This invention relates to profile change sensors and

is especially concerned with low energy profile change

sensors. By a "profile change sensor" is meant means for sensin

a change in the shape or relative position or both of at

least one surface, or a part thereof, of a body.

By a "low energy profile change sensor" is meant a

profile change sensor as above defined in which the energy dissipated in the sensor when in operation is such that the

danger of any inflammable material in the neighbourhood of the sensor being ignited as a result of the operation of the

sensor is small to non-existent. In such cases the inflamm¬ able material may comprise inflammable fluids (gases, liquid

or vapours) either on their own or mixed with solid part¬

icles. Inflammable mixtures of fluids and solids in the

neighbourhood of the body may also comprise inherently non¬

flammable fluids and inflammable solids or fluids and solids

both of which are inherently non-flammable but which togeth- er form an inflammable mixture.

The invention is particularly, but not exclusively, concerned with means for detecting the failure of bursting discs. At least one embodiment of the invention may also be

used for the detection of the creep of bursting discs and

of the lifting and/or chattering of pressure relief valves. Bursting discs are also known, inter alia, as "rupture discs"

"safety discs" and "frangible discs".

A known way of detecting the creep of a conventional

bursting disc, that is, one in which pressure is applied to

the concave surface of the dome of the disc, is to shine a

beam of light from one side to the other of the pipe in

which the bursting disc is located, the light being arranged

to fall on a slit in front of a photocell and the slit being

so positioned that the ribbon of light passing through it

just grazes or passes close to the topmost region of the

convex surface of the dome of the disc. If the disc creeps,

then the light beam will be progressively interrupted and

the signal from the photocell will change and this change

can be made to operate an alarm or indicator system.

It is not normally necessary or even practicable to

detect the creep of a "reverse"-type bursting disc, that is,

one in which pressure is applied to the convex surface of

the dome of the disc, because this is relatively small, but,

i order to be able to detect both the failure of a reverse

bursting disc and of a conventional bursting disc, it is necessary to arrange for the light beam to be interrupted by

the dome, so that upon failure of the disc, the photocell is

exposed to the light beam.

The disadvantages of arrangements of the type above___

described are that careful collimation of the beam is gener¬

ally required as well as accurate setting up of the slit,

especially for the purpose of creep detection. Further,

since only a proportion of the light energy in the beam will

pass through the slit, a relatively high intensity light

source will be required. This will normally mean that a good deal of energy is dissipated at the light source with

the consequent danger of inflammable material in the vicinit of the source being ignited, or the consequent need for special measures to be taken to prevent this. Again, prob¬

lems can arise as a result of the light being randomly

reflected inside the pipe, rather in the manner of an

integrating sphere, and then passing through the slit on to

the photocell and producing spurious signals. Still further,

it would be possible for a conventional disc to burst or

rupture in such a way that one or more flaps of dome mater¬ ial could obscure the slit and/or the light source and thus

prevent light from the source from reaching the photocell

so that the indicator or alarm system would not be triggered. Another disadvantage is that different arrangements are

required to detect on the one hand the creep of a convention¬ al disc and, on the other, the rupture or failure thereof.

Other methods of detecting dome creep and failure involve the use of strain gauges and of in erferometric

techniques. These two methods can be highly accurate and

are very suitable for use in the laboratory. They are now, however, well adapted for use in industrial installations.

As will readily be appreciated, these testing arrange-

ments can also be used in the detection of the lifting and

chattering of pressure relief valves.

The present invention aims to provide reliable means

for detecting, inter alia, changes in the condition of burst¬

ing discs such as their creep and/or failure and changes in

the condition of pressure relief valves such as their lifting

and/or chattering in which the disadvantages of conventional

arranges, such as those described above, are obviated.

According to the present invention, a profile change

sensor (as' herein defined) comprises a source of energy, at least one signal conveying member for establishing a signal path from the source of energy to a signal responsive member

with the signal conveying member or a further member assoc¬ iated therewith so disposed in relation to at least a part of a surface of a body that in the event of a change, greater

than a given magnitude and in a given direction of the posi- tion or the shape of the-said part of the body or both,

occurrence of the change results in the interruption or the establishment of a signal.

