GB2243220A - Measuring apparatus - Google Patents

Abstract

The separation of two surfaces (16 and 18) e.g. belonging to cylinder block (10) and head (12) of an i.c. engine is measured by determining changes in capacitance. The apparatus comprises an electrode (20), an elongated conductor (22) extending from the electrode, and an elongated shield (24, 26) at the same electrical potential as the electrode (20). The shield comprises two closely-spaced planar portions (24 and 26) with the conductor (22) therebetween. The electrode (20) forms a capacitor with another electrode provided by or movable with one of the surfaces (16 and 18). The electrode (20), the conductor (22), and the shield portions (24 and 26) are provided by layers of metal provided on electrically insulating substrates and secured together in a stack. The sensing end of the arrangement is accommodated in a hole and groove in the edge of cylinder head gasket (14), while the other end is connected to a coaxial cable (60). The arrangement is flexible and the dielectric (56) is resilient e.g. of silicone rubber. <IMAGE>

Description

MEASURING APPARATUS The present invention relates to an apparatus and method for measuring the separation of two surfaces by determining changes in related capacitance, the apparatus comprising at least one of two spaced electrodes of a capacitor, this electrode being movable with one of the two surfaces to be monitored, and the other electrode either being provided by, or being movable with the other surface. The apparatus also has a conductor extending from said one electrode to a point beyond said surfaces so that it can be connected to indicating and/or recording means associated with the apparatus.

It is not essential that the surfaces, the separation between which the apparatus in accordance with the present invention is to measure, should be parallel to each other, but usually these surfaces, and the electrodes of the capacitors, are parallel.

For convenience, the surfaces are referred to hereinafter as being parallel, but such statements are to be considered as referring also to appropriate arrangements of the surfaces which are not parallel.

It is required that when the measuring apparatus is operable, the capacitor is included within an electrical circuit, also having, in addition to the indicating, and/or recording means, at least a source of electrical energy. However, in this specification, any such additional means of the electrical circuit is not considered to be part of the measuring apparatus in accordance with the present invention.

The determining of changes of the capacitance related to the separation of two surfaces, in order to measure the separation of the two surfaces, is advantageous if the two surfaces are located in a hazardous environment, because the sensor provided is inherently strong and robust, and can resist high temperatures. However, each electrical signal generated within the electrical circuit, and representative of the instantaneous, inevitably small change of capacitance, inherently has a disadvantageously small voltage and/or current to be operated upon by the associated indicating and/or recording means. Consequently, it is required that such signals are not adversely affected by any electro-magnetic field generated between the electrode, or the conductor, and any adjacent surface maintained at zero potential, except the other electrode of the capacitor.Thus, the generation of any such field desirably is prevented.

In addition to being required to determine the separation of two surfaces in a hazardous environment, it may be required to determine the separation of the two closely spaced surfaces, for example, the opposing cylinder head, and cylinder block surfaces within an internal combustion engine. This implies that, if a change of the capacitance related to the separation of the two surfaces is to be determined, only a very small capacitor can be provided in the narrow space between the opposing surfaces and, for example, a capacitance in the range of 0.1 to 1.0 picofarad is to be determined.

Hence, in any such arra-ngement, it is particularly desirable to provide a shielded conductor extending from the capacitor to be beyond the opposing surfaces, the shield to be maintained at the same potential as the conductor and the electrode, so that electromagnetic fields, which otherwise would adversely affect the signals in the electrical circuit, are not generated. Such constraints on the construction of the measuring apparatus has implied that, previously, a capacitor has not been employed in an arrangement in which the separation of two closely spaced surfaces is to be measured.

Usually, in internal combustion engines, care is taken to ensure that the gasket is uniformly compressed by a required amount between the cylinder block and the cylinder head, and it is desirable to know the extent of the gasket compression. Thus, it is desirable to know the separation of the cylinder head from the cylinder block. Loading on the gasket is also important. At any point on the gasket, load and compression are related in a way capable of being determined empirically; and this relationship may vary with the age and the environmental history of the gasket. Hence, the separation of the cylinder head from the cylinder block is required to be measured statically. Initially, the measuring apparatus is calibrated, and then any difference between the instantaneous separation and a datum separation is determined.

