GB1586740A

GB1586740A - Hygro-extensometer apparatus

Info

Publication number: GB1586740A
Application number: GB2091178A
Authority: GB
Original assignee: Wiggins Teape Group Ltd
Current assignee: Wiggins Teape Group Ltd
Priority date: 1978-05-19
Filing date: 1978-05-19
Publication date: 1981-03-25

Description

(54) HYGRO-EXTENSOMETER APPARATUS (71) We, THE WIGGINS TEAPE GROUP LIMITED, a British Company, of Gateway House, Basing View, Basingstoke, Hampshire RG21 2EE, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to an hygroextensometer apparatus.

In paper-making it is often necessary to test the dimensional stability of paper when subject to varying conditions of humidity.

The method of testing used heretofore was manual and was both laborious and liable to operator error.

In one apparatus for performing this manual method a cabinet in which the humidity was controllable contained a number of test stations. Each station comprised an upper and a lower support point between which a strip of paper to be tested was fixed, the strip being of a predetermined length between the points of support. The upper support point was in turn supported on the shaft of a vertically disposed micrometer and by means of a spring loading the micrometer could be adjusted to provide a preset test force longitudinally of the strip. The lower support point included fixed and movable indexes which came into alignment when the preset force was applied across the predetermined length of strip.

In order to test a strip for changes in its length with changes in humidity, the tension in the strip was relaxed by releasing the micrometer, the humidity adjusted manually to a new value and the conditions allowed to stabilize, the tension then reapplied to the strip with the aid of the micrometer until the indexes aligned, and then the micrometer read. As the cabinet could contain some twenty such test stations, the procedure could be laborious with the risk that in the time taken to adjust and read all the micrometers the humidity could change. Additionally the alignment of the pairs of indexes and the reading of the micrometers was liable to operator error.

There will be described below, an embodiment of the present invention which by adopting automatic techniques enables the disadvantages of the prior apparatus to be substantially mitigated.

According to a first aspect of the present invention there is provided a hygroextensometer apparatus comprising a closable chamber having inlet means for introducing a gas of controlled humidity to the chamber, and a test station within the chamber including: a support means for receiving and suspending a strip of material to be tested; a displacement responsive transducer below said support means, said transducer having a movable element and being operable to provide an output signal that is a measure of the position of said element over a range of displacement thereof, said element being secured to means for attaching to a lower portion of such a suspended strip at a predetermined distance from the point of suspension at said support means such that, in operation, the transducer provides an output signal indicative of changes in the predetermined suspended length of the strip due to changes of humidity within the chamber.

According to a second aspect of the invention there is provided a hygroextensometer apparatus comprising: a closable chamber having inlet means for introducing a gas of controlled humidity to the chamber; and a plurality of test stations within said chamber each test station including a support means for receiving and suspending a strip of material to be tested; a displacement responsive transducer below said support means, said transducer having a movable element and being operable to provide an output signal that is a measure of the position of said element over a range of displacement thereof, said element being secured to means for attaching to a lower portion of such a suspended strip at a predetermined distance from the point of suspension at said support means such that, in operation, the transducer provides an output signal indicative of changes in the predetermined suspended length of the strip due to changes of humidity with the chamber.

The invention and its practice will now be further described with referene to the accompanying drawings, in which: Figure 1 shows a diagrammatic view of a test station in a chamber the humidity of which is controllable.

Figure 2 is a diagrammatic side view of a strip of material supported in the test station, and showing one form of support and attachment means; Figure 3 is a side section of the chamber, showing more details of construction and showing also a different form of support and attachment means; Figure 4 is a plan view of platforms associated with groups of test station in the apparatus; Figure 5 shows a humidity control arrangement for the chamber; and Figure 6 is partly schematic and partly block diagram of the circuitry used to select, switch, and process and display signals from individual transducers associated with the test stations, Figure 6A showing the switching portion of the circuit, Figure 6B the selection and decoding portion and Figure 6C the signal processing and display.

In the Figures, like parts bear like reference numerals.

Referring to Figure 1, there is shown a portion of a rectangular chamber 10 seen from a front access position. Access is through a door extending along the front wall of the chamber as will be subsequently described with reference to Figure 3. The chamber is sealed when the door is closed.