The body is preferably a bursting disc or a pressure relief valve. If desired:-

(a) at least a part of the or each signal-conveying-_

member may be made from a flexible material;

(b) at least a part of the or each signal-conveying

member may be bonded to or may be in contact with

or slightly displaced from, the surface of the

body or a part thereof;

(c) at least one signal-conveying member may be

suitable for conveying a low voltage electric

current;

(d) at least one signal-conveying member may be a

light conveying means, for example, a light guide;

(e) at least one signal-conveying member may be

suitable for conveying sonic energy.

Preferably, in a profile change sensor according to

the present invention, one or more light guides are employed.

The invention will now be described by way of example with reference to the accompanying drawings, in which:-

Figure 1 is a diagrammatic representation of a conven¬

tional bursting disc and an arrangement according to the invention showing light guides, a light source and a photo- sensitive device.

Figure 2 is a partially diagrammatic representation of a collar for holding the flange of a bursting disc, light guides and a portable test unit;

Figure 3 is a diagrammatic representation of an

arrangement according to the invention showing light guides

and a reverse bursting disc;

Figure 4 is a second arrangement according to the

invention in which light guides are located in a plane

slightly displaced from the dome of a conventional bursting

disc;

Figure 5 is a partial section of the arrangement of

Figure 4 showing the disposition of the light guides in

relation to the dome of the bursting disc; Figure 6 indicates diagrammatically and in section

the effect on the light guides of the failure, due to excess

pressure, of the bursting disc of the arrangement of Figure

4.

Figure 7 illustrates diagrammatically a third arrange-

ment according to the invention in which a further member comprising a scoop or umbrella is associated with light

guides;

Figure 8 is a diagrammatic representation of the

further member of Figure 7;

Figures 9 to 11 illustrate the effect on the further member of Figure 7 of a shock wave of fluid due, for example, to the failure of a bursting disc upstream from the member; Figure 12 illustrates a fourth arrangement according

to the invention in which a further member comprising a

bell-mouthed open-ended tube is associated with light guides;

Figure 13 illustrates a fifth arrangement according

to the invention in which the signal conveying member com¬

prises a loop of brittle light guide material located within

a pipe slightly downstream from a bursting disc;

Figure 14 is a detail of a modification of the loop of brittle light guide material illustrated in Figure 13, and

Figure 15 is a sixth arrangement according to the

invention in which a further member associated with light

guides comprises a hinged flap to which is secured a scoop or umbrella shaped member.

In Figure 1, light guides 1 and 2 are shown passing

through pressure seal 3 in the wall of a collar 4, which

constitutes the upper holder flange of a bursting disc assembly incorporating a conventional bursting disc. The

light guides rest on the dome, represented by the broken

line 5, of the bursting disc and at their opposite ends

enter a cavity 6 in the wall of the collar 4 via small holes

in a flexible diaphragm 7, secured across the entrance to the

cavity. Preferably those portions of the light guides which rest on the dome and pass through the flexible diaphragm 7 are circumferentially coated with a plastics material.

The cavity 6 is preferably substantially hemispherical

in form and the curved wall 8 is preferably provided with a

reflective coating. Preferably, the inner surface of the

flexible diaphragm is also made reflective. In use, the

light guides are pulled fairly taut across the dome of the

bursting disc and the arrangement is such that any light

passing down one light guide will emerge from the guide at

the cavity 6 end and be reflected from the wall 8. Part of

this light will enter the other light guide and be conveyed across the dome and through the seal 3.

In Figure 1, 9 represents a light-emitting diode which

is connected to a source of electrical energy via the wires

11 and which feeds light into the end of the light guide 1.

