnet 32 co-operates with the pressure plate 17 to provide a "snap" action, so that the forces acting on the diaphragm assembly 10 at the time of switch operation can be lower than is normally possible with a conventional pressure switch device, with only spring action effective on the diaphragm assembly. The switch operating movement of the assembly is limited by the free ends of the side limbs 26 of the bracket 25, which are engaged by the pressure plate 18 before any excess load can be applied to the switch plungers.

When the pressure differential across the diaphragm assembly 10 reduces or is reversed, the diaphragm assembly will move to the left (as shown), assisted by the return spring 36 if fitted, against the pull of the "hold on" magnet

32. Engagement of the reset plate 17 with the magnets 40,41 follows, so that the position shown in the figure has been restored.

The invention can of course be embodied in a variety of ways other than as specifically described and illustrated. For example, the number and relative locations of the "hold off" magnet means and reset device in the reset chamber can be selected according to requirements, as can be the number and location of the switch means and "hold on" magnet means in the switch chamber. The pressure fluid ports can be located elsewhere than in the side wall of the casing if preferred.