The chamber contains a plurality of test stations, say twenty, a representative one of which is shown in Figure 1. Each test station serves to support a strip of material, for example paper or textile material, with a predetermined load applied thereto and includes a transducer for measuring changes in the suspended length of the material due to changes in humidity imposed in the chamber 10 by means to be described.

Figure 1 shows a portion of the base 11 and top wall 12 of the chamber together with one end wall 13. A rail 20 runs the width of the chamber of the top thereof and is secured to the top and/or end walls. The rail 20 provides a support for a row of test stations spaced across the width of the chamber 10 and, as is seen from Figure 3 in order to accommodate more test stations in the chamber 10, two such rails are provided in parallel, each supporting half the total number of test stations.

As shown in Figure 1 each test station along rail 20 includes an upper support means 30 which is hung from a pin or screw 21 projecting from the rail. In this support means is clamped the upper portion of a strip 32 of material under test. To the lower end portion of the strip 32 another clamp 34 is attached. The point of attachment of clamp 34 is at a predetermined distance I from the point of suspension of the strip in the upper clamp 30. Thus the strip is freely suspended over the length I in the sense that it expands or contracts over this length when the humidity in the chamber 10 is changed. The sample is initially cut to provide a predetermined length I established outside the chamber by use of a gauge or rule.

From the suspended lower clamp 34 a rod 36 depends vertically. Part way down the rod a weight 38 is mounted. The rod has a lower portion 37a at the end of which is secured the movable element of a displacement-responsive transducer assembly 70 described further below. The upper portion 37b of the rod 36 extends through an aperture 41 in a part 40 formed as a platform. By means of a coupling diagrammatically indicated at 42, the platform 40 is vertically movable from a lower position shown to an upper position 40a indicated by dashed lines. It will be noted that in moving the platform to the upper position 40a it will engage the underside of clamp 34 and raise it to a position at which it, or more particularly its clamping point, is less than the distance I from the point of suspension of the strip 32 in the upper clamp 30.Thus in this position tension is removed from the strip which hangs limply. In the lower position of platform 40 it disengages the clamp 34 enabling the strip to be suspended and loaded over the distance I. The construction of platform 40 and its coupling 42 will be further described with reference to Figures 3 and 4.

The suspension length I of the sample strip under test may be conveniently chosen as 200 mm. but the chamber may be adopted to accommodate strips of different length. This is done by altering the height of rail 20, or inserting a new rail in the chamber 10, as by affixing it to brackets mounted part way up the end walls of the chamber. One bracket 24 on end wall 13 is shown, the rail being affixable to this bracket at two heights indicated by 20' and 20". The value oflis altered by changing the height of the upper rather than the lower clamp in order to maintain a predetermined central position of the movable element of the transducer as will be later described.

The chamber 10 has a gas inlet 50 through the other end wall 14, only a fragment of which is shown, and this inlet communicates with a pipe or conduit 52 that extends in a rectangular shape around the lower portion of the chamber, the pipe having a series of small apertures 54 therealong to allow gas of controlled humidity introduced into the pipe to flow into the chamber. The inlet 50 may be a stub pipe communicating with the short side of the apertured pipe 52 that runs adjacent end wall 14. The disposition of the inlet pipe in chamber 10 is also seen in Figure 3. The chamber is also provided with an outlet 56 through end wall 13 that allows gas to escape from the chamber and bleed valve 58. The pipe 52 itself is provided with a bleed valve 53.

The chamber 10 also contains a humidity sensor 60 that provides a humidity representing electrical signal on lead 61 that is applied to a humidity control system shown diagrammatically in Figure 5.

Various forms of humidity control systems are known and are available commercially so that only a brief description is given of the system.

Referring to Figure 5, the system 62 includes an automatic control arrangement 64 including a pump (not shown) and a valve device that enables a selection to be made of gas, ordinarily air, traversing a humidifier chamber 66 or a dessicant chamber 67. The automatic control arrangement is set to a selected humidity with the aid of an adjustable calibrated regulator 68 and provides humidified or dried air or a combination of both as required to its output 65 connected to the inlet 50 of the chamber 10. With the aid of the signal from sensor 60 the automatic control arrangement maintains the humidity at the preselected value.

Reverting to Figure 1, the predetermined suspended length I of the test strip 32, which was originally measured against an external gauge will in fact change with changes of humidity in the chamber 10. Since the lower end of the strip is freely suspended, the clamp 34 and parts attached to it will displace vertically with changes in 1, which changes will be referred to as Al. These displacements are sensed by the transducer assembly 70.