Further, in the operation of the internal combustion engine, the pressure acting on the cylinder head may cause movement of the cylinder head in relation to the cylinder block, and this movement desirably is monitored. Thus, the separation of the cylinder head from the cylinder block is also required to be measured dynamically, the speed of the dynamic measuring operation primarily being determined by the measuring apparatus. In particular, any change in the separation of the cylinder head from the cylinder block is required to be measured statically, or dynamically, by determining any corresponding change in the capacitance related to the separation of the cylinder head from the cylinder block.

It is an object of the present invention to provide a novel and advantageous form of apparatus of the type referred to above, for use in measuring the separation of two parallel surfaces, especially of two closely spaced surfaces; and particularly, but not exclusively, when such surfaces are located in a hazardous environment; by determining any change of capacitance related to the opposing surfaces.

The invention provides apparatus for use in measuring the separation of two surfaces by determining changes in related capitance, the apparatus comprising at least one of two spaced electrodes of a capacitor, this electrode being movable with one of said surfaces and the other electrode being provided by or being movable with said other surface, the apparatus also comprising an elongated conductor extending from said one electrode to a point beyond said surfaces, and an elongated shield electrically insulated from the conductor and maintained at the same electrical potential as said one electrode, the shield comprising two closely spaced planar portions with the conductor therebetween, wherein said electrode, said conductor and said shield portions are provided by layers of electrically conducting material provided on electrically insulating substrates and the layers and substrates are secured together in a stack.

Preferably, in apparatus in accordance with the invention, the conductor and the electrode are provided by the same layer which is preferably of metal. This provides a more compact structure and avoids the necessity of creating a connection between the electrode and the conductor.

Two of the conducting layers may be provided on opposite faces of the same substrate, thereby increasing ease of assembly.

Preferably, one of the shield portions of the apparatus extends to shield one face of the electrode, thereby ensuring that only one face of the electrode acts as a capacitor.

Since it may be necessary to insulate the apparatus, at least one of the portions of the shield may have a layer of insulating material secured thereto on the side thereof remote from the conductor.

The substrates may be made of a polyimide so that circuit board material may be utilised.

For example, the stack may be made up of a doublesided substrate, with a metal layer on one side forming the conductor and electrode and a metal layer on the other side forming one of the shield portions, a layer of insulating material adhesively-secured to the side of the substrate having the shield portion thereon, a single-sided substrate adhesively-secured to the other side of the double-sided substrate with a metal layer on the single-sided substrate facing away from the double-sided substrate and forming the other shield portion, and an insulating layer adhesively-secured to the side of the single-sided substrate which has the metal layer thereon.

The apparatus, preferably, also comprises an operational amplifier comprising part of an integrator, and the conductor is, preferably, coupled to the inverting input of the amplifier, and the shield is coupled to the non-inverting input of the amplifier.

The apparatus may comprise a coaxial cable, the conductor being connected to the core of the coaxial cable and a continuous electromagnetic shield being provided between the sheath of the coaxial cable and the shield of the apparatus.

The invention also provides a method of measuring the separation of opposed surfaces to two members by determining changes in related capacitance, the members having a deformable layer of insulating material between their opposed surfaces, the method comprising mounting apparatus according to the invention so that the electrode of the apparatus resides in a groove in a face of the deformable layer and the conductor and shield portions protrude from the groove beyond the surfaces, biassing the electrode of the apparatus towards one of said members by means of a resilient dielectric material residing in a hole through the deformable layer, and determining the capacitance between the electrode of the apparatus and an electrode formed by or movable with the other of the members.

The present invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a partly diagrammatic sectional view of a measuring apparatus in accordance with the present invention, the measuring apparatus being used in combination with, and to measure dynamically any change in the separation of parallel opposing surfaces provided by a cylinder block and a cylinder head, with a gasket therebetween, a capacitor, which is partially provided by the measuring aparatus, being located in the gasket; Figure 2 is a longitudinal section of each of the constituent components of the measuring apparatus of Figure 1; Figure 3 is a plan view of each of the constituent components, in the direction of the arrow X shown in Figure 2, Figures 2 and 3 together partly indicating the construction, and the manner of fabrication, of the measuring apparatus;; Figure 4 is a detailed enlargement of the interconnection between the conductor and shield of the measuring apparatus, and an associated coaxial cable, the interconnection being shown in a more general way within the dotted line Y in Figure 1; and Figure 5 is a circuit diagram of part of the electrical circuit including the measuring apparatus.