Item 10, on the other hand, is a photo-responsive device . which receives and responds to light which has emerged from

the opposite end of light-guide 1, and then after reflection by the surface 8 of cavity 6 and the inner surface of dia¬

phragm 7 has entered the other light guide 2 and passed

thence along this light guide to its opposite end. When

light leaves this end and falls on the photo-responsive

device 10, this produces an output signal which may be conveyed along wires 12 to means which will respond to a change in this output signal. Such a change might, for

instance, be made to activate an alarm or indicating device. With the light guides in the positions shown in Figure 1, light produced by the LED 9 will impinge on the photo-responsive device 10 and may be made to hold off an

alarm or indicating device. If the dome 5 were to creep

as a result of the pressure represented by the arrows P

BUR

exerted on its concave surface, one or both of the light

guides 1, 2 will eventually be pulled out of the diaphragm 7

and/or one or both of them will be broken. Any of these

events will have the effect of interrupting the light path

from LED 9 to photo-responsive device 10 and thereby cutting

off the supply of light to the photo-responsive device 10

and, for example, causing an alarm or indicator to be activ¬

ated.

If the dome bursts, one or other of the light guides

will be pulled out of the diaphragm 7 and/or broken much

more rapidly than in the previous case but the result will be the same and an alarm or indicator, may, for example, be activated.

The collar 4, which is not to scale, forms one of a

pair of so-called "holder flanges" between which the flange

or clamping face 13 of the bursting disc ^" is clamped to form a bursting disc assembly. The two holder flanges are common ly held together and clamped on to the flange 13 of the

bursting disc by means of so-called "Allen","cap-head" or "socket" screws which pass through one holder flange into

the other in a direction substantially parallel to their common axis of symmetry.

A part-diagrammatic view of the collar 4 is also given

in Figure 2. Here the light guides 1 and 2 on emerging from

ile pressure sβ .J pass into a d p o unit 14. This is

designed to receive, inter alia, the mating portion 15 of

a hand held, battery-powered test unit 16. This may comprise

an LED 17 and a photo-sensitive device 18, each being connec-

ted via lightguides 19 and 20 to mating portion 15 of unit

16. In use, portion 13 is plugged into the mating cavity 20

of unit 14 so that the ends of light guides 1 and 2 are in

optical contact with the ends of light guides 19 and 20

respectively. LED 17 is then switched on and, if both of

the light guides 1 and 2 are intact with their ends passing

through diaphragm 7 as.shown, light from the LED 17 will pass

into the photo-responsive device via light guides 19, 1, 2

and 20 and the resulting output from the device 18 will be

registered by some suitable indicating instrument (not shown)

forming part of the unit 16. The indication of an output

from device 18 then shows the operator that the light guides

1 and 2 are undisturbed and the bursting disc (5 in Figure 1)

accordingly intact.

In place of the photo-responsive device 18 the end of

the light guide may be presented to an eye-piece in the wall

of unit 16. On viewing the end of the light guide through

the eye-piece the operator will see the end of the light

guide lit up if guides 1 and 2 are not disturbed sufficiently

to pull one or the other out of diaphragm 7 and the end of

the light guide dark if guides 1 and 2 are so disturbed and/

or are broken. .—TΓ _. c-

In place of the arrangement shown in Figure 2, the

lightguides 1, 2 on emerging from pressure seal 3 could be

extended to a central control point where the state of the

light guides 1, 2 within collar 4, and hence the condition

of ^' the bursting disc, could either be continuously monitored or checked regularly according to a preset programme and/or

checked at will by means of a device of the type indicated

at 16 in Figure 2.