In this embodiment the assembly 70 uses a device known as a linearly variable differential transformer (LVDT).

This is an electrical device which provides a voltage output that is a highly linear function of the displacement of a core member over a certain range of displacement. Such devices are well-known and will not be described in detail here. A suitable device for the present purpose is that manufactured by Schaevitz em Limited of Slough under the model No.

D401/01+0.500. These devices are sold complete, with individual calibration data.

The LVDT comprises a winding portion 72 through which extends a bore 74. The winding portion is seated in an aperture l la in the base of the chamber 10 and carries a cap 76 through which bore 74 extends and which is secured to the base 11 as by screws.

The winding portion is fixed to the chamber base 11 to have the bore 74 vertical. The core member 78 of the LVDT is secured to the bottom end of the lower rod portion 37a.

It will be noted that the clamp 34, rod 36, weight 38 and core member 78 are freely suspended from the strip 32 so that core 78 moves substantially freely in bore 74 and is displaced from its initial position in accordance with changes Al in the strip length. The just-mentioned items also provide a predetermined weight to load strip 32. The loading may be changed by adding one or more radially-slotted weights 39 on the rod portion 37b above weight 38.

The dimensions of the test station are so chosen that core member 38 will normally lie at about the mid-point of the linear characteristic of the LVDT.

The lower end of the transformer winding portion 72 is located in an aperture 15a in a sub-base member 15 secured to chamber base 11 as will be described below with reference to Figure 3. The lower end of bore 74 is closed by a plug 75 to prevent leakage from the chamber through the bore 74.

The winding portion 72 contains the appropriate transformer windings. In the model quoted above, the transformed is energised by direct current on lead 79 and provides a direct current output on lead 80.

The winding portion 72 contains the necessary electronic - circuitry to convert the input d.c. to a.c. for the transformer and to convert the a.c. output indicative of the position of core 78 to d.c. on line 80. With other LVDTs additional external circuitry may be needed. The signal-processing for the transducer and more particularly for a set of twenty transducers is described later with reference to Figure 6.

Having described generally the construction of one test station in chamber 10, some further details will now be explained with reference to Figures 2, 3 and 4, the latter two figures showing details pertinent to the arrangement of the plurality of test stations.

Looking at Figure 2, this shows one form of support and attachment means for a strip 32 in a test station. The support means, which in Figure 1 was shown purely diagrammatically as a clamp 34, comprises an L-shaped bracket 90 apertured at 91 to be supported on pin 21 projecting from rail 20. The foot 93 of the bracket is secured centrally as by a nut and bolt or rivet 94 to the base of an inverted paper clip 95, which grips the upper portion of the strip 32. The lower attachment means (clamp 34 in Figure 1) is also a paper clip 96 centrally apertured to receive the upper end of rod portion 37b which may be threaded and secured by nuts 97. The distance I between the upper point of suspension and lower point of attachment is marked in Figure2.

Looking now at Figure 3 this shows a sectional side view in a practical embodiment of chamber 10.

The front wall 16 of the chamber contains a large access cut-out 16a extending the length of the rows of test stations. It is closed by a door 17 that, when closed, seats against a soft rubber sealing ring 18 extending around the cut-out 16a so as to seal the chamber 10. The door is secured by hinged toggle latches 19 acting between the door and the front wall 16 and adjoining portions of a front panel 117 of the apparatus. Such latches are commercially available items and three may be placed along each of the top and bottom edges of the door. Obviously any suitable closure means may be used. The chamber walls should be made sufficiently thick and insulating that a uniform humidity can be established in chamber 10 unaffected by conditions outside the chamber.This also applies to door 17 and conveniently it is made of transparent material to allow the test samples to be viewed at all times. The door can comprise a piece of half-inch thick Perspex (Registered Trade Mark).

Mention was made of arranging the test stations in parallel rows and Figure 3 shows two parallel support rails 20a, 20b for this purpose. The transducer assemblies are also arranged in parallel rows, two assemblies 70a and 70b being shown, one in each row. The sub-base 15 is shown secured to the chamber base 11 by bolts 81 carrying spacer pillars 82.

To support two parallel rails at a lower height in the chamber the end wall 13 carries fore and aft brackets 24a, 24b each being slotted to receive a respective rail and the two rails and brackets being apertured to be secured by common bolt 83. The other end wall carries like brackets.

