GB2356256A - Backing member for a surface roughness tester - Google Patents

Abstract

The invention provides a resilient backing member (1) for use in the measurement of surface roughness of flexible sheets (27), especially paper, by the air leak method or by an optical technique. The backing member (1) comprises a membrane of soft elastomeric material (3) supported on a fluid reservoir (2). In use, the elastomeric membrane (3) is pressed against the back surface of a sheet under test whilst gas flow rate/impedance or optical measurements are performed on the front surface of the sheet under test. The invention also provides an apparatus and method incorporating the backing member for measuring the surface roughness of sheet material.

Description

2356256 BACKING MEMBER FOR A SURFACE ROUGHNESS TESTER The present

invention relates to a backing member, in particular for use in an instrument for the measurement of surface roughness of flexible sheet materials such as paper. The present invention also relates to an instrument comprising such a backing member, and to methods of measurement of surface roughness of flexible sheet materials using such an instrument.

Printing quality onto paper is primarily related to the roughness of the paper, and it is therefore possible in principle to assess the printability of paper by measuring its roughness without actually performing a printing operation. This enables quality control of the printability of paper be performed at the paper making plant without reference to the printer.

Measurement of paper roughness to assess printability must take into account the flexibility of paper, and the fact that most common printing processes use deformable materials to contact the paper during printing. For example, in gravure printing, the paper is pressed by a rubber covered.backing roll against a,cylinder carrying the image to be printed in ink-filled engraved cells. Similarly, in the letterpress process, the plate is of metal with the image in relief and ink transfer is affected by pressing the paper against this plate by means of a pad of composite rubber or other deformable printing blanket. In the increasingly popular and versatile flexographic process, the ink image is formed on the relief areas of a rather soft rubber plate. In all these practical printing processes, therefore, use is made of a deformable member on one or other side of the paper web to improve the ink transfer to the paper. It is easy to show by experiment that, in the absence of a deformable material, print quality is altogether unacceptable.

There are two reasons for this importance of deformable materials in printing. In letterpress printing the deformable blanket is used to press the paper against the printing plate and thereby clearly reduces the gap between the two that must be filled with ink to obtain a good print. Thus it reduces the roughness of 2 the paper, almost certainly by flexing it. In gravure printing, it is the frequency of the contact between the paper and the printing plate that make consistent ink transfer possible to the paper from each engraved cell. Again, the rubber covered roller used to press the paper and image cylinder together appears to flex and 5 flatten the paper.

In contrast, in offset printing, where a rubber faced blanket carries the ink to the paper, or in flexography, where the printing plate itself is resilient, the ink bearing elastomeric transfer medium [probably] deforms in contact with the paper, bulges into each paper surface cavity, and thus reduces the thickness of the ink necessary to achieve good ink transfer. Moreover, wide surface cavities can be more readily bottomed by a deformable inked surface than narrow cavities of the same depth Thus it can be seen that the assessment of surface roughness for the purpose of printability must take into account not only the roughness of the unstressed paper sheet, but also the flexibility of the sheet and the way in which surface roughness is affected when the sheet is pressed against hard or soft surfaces. Preferably, the measurement should provide an estimate of the diameters of the paper surface cavities as well as their depths.

The roughness. of hard surfaces, such as metal surfaces, is conveniently determined by dragging a stylus lightly across the surface. The surface roughness is then defined and measured as some mean of the deviation of the distance between this real surface and some mathematically derived smooth version of the same surface. Unfortunately, this technique cannot successfully be applied to paper because the deformation of the surface caused by the stylus probably bears little relationship to the surface deformation that occurs during printing. This may explain why satisfactory indications of printing roughness can seldom be obtained by this means.

3 A means of characterising paper surfaces was described by S. M. Chapman in the Convention Issue of the Pulp and Paper Magazine of Canada, pages 140 to 150, in 1947. The Chapman tester measures the proportion of a paper surface that can be brought into optical contact with the face of a smooth glass prism against which it is pressed, usually by means of a deformable backing. Chapman noted that the softness of the material underlying the paper had a considerable effect on the results. A version of this instrument with modified optics, known as the PATRA-Chapman smoothness tester, is currently available from PIRA International at Leatherhead, Surrey.