In Figure 3 the flange or clamping face 13 of a revers bursting disc is shown clamped between collars or holding

flanges 4 and 21. A bursting disc of this type is, in use,

subjected to pressure on the convex surface of the dome 5 as

shown by the arrows P. When this pressure exceeds the desig pressure (or a value in the region thereof) the dome collap- ses inwardly on to a cruciform array of knives which cut the

material of the disc into four sector-shaped pieces and

thereby ensure an unhindered release of the pressure. Three of these knives are shown as items 22, 23 and 24 in Figure 3

In a a similar manner to flange 4 in Figures 1 and 2, flange 21 in Figure 3 is provided with a pressure seal 3

through which pass light guides 1 and 2 in a gas-tight manner. Within the collar 21 the guides 1, 2 pass, respect¬ ively, over the knife 24 and the knife (not shown) behind

it which forms the fourth arm of the cross, the light guide

2 where it passes behind knives 22 and 23 being shown

the broken line. At their ends opposite to seal 3, the light guides pass through the flexible diaphragm 7 which is

fitted across the opening of cavity 8. As before, the walls

of this cavity are reflective as is, preferably, the inner

surface of the diaphragm 7.

As previously indicated, when the pressure P applied

to the convex surface of the dome 5 exceeds the designed

bursting pressure of the bursting disc (or the value in

the neighbourhood thereof), the dome collapses inwardly on

to the cruciform array of knives indicated by items 22, 23

and 24 _. in Figure 3. When this happens, one or other of the

light guides 1 and 2 are pulled free of the diaphragm and/or are broken or cut on knife blade 24 and/or the fourth knife

blade (not shown) behind it so that the continuity of the

light path from point a, say, at adaptor box 14 via light guides 1 and 2 to point b is interrupted.

The bursting disc assembly shown in Figure 3 may be used in essentially the same way as described with reference

to Figure 2, adaptor box 14 being provided with the same

facilities as adaptor box 14 in the arrangements shown in Figure 2.

The pressure tight seal 3 in the arrangements shown in Figures 1, 2 and 3 may comprise a filling of plastics

material in a suitably shaped hole passing through the wall

of the holder flange 4 or 21, the two light guides

through the plastics material. Preferably, however, it is a

threaded plastics stopper which screws into a tapped hole in

the wall of the holder flange and which may be provided with

one or more 0-ring seals to ensure that the joint is gas-

5 tight. Such an arrangement has the advantage that a holder flange may easily be provided with a replacement unit

^' comprising an adaptor and light guides when a disc has

burst and the bursting disc assembly is being made.

It is by no means essential for two light guides to be

10 provided as shown in Figures 1, 2 and 3. Instead, only one

such guide may be used. In this case, the light guide would for example, pass through a holder flange wall via a seal such as 3 and then again pass through a flexible diaphragm

covering the entrance to a chamber approximately diametri-

5 cally opposed to the seal. The chamber in this case would

be smaller than before and would be so arranged that the end

of the light guide would come into optical contact with a further light guide passing through - .gas-tight seal in the remainder of the wall of the holder flange to suitable means 0 • for detecting the emission of light from the end of this said further light guide. - By **optical contact" both here

and elsewhere i this specification is meant that the ends

of the two light guides are so disposed in relation to each

other, with or without an interposed zone of some plastics

*-- • material, for example, of similar refractive index to the . /"BU

material of the light guides, that at least proportion of any

light emerging from the end of one light guide will enter the end of the other light guide. As before, the breaking

and/or displacement of the light guide results in the optical

path being interrupted and may provide an indication to the

operator that a bursting disc has crept or failed.

Yet again, in the arrangements shown in Figures 1, 2 and

3, the two light guides may be replaced by a single light

guide and pulse techniques used for determining the contin-

uity or otherwise of the optical path. By this is meant thsy

the light will be passed into the light guide in short pulses

and the reception ^' of the reflected light back along the light

guide detected by sophisticated measuring techniques.

As will be appreciated, bursting discs are normally

supplied in assemblies each comprising a disc and two holder flanges as shown in Figure 3. Such asseblies are normally

clamped in a gas-tight manner between pipe flanges and provide positive and predictable relief of pressure in a vessel or

pipe line when the pressure exceeds a predetermined value. Although the invention has been described above with reference to the monitoring of ,the condition of bursting discs by means of light guides, it is by no means so limited. For example, other signal-conveying means could be used and also,

both these and light guides could be used to provide a warn-

ing if, say, part of a pressure relieve valve should lift or

begin to chatter or if part of a machine tool or some other

equipment, such as an overhead crane, should move beyond a

predetermined position. Further, the invention could be used

as a burglar or intruder alarm to detect, for example, the

5 opening of a door or the movement of a fence.