It will be noted that in Figure 3 the support and attachment means for the test strips are of a different design to that shown in Figure 2. One test station supported from rail 20b is shown. The upper clamp 99 comprises a plate 100 apertured for support on a screw 21 set in rail 20b. The lower end of the plate carries a slotted member 101 in which the upper end portion of the test strip 32 is received and secured by a set screw 102 urging an insert 103 against the test strip.

The lower attachment means 99' is of similar design and is secured to the rod 36 leading to the transducer assembly 70b.

In the arrangement shown in Figure 3, the test stations, which as stated may be twenty in number, could be provided with a common lifting mechanism corresponding to platform 40 in Figure 1. However, it is preferred to divide the test stations into groups of four with a lifting platform for each group. Such a platform is denoted 110 in Figure 3 and a diagrammatic plan view of one such platform and part of two adjacent platforms is seen in Figure 4. The platform is arranged to engage and lift the attachment means of two test stations in the rear row and two adjacent stations in the front row.The platform is apertured at 11 it and 11 lb (Figure 4) to receive the respective rods 36 of the four stations and is supported at its centre by a rod 112 (Figure 3) secured at 112' to the centre of the platform and slidable in a vertical tube 113 having a flanged end 113a secured to the chamber base 11, the tube 113 depending from the base and the lower end of the tube 113 being received in an aperture 114 in the sub-base 15.The rod 112 is a close fit in tube 113 to prevent undue leakage from chamber 10 and is moved vertically in the tube by a Bowden cable mechanism 115, the wire of which is connected at one end to the bottom of rod 112 and that terminates at the other end in a manually operable control knob 116 prqjecting from the front panel 117 of the apparatus fixed to the front of chamber 10. In Figure 3, the platform 110 is shown in its lower position. By pushing control knob 116 inward, the platform is raisable to engage parts 104 carried by the attachment means and lift same to a position at which the strip 32 hangs limply without tension. To assist in guiding the vertical movement of the platform 110 it is provided with an aperture 118 (Figure 4) which embraces a vertical post 119 upstanding from chamber base 11 (Figure 3).

For convenience of access to the test stations, those in the front row from the door 16 are staggered relative to those in the rear row by half the spacing between adjacent stations in a row, the spacing being uniform. This is seen in Figure 4 where the platforms 110 each has the shape of a parallelogram to accommodate the stagger between the rear row apertures 11 lea and the front row apertures Illb.

Before discussing the signal-processing, the setting up of test strips in the hygroextensometer apparatus will be described.

In this particular embodiment up to twenty sample strips may be tested simultaneously.

The strips are supported with the required gauge length I in the test stations and the door 17 secured to close the chamber 10.

The appropriate platform or platforms are raised to remove the tension from the strips and the required humidity set by the regulator 68 of the automatic humidity control system (Figure 5). Sufficient time is allowed to establish the desired humidity in the chamber, whereupon the lengths of the strips are measured with the aid of their respective transducers to indicate extensions or contractions of length. The measurements can be repeated at various levels of humidity to ascertain the dimensional stability of the materials of the strips under varying conditions of humidity.

The processing of the transducer output signals will now be described with reference to Figure 6 which is a schematic diagram of the circuitry for selecting the output of a desired transducer, processing the selected output signal and digitally displaying the processed signal.

As stated above the LVDT type particularly specified is designated to be energised with d.c. and provides a d.c.

output signal. The specified LVDT is a four terminal device having the input and output terminals separated. In the circuit to be described the negative input terminal is grounded as is one of the output terminals.

The specified LVDT has a linear range of operation of +0.5 inches (approximately 12.5 mm.). At the midpoint of this range the output voltage is zero, the output voltage reversing polarity as the core 78 (Figure 1) passes through the midpoint position. Thus the output can be selected to be of a given polarity for expansion or contaction of the strip as required. The output voltage for a displacement of 0.5 inches from the midpoint (in either direction) is about 5 volts. It varies slightly from transducer to transducer and each device is supplied with appropriate calibration data giving the precise value for that device.

