This invention provides a bellows-operated device including a bellows having a fluid pre-s-sure connection to its interior and at one end contacting directly or indirectly a flexible diaphragm, and a closed housing surrounding said bellows and sealed by said diaphragm, the effective area of said diaphragm being larger than that of the bellows. The diaphragm may be connected to operate another device, ft.g. an electrical switch, in such a way that in normal operation increased pressure in the bellows places the switch in an 'off or safe condition. Should the bellows leak into said closed housing the increase .in pressure in the housing acts on the larger effective area of the diaphragm and tends to place said switch in said 'off or safe condition. , In a- preferred arrangement the closed housing is subjected to atmospheric internal pressure, so that should the diaphragm leak there is no appreciable effect on the pressure in the housing. • Two specific embodiments of the invention are shown in the accompanying drawings, in which:- Figure 1 is,a section through a irst embodiment of a bellows-operated switch in a normal operating position, Figure 2 is a section through the -switch of Figure 1 in a failed position, Figure 3 i-s a section through a switch arrangement of a second embodiment, and Figure is a section through a bellows-operated device for xise in the embodiment of Figure >.

Referring first to Figures 1 and 2, the switch as shown has a microswitch 11 the button of which is operated by one end of a pivotted lever 12. A coil spring 13 opposes upv/ard movement of the lever, which movement tends to open circuit the switch 11. A pre-ssure tapping 15 i-s connected to the interior of a metal bellows 16, which thereby expands and contracts as the sensed pressure varies. The bellows carries a pressure pad 17, which contacts a flexible diaphragm 18, e.g., a textile fabric reinforced synthetic rubber diaphragm or ^' a metal diaphragm. On the other side of the diaphragm is an operating member 19 comprising a chamfered disc part 20 which contacts the diaphragm and a button 21 which contacts a depression in the lever 12. Increasing pressure in bellows 16 moves the pressure pad and diaphragm upwardly which in turn pushes the operating member 19 upwardly and operates the switch 11 to open a circuit. The switch may therefore be used to maintain a pressure b-stween limits, the pressure being produced by means (not shown) powered through switch 11.

Pressure tapping 15 forms one end of a housing 23 surrounding the bellows 16. The other end of the housing is closed by the diaphragm 18, the edges of which are clamped onto the housing. The diameter of the diaphragm is approximately twice that of the bellows. A space 2k is thereby formed about the bellows and this space is subject to a pressure below atmospheric by evacuation. This reduced pressure tends to pull the diaphragm 18 down substantially into the position shown in figure 1*, the bellows moving the diaphragm upwardly to open the switch as described above, against the resistance of the diaphragm.

Should the bellows fail by beginning to leak, pressurized fluid will escape into space 2k. The degree of vacuum therein will therefore be reduced, the downward pull on the diaphragm will be reduced, until the diaphragm eventually moves up into the failed position shown in Figure 2. In this position the switch 11 is . permanently open circuit so that the apparatus to which it is connected cannot be operated. Should the diaphragm leak, air will leak into space 2k and again the diaphragm will move into the failed position.

The embodiment of Figures 3 and k is similar to that of Figures 1 and 2, but shows the construction in more detail. Figure k shows the bellows operator part of the device, which Figure 3 shows the electrical switch part, the bellows part fitting onto the underside 5 of the switch part.

Referring first to Figure k-. a metal bellows 30 has means 31 for attachment of a pre-ssure line so that the pressure is applied to the interior of the bellows. A pressure pad 3-2 is secured to the top of the bellows to move therewith and has a slightly rounded upper surface.

-10 ^' A flexible diaphragm 37 iβ moved'by said pressure pad so as to apply pressure to an operating button 33 secured to a push rod J>k. As shown in the drawing the bellows is mounted in a housing 35 which provides a T-section opening such that the effective diameter of the diaphragm is greater than that of the bellows, in this case of the order of 3 ∑ 1-

15 The housing 35 provides a space 36 around the bellows and on one side of the diaphragm and in this embodiment this space is not evacuated but normally subject to atmospheric pressure.

Since the interior of the bellows is subjected to greater than atmospheric pressure, any leakage of the bellows into space 36 increases

20 the pressure on the underside of the diaphragm. Since the effective area of the diaphragm is greater than that of the bellows, the net effect is that the diaphragm is urged upwardly in the failed condition.

When secured to the electrical switch part of Figure 3 _t the push rod contacts a cranked lever kO pivotted at kΛ which may be rotated

25 upwardly against spring k2. Lever --+0 moves an adjustable actuator kj> which contacts the end of a rod kk which moves a member **+5 carrying electrical contacts 46, k7. Movement of push rod J>k therefore makes and breaks the contacts. The point of making and breaking the contacts may be adjusted by adjusting the position of pivot point kΛ by manual

30 rotation of knob k8 which thereby effectively selects the pressure at which the switch contacts change over.

. While the invention has been described above as applied to a pressure-sensitive switch, it could equally well be applied to other devices, e.g. , pressure-operated valves. The pressure applied to the

35 interior of the bellows may as described be a direct bleed from a pressurized line or it may be related to a temperature sensed for instance by a phial positioned at a location of which the temperature is to be controlled.