Yet again, the invention could be used to determine

when fluid begins to flow along a pipe or when the rate of

flow of such a fluid along a pipe increases. This could be done, for example, by replacing part of the pipe wall with a

10 flexible diaphragm and using the methods of the invention to sense any disturbance, greater than a certain magnitude, of the diaphragm surface resulting from the fluid beginning to

flow along the pipe or from an increase in its rate of flow along the pipe. Still further, two signal conveying members

15 such as light guides might be stretched across the pipe in

accordance with the methods of the invention, with a "body"

comprising a web of some suitable material such as "FEP"

(fluorinated ethylene propylene) between them. Any flow of fluid along the pipe at a velocity higher than a certain

20 threshold value would then move the body so as to cause at

least one signal conveying member to be disconnected and the associated signal path to be interrupted.

In Figures 4, 5 and 6 similar parts to those of the

arrangements of Figures 1, 2 and 3 have the same reference

25. numbers. In the arrangement, of Figures 4, 5 and 6 however a ^■ "

single continuous light guide 25 passing in and out of the

arrangement through adaptor box 14 replaces the two separate light guides 1 and 2 of Figures 1, 2 and 3 and follows a path

defined by legs 26, 27, 28, 29 and 30. As seen from Figure 5,

the light guide 25 legs 26, 28 and 30 are stretched tautly and

substantially diametrically across a subsidiary annular hold¬

er 4A. The subsidiary holder flange 4A is, as shown, seated

on the holder flange 4 and, consequently, the legs 26, 28 and

30 of the light guide 25 are axially displaced a short dis-

tance away from the bursting disc 5. It will be seen from

Figure 4 that the legs 27 and 29 (shown dotted) follow the

curvature of the bore of subsidiary holder flange 4A and these legs may be secured to or embedded in the flange 4A as desired. Figure 4 also shows the arrangement mounted between

two flanged pipes 31 and 32.

The adaptor box 14 used in this arran ement may be

provided with the same facilities as the adaptor box 14 of

Figure 2. Alternatively, the adaptor 14 may be used to

correct the two ends of light guide 25 to a central control point where the s ^' tate of the limbs 26, 28 and 30 could be

either continuously monitored or checked regularly according to a pre-set programme as previously described. Figure 7

shows a part of the pipe 32 in which an umbrella-like member _.

33 is mounted of shock waves which would result from rupture

of the burstin

the umbrella member 33. The umbrella member 33 is carried

by an arm 34 which has an aperture 34A and which is slidably

mounted within a housing 35. The housing is, in turn, suppor

ed on the wall of pipe 32. The two light guides 36 (input)

537 (output) are located in the housing 35 as shown and, depen

ding upon the position of the arm 34, the aperture 34A will or will not be in alignment with arms 36A, 37A of the light

guides 36 and 37 respectively. When the arms 36A, 37A are in alignment' with the aperture 34A light may pass from arm

36A to 37A through the apertures ^' 34A (see Figure 8). In a

position of non-alignment, light cannot pass between the two

light guides 36 and 37.

Figures 7, 8 and 9 show the arrangement with the

umbrella member 33 in an "undisturbed" position so light can

pass from arm 36A to arm 37A unhindered. When the bursting disc 5 (not shown) is ruptured, a shock wave passes along the pipe 32 in the direction of the arrows 38 and impinges

on the umbrella-member 33 causing the same to pivot as shown in Figures 10 and 11. Such pivot movement of the umbrella member 33 causes the arm 34 to move to the right in Figure so that the aperture 34A is displaced out of alignment with the arms 36A, 37A. The result is that light is prevented from travelling from arm 36A into 37A and any light signal

previously transmitted between is interrupted indicating

pivotal displacement of the umbrella member 33 consequent

_

- 18 -

upon the rupture of the bursting disc 5. As in the previous

arrangements the interruption of the light signal can be made to actuate an alarm signal. Alternatively, the arrangement

may operate so that in the rest position light is prevented

from travelling from arm 36A to arm 37A since the aperture 34A is out of alignment with these two arms. Upon pivotal

displacement of the umbrella member 33, however, the aper¬

ture (elongated if necessary) is brought into alignment with

the said arms permitting light to pass therebetween.