For the convenience of the operator the test stations in chamber 10 are numbered from (1) to (20); for example, the station shown in Figure 1 is No. (3). The respective output leads 80 (Figure 1) of the transducers, i.e. twenty in all, are taken to respective inputs of an analog switching circuit which in Figure 6A are marked (1)' to (20)' in conformity with the station numbering. Each input goes to a respective potentiometer network of an input network 120. Each potentiometer network comprises a fixed resistor R1 in series with a pre-set potentiometer R2 from the wiper of which an adjusted output signal level is obtained as will be explained. The wiper outputs of input network 120 are taken to respective inputs SI to S20 of the multiplexer or switching network 122 through which a selected adjusted transducer output is applied to a line 123.The network 122 uses analog multiplexers type AD7501K each of which is capable of handling eight inputs.

Three devices Cl to C3 are thus needed.

The selection among the multiplexers is controlled by a selection circuit 124 shown in Figure 6B which is controlled by the operator by means of a pair of decade thumbwheel-operated switches 125 and 126 of the type having a BCD (binary-coded decimal) output. Switch 125 selects the tens of the station number-in this embodiment only numbers 0, 1 and 2 are needed while switch 126 selects the units. The BCD outputs of the switches 125, 126 are decoded to three bit binary signals by BCD-to-binary converters A, and A2 (type SN74184) devices. The three bits, representing one of eight possible selections, appear on respective lines 127-9 and are applied in parallel to selection inputs Ao to A2 of the multiplexers Cl to C3 (Figure 6A). This controls which input of each multiplexer is selected.

It is also necessary to select only one of the multiplexers at a time which is done by signals applied to their respective enable inputs EN. The decoding effected by devices A, and A2 of the selection network 124 is such that through an output Y2 of device A1, only multiplexer C3 is enabled via a line 130 for the numbers (17) to (20) set upon the thumbwheel switches. For the numbers (9) to (16) an output Y1 of device A, is activated to enable multiplexer C2 on line 131. If, and only if, neither output Y, or Y2 is actuated, then via a NOR-gate 133, multiplexer Cl is enabled for numbers (1) to (8) via line 132.

The three outputs OUT of the multiplexers are connected in parallel to line 123 and the selected adjusted signal from input network 120 is applied to signal-processing network 140. The network comprises a voltage-follower stage 141 using a type 741 operational amplifier. This stage provides a high input inpedance that does not significantly load the multiplexer outputs and, therefore, does not modify the value of the analog signal transmitted by the selected multiplexer. The output of stage 141 goes to a low pass amplifier stage 142 using another type 741 device, the stage having unity gain and the capacitor C in the feedback loop providing a low pass characteristic to reduce transients etc.

Because the transducer outputs may assume either polarity, the multiplexer stage 122 and the amplifier stages 141, 142 must be capable of handling either polarity. The circuitry shown is capable of doing this and the smoothed output from stage 142 will swing positive or negative depending on the polarity of the transducer output connected to it at that time.

The output of stage 142 is applied to a digital display device, e.g. an Analog Devices type 2010, which provides a 3+ digit display and which also senses and displays polarity. The display and preceding electronic circuitry are set up to display a zero value for zero input volts. The cores 78 of the LVDTs can now be successively displaced by 0.5 inches, the appropriate transducer output successively selected, and the associated input potentiometer network R1, R2 adjusted until this displacement shows on the display. It is preferred to display in a different standard unit of length, namely metric, and the 3T digit display is set to show 12.50 mm. The display is conveniently mounted on the front panel 117 (Figure 3) of the apparatus so as to be readily seen by the operator.

Thus by adjusting the respective potentiometer networks of the transducers to produce the same change in reading on the display 145 for the predetermined displacement, the slightly different output sensitivities of the transducers are compensated and equal sensitivities are obtained in respect of the adjusted signal levels at the wipers of potentiometers R2 for subsequent processing in the following circuitry.

In operation, the described apparatus is simple to use in that the operator can easily make a set of measurements at a given humidity level by dialling each test station number required on the thumbwheeloperated switches and reading the digital display.

To complete the electronics circuitry, Figure 6C also indicates suitable power supplies 150. The transducers are energised by a 24 volt supply at terminals 152 which should be well-regulated for accurate operation. The multiplexers and the operation amplifiers require a +15/01-15 supply provided at terminals 153. The logic in the selection circuit and the display require respective five volt supplies at terminals 154 and 155. The design of suitable supplies need not be discussed in detail.

In Figure 6, pin numbers of the devices used are also marked together with relevant component values.