Shown in Figure 1 is part of an internal combustion engine, and in particular, a part of the cylinder block 10, and a part of the cylinder head 12, with a gasket 14 of a conventional form clamped therebetween, the gasket being in the form of a deformable layer of material. The gasket 14 has a uniform thickness, and separates parallel, opposing surfaces 16 and 18, respectively, of the cylinder block 10 and of the cylinder head 12.

It is desirable, at least under test conditions, to monitor the behaviour of the gasket 14 possibly over an extended period of use. In particular, it is desired to measure variations in the separation of the surfaces 16 and 18, in a dynamic manner.

Such a measurement is required to be made in the hazardous environment associated with the internal combustion engine. Whilst various ways of making such a measurement are known, because of the hazardous environment, it is desirable to monitor the separation of the cylinder block 10 and the cylinder head 12 by measuring any changes of the capacitance of a capacitor provided between the surfaces 16 and 18, the separation of the two electrodes of the capacitor being related to the separation of the surfaces.

However, the cylinder block 10 and the cylinder head 12 are closely spaced, which implies that only a small capacitor can be housed in the gasket 14, and it is difficult to provide a required conductor extending from one electrode of the capacitor to be beyond the opposing surfaces 16 and 18. Further, the provision of the capacitor, and the conductor connected thereto, between the cylinder block and the cylinder head is required to be in a manner which does not significantly impair the performance of the gasket. In addition, also because of the small separation between the cylinder block and the cylinder head, the capacitance of any capacitor provided therebetween is inevitably very small.Thus, any change in a signal generated in an electrical circuit ineluding the capacitor, and representative of an instantaneous change in the cpacitance of the capacitor, inevitably comprises a small voltage, and/or a small current. Consequently, it is desirable to provide a shield maintained at the same potential as the conductor, and the electrode, to prevent the generation of electro-magnetic fields between the electrode or the conductor, and any adjacent surface maintained at zero potential, except the other electrode of the capacitor, and thus to protect such signals from the adverse modifying effects of any such electromagnetic fields. Thus, it has not been known previously to monitor the separation of a cylinder block and a cylinder head by employing a capacitor provided therebetween.

Therefore, the apparatus in accordance with the present invention, includes some of the constituent electrically conducting elements of an electrical circuit having a capacitor. In particular, the elements provided by the apparatus comprise one, 20, of the two spaced electrodes of the capacitor, this electrode 20 being movable with the surface 18. The other electrode of the capacitor is provided by the cylinder block 10, although it may, alternatively, be provided by an electrode movable with the surface 16. The apparatus also comprises an elongated conductor 22 extending from said electrode 20 to a point beyond the two parallel surfaces 16 and 18, and an elongated shield, electrically insulated from the conductor 22 and maintained at the same electrical potential as the electrode 20. The shield comprises two closely spaced planar portions 24 and 26, secured together with the conductor 22 therebetween. The relative dimensions of the components of the apparatus, and the parts of the internal combustion engine have been exaggerated in Figure 1 for the sake of clarity.

The construction of the measuring apparatus of Figure 1, and its method of fabrication, are indicated in greater detail in Figures 2 and 3. Figure 2 shows a longitudinal section of each component of the measuring apparatus; and Figure 3 shows a plan view of each component, in the direction of the, arrow X of Figure 2.

The four constituent components of the apparatus are designated A, B, C and D. Component A is a layer of electrically insulating material.

Component B is a double-sided piece of printed circuit board of a conventional construction, having provided on the major face 30 of the electrically insulating substate 32 (which is made of a polyimide) to be opposite to the component A, a metal layer, for example, of copper, and comprising one portion 24 of the shield. On the other major face 34 of the substate 32 is printed a metal layer comprising both the circular in plan electrode 20, and the conductor 22 extending therefrom. Thus, the conductor 22 and the electrode 20 are provided by the same layer of metal.

Component C is a single-sided printed circuit board, otherwise of the same construction as component B, and having provided on the major face 40 of the electrically insulating substrate 42 (which is made of a polyimide) to be spaced from the component B, a metal layer comprising the other portion 26 of the shield. Component D is a layer of electrically insulating material opposite to the shield portion 26. An end region 44 of the shield portion 24 is also circular in plan, and is opposite to the electrode 20 on the same substrate 32. The shield portion 26 does not extend to be opposite to the electrode.