Figure 12 is a modification of the arrangement of

Figures 7 to 11 in which the arm 34 is replaced by a tubular

member 39 with a bell-mouthed end 40 disposed towards the

bursting disc 5 (not shown). As piston-like plug 41 having

an aperture 42 is displaceable within the tube 39. In the

event of a shock wave represented by arrows 38 (upon rupture of a bursting disc 5) impinging on the bell-mouthed end 40

of the tube 39, the piston-like plug 41 is displaced to the left in the Figure and movement is restrained by a perforated or other stop 43. In this restrainer position, the aperture 42 is displaced out of alignment with arms 36A, 37A of the light guides 36 and 37 so that any signal between these arms

is interrupted. Alternatively, the aperture 42 may be brought into alignment with the said arms when the piston 41 is in

the displaced and restrained position thereby permitting a

signal to pass between arms 36A and 3 A.

In the arrangement of Figure 13, a single and contin¬

uous light guide is mounted in the housing 35 and extends int

the pipe 32 at 44. At least that part 44 of the light guide

is brittle so that impingement thereon by a shock wave 38 or

5 a portion of a ruptured bursting disc causes fracture thereof

and the interruption of a light signal along the guide 25.

Preferably, the arrangement of Figure 13 is turned through

90 so that the arms or part 44 are located in a plane at right angles to the plane of the Figure as shown in Figure 0 14. Figure 14 shows a modification of Figure 13 in which an

umbrella member 45 is secured to the arms 44. Alternatively

the umbrella member 45 may be replaced by one or more turns

of a strip of plastics or other material wound around the leg

of part 44.

5 Figure 15 shows yet a further arrangement according to the invention in which the housing 35 is generally cylindrica

having a tubular end 46 defining a cylindrical cavity 47. A

disc 48 is pivotally mounted to close the cavity 47 and on

one face thereof causes a concave reflecting body 48 and on

0 the other face an umbrella member 50. In the undisturbed

position of the disc 48, an annular array of input light ^• guides 36 conveys light to the reflecting body 49 from whence the light is focussed onto a central outlet light guide 37. The underlying principle of a displaceable reflecting

5 body such as the disc 48 of Figure 15 represents a feature of

the present invention and may be realised in a number of

different ways of which the arrangement of Figure 15 repre-

sents only one. For example, the arrangement of Figure 15 may be modified so that the right hand ends of the light

guides terminate in a plane which is inclined relatively to

the axis of the pipe 32. A flat reflecting body is disposed

in a plane parallel to the plane containing the ends of the

light guides so that it reflects input light signals into

one or more output light guides. A portion of the flat

reflecting body extends into the path of a shock wave repre¬

sented by arrows 38 so that upon the rupture of a bursting disc, the flat reflecting body is displaced so that signals are interrupted. This underlying principle is shown in Figure 16. The light guides referred to in all of the foregoing

may take the form of optical fibres. Alternatively, sonic

signals may be passed through or along sonic energy convey¬ ing members or, yet again, low voltage electrical signals may be passed along electrical conducting members.

Additional embodiments of a profile change sensor and

ancillary equipment in accordance with the present invention

are shown in Figures 17 _f 18, 19, 20 and 21. If desired the

profile change sensor with form of a bursting disc unit

shown in Figures 17 to 20 is a secondary unit and is dispersed

upstream of a primary or main bursting disc unit. In such an, _,

arrangement, the bursting disc of the secondary unit is

substantially flat. The bursting disc of the secondary unit is preferably made from a material weaker than that of the

main bursting disc and preferably ruptures at a pressure 5% lower than the diagonal rupturing pressure of the main burst¬

ing disc.