In the above description, a rectangular shaped chamber 10 has been described with two rows of straight and parallel test stations. The chamber could be of cylindrical form with the test stations arranged in concentric circular rows about the cylinder axis.

The foregoing description has been particularly concerned with testing the dimensional stability of paper. The invention is equally applicable to testing other materials, for example textiles.

WHAT WE CLAIM IS: 1. A hygro-extensometer apparatus comprising a closable chamber having inlet means for introducing a gas of controlled humidity to the chamber, and a test station within the chamber including: a support means for receiving and suspending a strip of material to be tested; a displacement responsive transducer below said support means, said transducer having a movable element and being operable to provide an output signal that is a measure of the position of said element over a range of displacement thereof, said element being secured to means for attaching to a lower portion of such a suspended strip at a predetermined distance from the point of suspension at said support means such that, in operation, the transducer provides an output signal indicative of changes in the predetermined suspended length of the strip due to changes of humidity within the chamber.

2. A hygro-extensometer apparatus as claimed in Claim 1 further comprising a part mounted for movement between an upper position supporting the attachment means at less than said predetermined distance from said point of suspension, and a lower position allowing said attachment means to be suspended freely at said predetermined distance from said point of suspension from a strip of material suspended from said support means; and manually-operable means external to said chamber and coupled to said part for moving said part from one position to the other.

3. A hygro-extensometer apparatus as claimed in Claim 2 in which said attachment means includes as rod securing it to the movable transducer element, the rod being vertical when the attachment means is freely suspended as aforesaid, and said part comprises a platform having an aperture therein through which the rod extends.

4. A hygro-extensometer apparatus as claimed in any preceding Claim further comprising an adjustable humidity control system connected to said chamber inlet means, and a humidity sensor located in said chamber to provide a signal representing the humidity therein, said humidity control means including automatic means responsive to said humidity-representing signal to maintain a pre-selected humidity in the chamber.

5. A hygro-extensometer apparatus as claimed in any preceding claim in which said transducer comprises a linearly variable differential transformer having an energisable winding portion with a bore therein, the winding portion being fixedly secured with respect to the chamber to have the bore vertical, and said movable element constituting the core member of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The output of stage 142 is applied to a digital display device, e.g. an Analog Devices type 2010, which provides a 3+ digit display and which also senses and displays polarity. The display and preceding electronic circuitry are set up to display a zero value for zero input volts. The cores 78 of the LVDTs can now be successively displaced by 0.5 inches, the appropriate transducer output successively selected, and the associated input potentiometer network R1, R2 adjusted until this displacement shows on the display. It is preferred to display in a different standard unit of length, namely metric, and the 3T digit display is set to show 12.50 mm. The display is conveniently mounted on the front panel 117 (Figure 3) of the apparatus so as to be readily seen by the operator. Thus by adjusting the respective potentiometer networks of the transducers to produce the same change in reading on the display 145 for the predetermined displacement, the slightly different output sensitivities of the transducers are compensated and equal sensitivities are obtained in respect of the adjusted signal levels at the wipers of potentiometers R2 for subsequent processing in the following circuitry. In operation, the described apparatus is simple to use in that the operator can easily make a set of measurements at a given humidity level by dialling each test station number required on the thumbwheeloperated switches and reading the digital display. To complete the electronics circuitry, Figure 6C also indicates suitable power supplies 150. The transducers are energised by a 24 volt supply at terminals 152 which should be well-regulated for accurate operation. The multiplexers and the operation amplifiers require a +15/01-15 supply provided at terminals 153. The logic in the selection circuit and the display require respective five volt supplies at terminals 154 and 155. The design of suitable supplies need not be discussed in detail. In Figure 6, pin numbers of the devices used are also marked together with relevant component values. In the above description, a rectangular shaped chamber 10 has been described with two rows of straight and parallel test stations. The chamber could be of cylindrical form with the test stations arranged in concentric circular rows about the cylinder axis. The foregoing description has been particularly concerned with testing the dimensional stability of paper. The invention is equally applicable to testing other materials, for example textiles. WHAT WE CLAIM IS:

1. A hygro-extensometer apparatus comprising a closable chamber having inlet means for introducing a gas of controlled humidity to the chamber, and a test station within the chamber including: a support means for receiving and suspending a strip of material to be tested; a displacement responsive transducer below said support means, said transducer having a movable element and being operable to provide an output signal that is a measure of the position of said element over a range of displacement thereof, said element being secured to means for attaching to a lower portion of such a suspended strip at a predetermined distance from the point of suspension at said support means such that, in operation, the transducer provides an output signal indicative of changes in the predetermined suspended length of the strip due to changes of humidity within the chamber.

transformer and being received in said bore and freely suspended therein when the attachment means is attached as aforesaid to a strip of material suspended from the support means.