Conveniently, the substrates 32 and 42, and the electrically insulating layers A and D, are of a heat-resistant polimide, such as that sold under the Registered Trade Mark UPILEX of Ube Industries Limited. The components A, B, C and D of the apparatus may be secured together by an acrylic adhesive, such as that sold by GTS Flexible Materials Limited.

Each provided electrically insulating layer A and D, and any acrylic adhesive employed, initially is on a backing layer (not shown), and the backing layer is removed when the apparatus is assembled. Each printed circuit board is formed by employing conventional photolithographic techniques on a foil of electrodeposited copper, or rolled annealed copper, secured to a substrate 32 or 42 by an acrylic adhesive, such as the acrylic adhesive referred to above. Each constituent printed circuit board B and C, each constituent electrically insulating layer A and D, and the measuring apparatus formed therefrom, is flexible.

Initially, the electrically insulating layers A and D, and the substrates 32 and 42, have the same dimensions in plan, and not as shown in Figures 2 and 3. Also the electrically conductive elements 20, 22, 24 and 26 are provided wholly on the substrates, and not as shown in these Figures. The width of each elongated shield portion 24 or 26 is less than the width of the substrate, respectively, 32 and 42, on which it is provided; and the width of the elongated conductor 22 is less than that of each shield portion. Further, the circular periphery of the end region 44 of the shield portion 24 is wholly beyond the circular periphery of the opposing electrode 20. The shield portion 24, thus, extends to shield one face of the electrode 20.For the sake of clarity, the relative dimensions of the electrically conductive elements 20, 22, 24 and 26, the substrates 32 and 42, and the electrically insulating layers A and D, are exaggerated in Figures 2 and 3.

When the four components A, B, C and D of the apparatus are brought together, in the manner referred above, they are secured together by the application thereto for 60 minutes of pressure at 2.5 MPascals, at a temperature of 170 0C, to form a laminated structure.

The major face of the electrode 20 remote from the shield portion 24, in the completed apparatus may be covered with electrically insulating material; or portions of the substrate 42, and possibly also of the electrically insulating layer D, are removed, for example, by a stripping action, to expose selectively this major face of the electrode.

The electrode 20, the conductor 22 and the shield portions 24 and 26 are, thus, provided by layers of metal provided on the electrically insulating substrates 32 and 42 and the layers and substates are secured together in a stack by ahesive.

This construction for the measuring apparatus is inherently strong and robust; is capable of operating at high temperatures; and is compatible with the composition of the gasket 14.

As shown in Figure 1, the measuring apparatus, of the laminated construction, and at least substantially similar to the construction of a multi-layer printed circuit board, is placed in a channel-section groove 50 of the surface 52 of the gasket 14 opposite to the surface 18 of the cylinder head 12. The required shape in plan of the measuring apparatus is obtained by punching out this shape from the structure described above with reference to Figures 2 and 3.

The measuring apparatus resides partially in the groove 50, but extends therefrom to be beyond the gasket 14. At the end of the groove 50 within the gasket 14 is a hole 54 through the gasket, and the electrode 20 provided by the measuring apparatus is located over this hole. A layer of resilient dielectric material 56, for example comprising a silicone rubber, resides partially in the hole 54 in the gasket, and protrudes therefrom into the groove 50. The resilient dielectric material 56 biasses the electrode 20 towards the cylinder head 12, and so the electrode moves with the surface 18 of the cylinder head. The major face of the electrode 20 remote from the opposing end region 44 of the shield portion 24 is opposite to the dielectric material 56.The electrically conductive members 20, 22, 24 and 26 of the apparatus are electrically insulated from the cylinder head 12, and at the interface between the apparatus and the gasket 14.

In this manner, the required capacitor is provided between the cylinder block 10, and the cylinder head 12, the second electrode of the capacitor being provided by the cylinder block. The cylinder block is arranged to be maintained at zero electrical potential.

Conveniently, and as illustrated, the measuring apparatus described above is connected to a convention al coaxial cable 60. The measuring apparatus in accordance with the present invention also is advantageous in that it has a form suitably adapted to be connected to a coaxial cable.