GB-A-1063657 describes an air leak instrument and method for measuring the roughness of printing papers. According to this method, the face side of a test piece of paper or board, usually cut from a larger sample, is pressed against a hard smooth reference plane. The mean gap between the face of test. piece and the surface of the reference plane is then measured by the resistance of this gap to the flow of air. Specifically, the reference plane is penetrated by two or more long narrow parallel ducts separated by smooth hard metering lands in contact with the surface under test. Air is supplied to or removed from the ducts and flows through the gap between the paper and the surface of the measuring land. The paper is pressed against the reference plane by means of a deformable pad, usually a piece of printing blanket. Both the clamping pressure and hardness of the backing member can be selected to correspond to those used in a traditional printing process, such as gravure printing.

The above instrument, known as the Parker Print-Surf (Registered Trade Mark) Roughness Tester, is commercially available from Messmer Instruments Ltd. (now trading as Messmer BOchel).

In its normal configuration the sensing head and reference plane of the above mentioned instrument are above the test piece whilst the deformable backing lies below it. In the majority of Print-Surf instruments the sensing head is circular and has an annular reference plane; such instruments correspond to the 4 first embodiment described in patent specification GB-A-1063657 and are also as described in the international standard ISO 8791-4. In current versions of the Parker Print-Surf Roughness Tester, a fluidic impedance measurement is used to determine the air flow resistance of the paper surface by the method described in GB-A-1 389947, and thence the roughness of the sample may be calculated.

In this PPS instrument the paper sample, reference plane and backing member are pressed together by air pressure applied to a membrane located below the backing member. This simple pneumatic press does not constrain the backing member to move parallel to the operative face of the sensing head although, when clamping pressure is applied and the backing surface and reference surfaces make contact, they align themselves whether or not a paper test piece lies between them.

As was noted above, the backing members used in the air leak roughness tester described in GB-A-1063657 are often made from portions of commercial printing blanket bonded to a metal plate. Their hardness is usually specified by their apparent Shore A hardness, that is, by the Shore a hardness measured on a single thickness of blanket as a test piece. (The Shore A hardness tester is a less accurate version of the IRDH tester, but is commonly used instead in practical situations). Such printing blankets are normally 1-4 mm thick and compdse a facing of rubber backed by one or two layers of a woven textile in a rubber matrix. An alternative harder backing, formed from a sheet of granulated cork covered by a polyester film, is suggested for the measurement of certain letterpress papers Unfortunately, when a roughness measurements is made with the aforementioned circular sensing head, its rather narrow annular reference plane locally flexes and indents both the backing member and the sample. Especially if the sample is of board or is relatively stiff, this flexing causes a serious reduction in the clamping pressure in the region of the metering land. In consequence the measured roughness is erroneously high. This defect is only partially corrected by supporting the deformable backing from below only where it is immediately opposite the reference plane.

The uniformity of the backing member used in the above-described air leak roughness tester is critical to accurate and reproducible measurements Its possible non-uniformity is now a source of concern.

The present invention is based on the discovery that the use of softer and more flexible elastomers for the backing member can provide important advantages in terms of information content as well as in air leak measurement consistency. It can further be shown that, by carrying out a series of measurements on paper test pieces at different clamping pressures whilst using a very soft backing member, the results obtained would provide rather detailed information about the widths of the cavities in the paper surface.

The present invention provides a resilient backing member for backing a sheet material in a surface roughness tester comprising a sheet elastomeric material and a reservoir of fluid by which the elastomeric membrane is supported.

The backing member is preferably specifically adapted for use with a sensing head for the measurement of paper or board roughness. Whilst each roughness measurement is being made, the elastomeric membrane is forced by fluid pressure against the test piece, which in turn makes contact with a reference plane of the sensing head.

Preferably, the reservoir and elastomeric membrane are circular or rectangular in plan. Preferably, the reservoir comprises a base and side walls extending around the base with the elastomeric membrane extending across the reservoir between the side walls in fluid-tight fashion. Preferably, the elastomeric membrane is secured in fluid4ight fashion to the side walls of the reservoir. More preferably, the edges of the elastomeric membrane are secured to the side walls in gasket fashion by means of a clamping ring, this clamping ring' forming part of 6 the side wall of the reservoir. Preferably, to allow limited vertical movement of the membrane relative to the walls of the reservoir, the inner corners of both the reservoir walls and clamping ring are also rounded where they contact the membrane.