6. A hygro-extensometer apparatus as claimed in Claim 5 when dependent on Claim 3 in which said core member is rigidly secured at the lower end of said rod.

7. A hygro-extensometer apparatus as claimed in any preceding claim comprising a signal-processing circuit connected to the output of said transducer to provide a display indicative of the length of a strip suspended between point of suspension and the attachment means as sensed by said transducer.

8. A hygro-extensometer apparatus comprising: a closable chamber having inlet means for introducing a gas of controlled humidity to the chamber; and a plurality of test stations within said chamber each test station including a support means for receiving and suspending a strip of material to be tested; a displacement-responsive transducer below said support means, said transducer having a movable element and being operable to provide an output signal that is a measure of the position of said element over a range of displacement thereof, said element being secured to means for attaching to a lower portion of such a suspended strip at a predetermined distance from the point of suspension at said support means such that, in operation, the transducer provides an output signal indicative of changes in the predetermined suspended length of the strip due to changes of humidity within the chamber.

9. A hygro-extensometer apparatus as claimed in Claim 8 further comprising a part mounted for movement between an upper position supporting the attachment means of said plurality of test stations or an adjacent group thereof at less than said predetermined distance from the associated point of suspension; and a lower position allowing each of said attachment means to be suspended freely at said predetermined distance from the associated point of suspension from a respective strip of material supported at the respective test station; and manually-operable means external to said chamber and coupled to said part for moving said part from one position to the other.

10. A hygro-extensometer apparatus as claimed in Claim 9 in which each attachment means of said plurality of test stations or group thereof, as the case may be, includes a rod securing it to the movable transducer element, the rod being vertical when the attachment means is freely suspended as aforesaid, and said part comprises a platform having a plurality of apertures therein, each rod extending through a respective aperture.

11. A hygro-extensometer apparatus as claimed in Claim 8, 9 or 10 further comprising an adjustable humidity control system connected to said chamber inlet means, and a humidity sensor located in said chamber to provide a signal representing the humidity therein, said humidity control system including automatic means responsive to said humidity-representing signal to maintain a pre-selected humidity in the chamber.

12. A hygro-extensometer apparatus as claimed in any one of Claims 8 to 11 in which each transducer comprises a linearly variable differential transformer having an energisable winding portion with a bore therein, the winding portion being fixedly secured with respect to the chamber to have the bore vertical, and said movable element constituting the core member of the transformer and being received in said bore and freely suspended therein when the associated attachment means is attached to a strip of material suspended in the associated test station.

13. A hygro-extensometer apparatus as claimed in Claim 12 when dependent on Claim 10 in which each core member is rigidly secured at the lower end of the said rod of the associated attachment means.

14. A hygro-extensometer apparatus as claimed in any one of Claims 8 to 13 comprising a selection circuit connected to the respective outputs of said plurality of transducers select the output of the transducer of a selected test station, and signal-processing means responsive to the selected output to provide a display indicative of the length of the strip suspended at the selected test station as sensed by the associated transducer.

15. Apparatus as claimed in Claim 14 in which said selection circuit includes a manually-operable selection arrangement mounted externally of said chamber and having a plurality of selection positions respectively corresponding to said test stations, and a switching circuit having a plurality of inputs respectively connected to said transducer outputs and an output connected to said signal-processing means, said switching circuit being connected to said selection arrangement to transfer to its output the transducers output of the selected test station.

16. Apparatus as claimed in Claim 14 in which the transducer outputs are connected to the inputs of the switching circuit through respective adjustable circuits for equalising the signal levels obtained from the transducers.

17. Apparatus as claimed in Claim 7, 14, 15 or 16 in which the display provided by said signal-processing means is a digital display in a recognised unit of length.

18. A hygro-extensometer apparatus substantially as hereinbefore described with reference to Figures 1 and 3 to 6, or Figures I and 3 to 6 as modified by Figure 2 of the accompanying drawings.