A suitable form of interconnection between the measuring apparatus and the coaxial cable 60 is shown in a general way within the dotted line Y in Figure 1. The construction of this interconnection is shown in greater detail in Figure 4.

The end of the measuring apparatus beyond the gasket 14, initially, is prepared for connection to the coaxial cable 60. Thus, the extremity 62 of the conductor 20, and the extremities 64 and 66, respectively, of the shield portions 24 and 26 are exposed selectively by removal of the appropriate portions of the substrates 32 and 42, and of the electrically insulating layers A and D. This exposure also is shown in Figures 2 and 3, but could be obtained after the measuring apparatus has been assembled.

The adjacent end of the coaxial cable 60 also is prepared initially by exposing selectively the end 70 of the central conductive core 72, and the extremity 74 of the outer conductive sheath 76, by removing the end regions of the outer electrically insulating coverings 78, and of the electrically insulating cylinder 80 between the conducting core 72 and the sheath 76.

The end 70 of the central core 72 of the coaxial cable is then soldered to the extremity 62 of the conductor 22, as indicated at 82, and electrically insulating epoxy resin 83 is provided over the otherwise exposed conductive ends 70 and 62, and the interconnection 82 therebetween. An electrically conducting shield 84, initially in the form of a hollow cylinder, is slid over the coaxial cable 60, and is crimped on to, and soldered to, the extremities 64 and 66 of the shield portions 24 and 26, respectively, and as indicated at 86 and 88. Then the shield 84 is crimped on to, and is soldered to, the extremity 74 of the sheath 76, as indicated at 90. In particular, the arrangement is such that a continuous shield 84 is formed between the cylindrical sheath 76 of the coaxial cable 60, and the two planar shield portions 24 and 26 of the measuring apparatus.Finally, an electrically insulating coating 92, initially in the form of a tube, is provided on the sheath 76, and is heat shrunk to conform to the shape of the sheath. The coating 92 also extends on either side of the sheath to protect the interconnection, and to ensure there is no electrical short-circuit thereto.

The electrical circuit including the measuring apparatus may have any suitable form for determining any change of the capacitance related to the separation of the surfaces, and caused by a variation in this separation. Thus, the electrical circuit is required to produce a signal having a voltage and/or a current representative of the change of capacitance. Such signals are supplied to means, also included in the circuit, and to indicate, and/or to record, any change of the capacitance related to separation of the surfaces.

Hence, the measuring apparatus may be included, for example, in the electrical circuit shown partially at 100 in Figure 5. The electrical circuit 100 includes an integrator having an operational amplifier 102.

The output of the operational amplifier 102 is connected to a point maintained at zero electrical potential.

The capacitance associated with the integrator partially is provided by the variable capacitance C1 between the electrode 20 provided by the measuring apparatus and the cylinder block 10. The electrode 20 provided by the measuring apparatus is coupled to one of two terminals of an oscillator 104 and by a reference capacitor C2. The electrode 20 is also connected to the inverting input of the amplifier 102 by the conductor 22 of the measuring apparatus, and the conductive core 72 of the coaxial cable 60. There can be considered to be a capacitive coupling C3 between the combination of the shield 24, 26 of the measuring apparatus and the sheath 76 of the coaxial cable 60; and the combination of the conductor 22 and the conductive core 72 of the coaxial cable 60.

The sheath 76 of the coaxial cable is connected directly to the non-inverting input of the amplifier 102.

The non-inverting input is connected to the output of the electrical circuit, this output also being connected to the other te-rminal of the oscillator 104; and to the indicating, and/or recording, means (not shown) The output voltage VO from the illustrated electrical circuit 100 is approximately C2 i dt divided by C1 where i = i sin wt, i is the current drawn from the oscillator 104, and the frequency W divided by 2Pi is approximately 100 KiloHertz.

Thus, the output voltage VO is inversely proportional to the variable capacitance C1 between the electrode 20 and the cylinder block 10. The output voltage is supplied to the indicating, and/or recording, means.

Since the operational amplifier 102 must maintain both its inputs at the same potential, the shield 24, 26 is at the same potential as the conductor 22, of the measuring apparatus, as is desired.

Initially, the measuring apparatus is calibrated so that subsequently there is indicated, and/or recorded, the instantaneous actual value of the separation of the surfaces.