Preferably, one or more dams are supported on an edge of the elastomeric membrane within the reservoir to prevent extrusion of the elastomeric membrane when it is pressed against a test piece. Preferably, the one or more dams extend around the edges of the elastomeric membrane inside the clamping ring, more preferably the one or more dams extends around substantially all the said edges, and most preferably a single annular dam extends around substantially all the edges of the membrane inside the clamping ring. Furthermore, the cross-section of the dam is preferably configured such that the edge of the elastomeric membrane inclines upwardly below the dam at an angle towards a central test region of the sheet. Preferably, the cross-section of the dam is substantially triangular but with rounded external comers.

Preferably, the dam can move vertically together with the elastomeric membrane in response to the application of clamping pressure. More preferably, the vertical movement of the dam is restricted by rebates in periphery of the dam that engage with complementary rebates in the inner surface of the reservoir clamping ring. Preferably, in the absence of other constraints, the rebates limit the extension of the dam above the top of the sealing ring on the rim of the reservoir to 1.0 to 3.0 mm.

A further advantage of the annular dam is that it provides a clearly defined clamping surface area, whereby a clamping pressure applied to the backing member is distributed over a constant area of membrane-plus-dam, making the calculation of clamping pressures much easier and more reliable. This is important in cases where the backing member is sealed (i.e. constant reservoir volume) and pressed against a sheet under test by an external clamping element, such as a hydraulic press.

7 Preferably, the depth of fluid supporting the membrane is from 1 to 10 mm. Preferably, the maximum width or diameter of the test contact region elastomeric membrane is 10 to 100 mm for both rectangular and circular reservoirs.

Preferably, to aid the initial filling of the reservoir with fluid and the removal of air bubbles, and also to support the membrane when it is retracted by reduction of the fluid pressure, a metal or plastic plug may be centrally located on or integral with the base of the reservoir. Preferably, the upper surface of the plug is shaped to correspond to the contours of the elastomeric membrane in use, whereby a substantially constant thickness of fluid supports the membrane in use. For example, the plug is preferably substantially cylindrical with chamfered upper edges. The plug preferably reduces the effective depth of fluid supporting the membrane to 0.5 to 20 mm. and more preferably to 2 to 15 mm. More preferably, the plug is also pierced by a central hole for the removal of air when the reservoir is filled through a filler hole located near the periphery of the base of the reservoir.

Preferably, the elastomeric membrane has a thickness in the range of from 0.1 to 5.0 mm, more preferably from 0.2 to 4.0 mm and most preferably from 0.5 to 2.0 mm.

Preferably, the elastomeric membrane has a hardness in the range 20 to 80 IRDH, more preferably from 30 to 40 IRDH.

For measurement of the roughness of some types of flexible sheet it may be desirable to increase the stiffness of the elastomeric membrane, and accordingly the backing member may further comprise a stiffening sheet applied to the surface of the elastomeric membrane opposite the fluid reservoir, wherein the stiffening sheet is formed from a stiffer material than the elastomeric membrane, for example from a thermopolymer film.

8 Preferably, the backing member according to the present invention further comprises a pressure sensor for measuring the pressure of the fluid in the reservoir.

Preferably, the fluid within the reservoir is a substantially incompressible liquid. Preferably, the liquid is inert and non-corrosive, for example a polyhydric alcohol or a silicone oil. More preferred are propylene glycol, ethylene glycol and diethylene glycol. Alternatively, for particular applications, the reservoir may be filled with gases, or with substantially incompressible substances other than simple liquids, such as a highly viscous liquid or very soft elastomer. The reservoir may be sealed, or may comprise means to vary the amount of fluid therein.

Preferably, the backing member according to the present invention comprises means to regulate the pressure of the fluid in the reservoir. Such means may either vary the amount of fluid in the reservoir and/or vary the volume of the reservoir. Of course, the resilience of the elastomeric membrane means that it moves in response to such pressure, thereby potentially varying the volume of the reservoir. More preferably, the backing member comprises means to vary the amount of fluid in the reservoir operatively associated with pressure sensing means to measure the pressure of the fluid in the reservoir so as to achieve a desired pressure independent of changes in the volume of the reservoir.