The profile change sensor, whether associated with a

primary bursting disc and or a secondary bursting disc unit

comprises a flexible strip 49 of material which permits the

passage therethrough of electromagnetic radiation within a

predetermined wavelength band. Preferably, the said strip is

disposed diametrically across the bursting disc (primary or

secondary) and is clamped at one end 50 (Figure 17) whilst the other end passed freely through holder 4 into a tube 51

and thence between apertures 52/53 so as to be interposed in a

beam of light passing from a light source 54 through the aper¬

tures 52/53 to a sensor (not shown). Thus the wave length

distribution of the electron magnetic energy reaching the sensor is changed. For example, the flexible strip 49 may be a strip of "Kapton" polyimide film which has a translucent amber colour.

Figure 18 shows the main bursting disc 5, the secondary bursting disc 5A and the flexible strip 49. When the main

disc 5 ruptures, the secondary disc 5A ruptures shortly

thereafter, the flexible strip 49 is pulled out of the path -of

the beam from the light source 54 and the light energy from

aperture 53 is no longer filtered by the polyimide strip 49

as indicated in Figure 19. This change in the light energy

impinging on the sensor (not shown) occurs upon the rupture

of the main and secondary bursting discs.

Figure 20 shows an arrangement of how the strip 44 may

be supported on a guide arm 55 and diametrically across a

carrier ring 56. When the carrier ring 56 and the secondary

disc 5A are clamped between conventional clamping collars, a handle 57 is removed by twisting which results in fracture

across cut-outs 58.

Figure 21, the sensor not shown in Figures 17-20 is dissipated generally at 57. In practice, light from a light

source 54 passes along flexible light guide 58 via tube 52,

strip 49, and emerges from tube 53 and thence passes along

light guide 59. Light from the source 54 also passes along

light guides 60 and 61. Light emerging from light guide 60

passes through an orange filter 62 in a panel 63 and thence

through a blue filter 64 so as to give a green light through aperture 65 on the panel 63. Light from aperture 65 is a

reference green light. Light from source 54 also passes along light guide 61 through filter 64 and blue light is

therefore visible in aperture 66. This is the blue reference light.

When the amber polyimide strip 49 is interposed on the

beam between the apertures 52, 53 amber light passes along

the light guide 59 and thence through blue filter 64 so that

a green light is visible through aperture 67.

If therefore follows, that the underdisturbed condition

5 of the pull-out strip 49 (and, also of the bursting disc with

which it is associated) as indicated by a green light (formed

from the light passing through the amber strip 49 and the blu

filter 64) is visible in aperture 67.

When the pull-out strip 44 is removed from the light

10 beam passing through apertures-52 and 53, the light passing

along light guide 59 then only passes through blue filter 64

and a blue light is visible in aperture 64. Blue light in aperture 67 corresponds to the disturbed condition of the

strip 49 and the burst condition of the disc with which it is

15 associated.

As indicated in Figure 21A, with the disc intact, green light is visible in apertures 65 and 67.

When the disc is burst, blue light is visible in aper¬

tures 66 and 67. As will be appreciated the green and blue 20 light visible in apertures 65 and 66 is provided to enable

the operator reading to identify the light visible in aper¬ ture 67 as green (disc intact) or blue (disc ruptured).

High efficiency, lower power fibre optic systems oper¬

ate in the infra red region with a wave length of about

x— > Jc cj 11 • iuuώ . -L v_ _- - cuiu bt. ___._ _.-__.-<_/._. _-i._c, μμj.jn,α _u .lύπ3 _■ Hi-. r __ £t__ ^m _^- — PΓT

is to be preferred to the visible light used with the fibre

optic system described with reference to Figure 21.

The Kapton film 49, may be made translucent to infra red radiation by sand blasting to produce a matt surface. Such a

modification makes electronic discrimination between the disc

intact and disc ruptured states relatively simple. A battery powered remote unit is indicated in Figure 22 and the assoc¬

iated circuitry in Figure 23.