Any change in the output voltage VO is representative of any corresponding change of the variable capacitance C1, and hence is representative of any instantaneous change in the separation between the pair of opposing surfaces 16 and 18, respectively, of the cylinder block 10, and of the cylinder head 12. The output voltage VO varies linearly with any change of the separation between the pair of opposing surfaces 16 and 18.

Instead of measuring the separation between the pair of opposing surfaces 16 and 18, of the cylinder block 10, and of the cylinder head 12, in a dynamic manner, as described above, the separation may be measured statically.

Measuring apparatus in accordance with the present invention may be employed between two parallel surfaces provided by any two members, the separation of which members is required to be measured. It is not essential that the measuring apparatus is employed between two members in a hazardous environment; nor that the two members are closely spaced.

One of the electrodes of the capacitor, instead of being provided by one of the members, may be separate therefrom, but is arranged to move therewith.

More than two printed circuit boards may be included in the measuring apparatus. More than one printed circuit board may have metal layers on both major faces of the substrates; and/or more than one printed circuit board may have a metal layer on only one major face of each substrate. Initially, separate electrically insulated layers may not be included in the laminated construction of the measuring apparatus.

The measuring apparatus, if desired, may not be flexible in construction.

In a method of measuring the separation of the opposed surfaces 16 and 18, according to the invention, by determining changes in related capacitance, the above-described apparatus according to the invention is mounted so that the electrode 20 resides in the groove 50 and the conductor 22 and shield portions 24 and 26 protrude from the groove beyond the surfaces 16 and 18. The electrode 20 is biassed towards the head 12 by the resilient dielectric material 56 which resides in the hole 54. The capacitance between the electrode 20 and the electrode provided by the block 10 is measured to determine separation.

Claims (11)

1. Apparatus for use in measuring the separation of the two surfaces by determining changes in related capacitance, the apparatus comprising at least one of two spaced electrodes of a capacitor, this electrode being movable with one of said surfaces and the other electrode being provided by or being movable with said other surface, the apparatus also comprising an elongated conductor extending from said one electrode to a point beyond said surfaces, and an elongated shield electrically insulated from the conductor and maintained at the same electrical potential as said one electrode, the shield comprising two closelyspaced planar portions with the conductor therebetween, wherein said electrode, said conductor and said shield portions are provided by layers of electrically conducting material provided on electrically insulating substrates and the layers and substrates are secured together in a stack.

2. Apparatus according to Claim 1, wherein the conductor and the electrode are provided by the same layer of metal.

3. Apparatus according to either one of Claims 1 and 2, wherein two of the conducting layers are provided on opposite faces of the same substrate.

4. Apparatus according to any one of Claims 1 to 3, wherein one of said shield portions extends to shield one face of said electrode.

5. 'Apparatus according to any one of Claims 1 to 4, wherein at least one of the portions of the shield has a layer of insulating material secured thereto on the side thereof remote from the conductor.

6. Apparatus according to any one of Claims 1 to 5, wherein the substrates are made of a polyimide.

7. Apparatus according to any one of Claims 1 to 6 wherein the apparatus also comprises an operational amplifier comprising part of an integrator, and the conductor is coupled to the inverting input of the amplifier, and the shield is coupled to the noninverting input of the amplifier.

8. Apparatus according to any one of Claims 1 to 7, wherein the apparatus also comprises a coaxial cable, the conductor being connected to the core of the coaxial cable and a continuous electromagnetic shield being provided between the sheath of the coaxial cable and the shield of the apparatus.

9. Apparatus for use in measuring the separation of two surfaces substantially as hereinbefore described with reference to and as shown in the accompanyinq drawings.

10. A method of measuring the separation of opposed surfaces to two members by determining changes in related capacitance, the members having a deformable layer of insulating material between their opposed surfaces, the method comprising mounting apparatus according to any one of Claims 1 to 7, so that the electrode of the apparatus resides in a groove in a face of the deformable layer and the conductor and shield portions protrude from the groove beyond the surfaces, biassing the electrode of the apparatus towards one of said members by means of a resilient dielectric material residing in a hole through the deformable layer, and determining the capacitance between the electrode of the aoparatus and an electrode formed by or movable with the other of the members.

11. A method according to Claim 10 of measuring the separation of the cylinder block and the cylinder head of an internal combustion engine, wherein the deformable layer comprises a gasket between said members.