The present invention further provides an instrument for measuring the roughness of a flexible sheet material comprising a backing member according to the present invention.

Preferably, the instrument is substantially as described in GB-A-1063657. That is to say that, in addition to the backing member (which requires a modified supporting or clamping system), the instrument comprises a sensing head having an operative face against which the test piece is pressed by the backing member, a channel opening to the face and divided into one or more inlet chambers and 9 one or more outlet chambers by one or more metering lands, and ducting leading from each chamber permitting the inlet chamber(s) to be connected to a source of fluid pressure and the outlet chamber(s) to be connected to a means of measuring fluid flow or fluidic impedance when the instrument is in use Preferably, the head comprises a series of parallel plates plied together with spacers between them. More preferably, the spacers are shaped so as to form between each pair of plates a chamber which has an opening on one side of the assembly; the plates and spacers are perforated and arranged so that, when the instrument is connected to a source of compressed air, air under pressure is supplied to at least one of the chambers and this air is collected from at least one of the remaining chambers; the plates and spacers are clamped together to form a rigid assembly; and the face of the assembly to which all the chambers open is smooth and flat.

In order to clamp the sample against the reference plane of the sensing head, the whole backing member may be constrained to move parallel to the operative face of the sensing head and then be forced against it by pneumatic or other means to generate the pressure in the reservoir fluid that is necessary to press the test piece against the sensing head whilst a roughness measurement is made. Care must be taken when carrying out such clamping, because the backing member will not be self-aligning. The backing assembly should therefore be guided such that it is always substantially parallel to the sensing head. The presence of a dam provides a well- defined area for calculating the clamping pressure from a given clamping force, as discussed above.

More preferably, the backing member is supported substantially fixedly relative to the sensing head with the resilient membrane located 0.5 to 1. 5 mm below the reference surface when the reservoir is at zero gauge pressure, whereby the resilient membrane can be brought into engagement with the test piece simply by pressurizing the reservoir to displace the membrane upwards. The reservoir is constantly connected to a source of fluid of which the pressure may be adjusted in order to clamp or release the test piece required. The reservoir fluid may be either a lower viscosity liquid or, for low pressure applications, a gas. Preferably, the reservoir clamping ring and the reference surface are substantially parallel and the gap between them is in the range 0.5 to 1.5 mm, depending on 5 the thickness of the sheet material to be tested.

The present invention further provides a method of measuring the roughness of a test piece of sheet material comprising: clamping the test piece between the backing member and the operative face of an instrument according to the present invention; supplying a gas under pressure to the said one or more inlet chambers; and measuring the gas flow rate and preferably the gas fluidic impedance between the said one or more inlet chambers and the said one or more outlet chambers.

Preferably, the equipment used and the method of measurement are substantially as described in GB-A-1063657 and GB-A-1389947. Preferably, the sheet material is paper or board.

Preferably, a plurality of measurements are carded out at different clamping pressures between the backing member and the operative face. This can give additional information about the geometry of the surface roughness as discussed in more detail below.

A specific embodiment of the present invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a longitudinal cross section through a backing member according to the present invention, and 11 Figure 2 shows a longitudinal cross section through an instrument according to the present invention, and also shows a sheet of paper under test by the instrument.

Referring to Figure 1, the backing member 1 comprises a reservoir 2 of liquid across the top of which is stretched a membrane membrane 3 of a soft elastomer. The soft elastomer is about I mm thick and is formed from a synthetic rubber such as neoprene having a hardness of approximately 40 IRDH. Such a membrane will have a bending stiffness less than 1 % of the bending stiffness of a sheet paper, so that it will conform to the underside shape of the paper and, regardless of any lack of flatness, transmit the hydraulic pressure of the reservoir fluid substantially uniformly to the paper sample.

The reservoir 2 is bounded by a base 4 and a substantially cylindrical side wall 5 machined from stainless steel or other suitable alloy. The edge 6 of the membrane is clamped circumferentially into the side wall 5 by a gasket-type clamp formed by an annular clamping ring 7 screwed into the side wall 5. The inner edges 8,9 of the clamping ring 7 and reservoir wall 5 surfaces between which the membrane is clamped are rounded.

A filling hole 10 is provided for introducing liquid into the reservoir 2. The filling hole 10 is connected to a pressure sensor (not shown) for measuring the hydrostatic pressure of the liquid in the reservoir and means (not shown) for varying the amount of liquid in the reservoir in order to regulate the said pressure and move the membrane upward into engagement with a test piece.

A plug 11 of metal or plastic is centrally located in liquid reservoir 2 and fixed to its base 4 by mating engagement with a central lug 12. The base 2 and plug plug 11 are centrally pierced by the vent hole 13 through which gas bubbles are purged when the reservoir 2 is initially filled with liquid. The upper surface 14 of the plug 11 is profiled such that when the membrane 3 is in the raised position, as when it is being pressed against a test piece, the gap between the membrane and the reservoir plug 11 has a substantially uniform thickness of about 3mm. When the membrane is lowered by reducing the fluid pressure in the reservoir, it may then may rest on the upper surface 14 of the reservoir plug 11.

An annular dam 15 of substantially triangular cross section is provided on top of the membrane 3 extending around the inside of the reservoir clamping ring 7. The upper and lower outside comers 16,17 of the dam 15 are rounded. The dam is vertically movable, but held in place laterally by the clamping ring 7. Its vertical movement is also restricted by complementary rebates 18,19 formed in the dam itself and in the reservoir clamping ring. The dam prevents the membrane from bulging over the edges of the clamping ring 7, and enables a well defined test area of the membrane 3 to be presented to the test piece.

A thin plastic film (not shown) may optionally be provided on top of the surface of the membrane 3. The plastic film is typically 0.01 to 0.5 mm thick and has a bending stiffness greater than the stiffness of the membrane. This enables the membrane to present a surface of greater bending stiffness to the material under test in certain circumstances where this may be desirable.

Referring to Figure 2, the instrument according to the present invention comprises a backing member 1 according to the present invention operatively associated with a sensing head 20 substantially as described in GB-A- 1063657 and with a fluidic impedance measuring system as described in GB-A- 1389947, The sensing head 20 comprises a plurality of alternate plates and spacers [24] that define a succession of usually alternate inlet and outlet airways 22 and 23. The whole is held together by end pieces 25 through which airways also pass.

The lower edges of the plates 21 form smooth flat lands 26 that are mutually coplanar and are also coplanar with the lower faces of the end pieces 25. The sensing head 20 is connected to a source of filtered, pressurised air (not shown) for passing air initially through a fluidic impedance of known value and thence into the air inlet ducts 22. The sensing head 20 is rigidly supported such that its 13 operative face is parallel to and about 1.0 mm from the top surface of the reservoir clamping ring 7. No pneumatic ram is necessary. Test pieces may be pressed against the sensing head by suitably increasing the pressure of liquid in the reservoir 2 under the membrane 3.

In use, the sensing head 20 and backing member 1 are clamped in fixed spatial relationship on either side of the paper sample 27. The clamping is carried out by varying the hydrostatic pressure in the reservoir 2 to achieve a predetermined clamping pressure, typically about 1 MPa, which is further adjustable. Pressurised air at a gauge pressure of about 0.25 atmospheres first applied to the fluidic impedance from which it discharges into the inlet ducts of the sensing head 20. The gauge pressure of this air is measured both at the inlet to the fluidic impedance and at the inlet to the sensing head. The air from the outlet ducts of the sensing head discharges freely at atmospheric pressure for which a nominal value is commonly assumed. The impedance to air flow of the gaps between the test piece and the smooth bottom faces of the lands is then calculated as described in GB-A-1389947.

The above calculations of impedance and of roughness may be understood 20 as follows. First, we note that if some dimensions of the sensing head are known; specifically, (a) the width t of the metering lands, measured in the direction of flow, and (b) the total length L of all the lands in the operative face across which air should flow. Suppose that the roughness of the test piece, measured as the cube root of the mean cube gap between the paper surface and the lands, is G, while the viscosity of air is i, then the impedance Z of this mean gap to air flow may be found from a rearrangement of the equation:

G 3 = 12jit/LZ The impedance Z, is defined by the equation:

Z = APIQ... (2) 14 Where Q is the volume rate of flow of air through the impedance, measured at any convenient pressure p,, and Ap is the differential pressure measured across the impedance. It is important, in calculating this differential pressure, that due allowance should be made for the compressibility of the air. To do this, Ap should be found from the equation:

AP (p, - PI) (R' + P-)... (3) PM Where p. and p, are the absolute pressures upstream and downstream from the impedance.

When a known impedance Z. is connected upstream and in series with 15 an unknown impedance, such as a sensing head against which a test piece is being pressed, and P3 I P2 and p, are the upstream, intermediate and downstream absolute air pressures then the impedance Z, of the paper surface under test may be found from the equation ZI Z2 PI 2. P1 2 (4) P2 2. p32 The above relationships only hold if the measurements are made under conditions such that the flow Q through each impedance, including that due to the paper surface, is proportional to the differential pressure Ap. It is wise to check by experiment that this condition is satisfied under all possible conditions of use of the roughness testing instrument, for all the types of paper and board that may be tested.

It is customary to quote G in micrometers. The roughnesses of printing papers measured under simulated printing conditions at I Mpa are usually in the range 0.6 to 6.0tm.

One advantage of the soft elastomeric backing member according to the present invention is that it reduces errors due to air passing through the thickness of the paper by achieving closer conformity to the back surface of the paper. The elastomeric backing sheet is highly conformable both because of its inherent softness and because of the conformability of the hydraulic reservoir with which it is supported.

Because the Young's modulus for soft rubber is very much less than that of a sheet of paper of normal thickness, and because the amplitude of flexing both of the elastomeric membrane and the paper would be of similar magnitudes, it is considered that the pressure applied to the paper by the backing member would be substantially uniform. Furthermore, if a sensing head is used with a flat lower surface, such the linear head constructedfrom flat plates as previously described, then pressure can be applied by the backing member of the present invention substantially without indenting the test piece.

A further advantage that is envisaged for the backing member according to the present invention is the use of multiple measurements at different hydraulic pressures to determine the width of cavities in the face sides of the paper test pieces in addition to the basic depth (surface roughness) of the paper. The explanation is as follows. The resistance to bending of a beam is proportional to the product El, where E is its Young's modulus and I is moment of inertia. If load w per unit length is applied to a beam that is clamped at both ends the deflection y at the centre is:

y -WL'... (5) CEI where the constant C has the value 384. For paper, if I is calculated per unit width and w is measured per unit area, then El may be defined as the paper stiffness. Formula (5) may then be applied in two dimensions to determine the 16 deflection y at the centre a circular disk of such paper of diameter L that is clamped at all points around its circumference, if the value of C is taken as 512 and it is assumed that Poisson's ratio for paper is zero.

If a narrow paper strip is placed on a flat plane surface and uniformly loaded, the mean distance L between the points at which it makes contact with the plane may be found from the rate at which the mean gap G between the paper and the plane, that is, the roughness, decreases as the load w is increased. If it is assumed that the maximum of this gap between each pair of contact points is y as in equation (5), and that y is roughly equal to 2G, then, by differentiation, L 4 = - 0.5 (dG/dw) CEI... (6) It has been suggested that L is a quantity of interest in the study of offset 15 and flexo printability. To estimate L accurately it is necessary to apply a load that is known to be substantially uniform to the obverse of a paper sample under test. This requires the use of a backing of negligible stiffness, relative to the paper, such as is provided by the backing member according to the present invention.

The above embodiment has been described by way of example only. Many other embodiments failing within the scope of the accompanying claims will be apparent to the skilled reader. For example, it will be appreciated that the backing member according to the present invention can be used not only in a fluid flow/fluid impedance type of roughness tester, but also in an optical roughness tester of the type described by S.M. Chapman, op. cit. Such testers further comprise a glass prism having a smooth face that is pressed against the surface under test. The extent of optical contact between the smooth face and the paper is then determined by measuring the reduction in total internal reflection from this face. The spatial distribution of the contact areas can also be recorded photographically.

17

Claims (1)

1. A resilient backing member for backing a sheet material under test in a surface roughness tester, said backing member comprising a membrane of 5 elastomeric material supported by a fluid reservoir.

2. A backing member according to claim 1, wherein said reservoir comprises a rigid base and side walls extending around said base, and said elastomeric membrane extends across said reservoir between the side walls in fluid4ight fashion.

3. A backing member according to claim 2, wherein an edge of the elastomeric membrane is secured to the side walls in gasket fashion by a clamping ring.

4. A backing member according to any preceding claim, wherein the fluid is a substantially incompressible fluid, such as a liquid.

5. A backing member according to claim 4, wherein the fluid is selected from the group consisting of polyhydric alcohols and silicone oils.

6. A backing member according to any preceding claim, wherein the elastomeric membrane has a thickness in the range of from 0.2 to 4.0 mm.

7. A backing member according to any preceding claim, wherein the elastomeric membrane has a hardness in the range 20 to 80 IRDH.

8. A backing member according to any preceding claim, further comprising a stiffening sheet applied to the surface of the elastomeric membrane opposite the fluid reservoir, wherein the stiffening sheet is formed from a stiffer material than the elastomeric membrane.

18 9. A backing member according to any preceding claim, wherein the elastomeric membrane comprises one or more vertically movable dams located proximate to one or more edges of the sheet to prevent extrusion of the elastomeric membrane when it is pressed against a surface to be supported.

10. A backing member according to claim 9, wherein a solid annular dam extends around a central contact region of the elastomeric membrane.

11. A backing member according to claim 9 or 10, wherein the dam has a substantially triangular cross-section, whereby the edges of the elastomeric membrane slope upwardly towards a central portion test region of the membrane.

12. A backing member according to claim 9, 10 or 11, wherein vertical movement of the dam is restricted by a rebate in the periphery of the dam that engages with a rebate in the inner surface of the reservoir clamping ring.

13. A backing member according to any preceding claim, wherein the fluid reservoir is sealed so as to contain a constant amount of fluid.

14. A backing member according to any one of claims 1 to 12, further comprising means to vary the amount of fluid in the reservoir. 15. A backing member according to any preceding claim, further comprising a plug located on the base of the reservoir, wherein the plug has a profiled upper surface to provide a substantially constant thickness of fluid under the membrane.

16. A backing member according to claim 15, wherein the plug is pierced by a central hole for removal of air when fluid is introduced into the reservoir.

17. A backing member according to any preceding claim, further comprising a pressure sensor for measuring the pressure of the fluid in the reservoir.

19 18. A backing member according to any preceding claim, wherein the membrane is supported by a fluid layer of thickness of from 0.5 to 20 mm.

19. A backing member according to any preceding claim, wherein the elastomeric membrane comprises a central test region having a maximum width or diameter of from 10 to 100 mm.

20. A backing member substantially as hereinbefore described with reference to Figure 1.

21. An instrument for measuring the surface roughness of a sheet material comprising a backing member according to any one of claims 1 to 20.

22. An instrument according to claim 21, further comprising:

a head having an operative face against which the sheet material is held by the backing member, a channel opening to the face and divided into one or more inlet chambers and one or more outlet chambers by one or more metering lands, and ducting leading from each chamber permitting the inlet chamber(s) to be connected to a source of fluid pressure and the outlet chamber(s) to be connected to a fluid flow meter or fluidic impedance when the instrument is in use.

23. An instrument according to claim 22, in which the head comprises a series of parallel plates plied together with spacers between them.

24. An instrument according to claim 23, in which the spacers are shaped so as to form between each pair of plates a chamber which has an opening on one side of the assembly; the plates and spacers are perforated and arranged so that, when the instrument is connected to a source of fluid pressure, fluid under pressure is supplied to at least one of the chambers and fluid is collected from the remaining chambers; the plates and spacers are clamped together to form a rigid assembly; and the face of the assembly to which all the chambers open is flat.

25. An instrument according to claim 21, further comprising an optical prism having a smooth faced adapted to be pressed against the surface of the sheet material under test.

26. A method of measuring the roughness of a sheet material comprising: clamping the sheet material between the backing member and the operative face of an instrument according to any one of claims 21 to 24; supplying a fluid under pressure to said one or more inlet chambers; and measuring the flow of fluid from said one or more outlet chambers.

27. A method according to claim 26, wherein the sheet material is paper or board.

28. A method according to claim 26 or 27, wherein a plurality of measurements are carried out at different clamping pressures between the backing member and the operative face.