Measuring head for the measurement of the porosity of a moving web

p0001The invention relates to a measuring head for the measurement of the porosity of a moving web of porous material, with a measuring chamber in which there is a negative pressure, a measuring plate covering this measuring chamber, over which the web is moved, measuring openings provided in this measuring plate, The porous web being sucked in, a measuring air nozzle through which the sucked-in air is sucked out of the measuring chamber, and a device arranged around the measuring plate for sucking the web onto the edge of the measuring plate.

p0002Such a measuring head is used for the porosity measurement on the running track. Particularly in the case of papers, for example filter papers, and woven materials, porosity is an important quality parameter. Its continuous monitoring by means of a one-line measuring method, for example for monitoring and controlling the production process, is therefore of particular importance. For an impeccable porosity measurement, the upper edge of the measuring chamber must be sealed as far as possible airtight against the path moved thereover so that air sucked exclusively by the porous web itself passes into the measuring chamber. For this purpose, the web sliding over the measuring head must be pressed onto the edge of the measuring plate.

p0003In the measuring head for porosity measurement disclosed by US Pat. No. 3,466,925, additional holes are arranged around the measuring openings provided in the measuring plate in a plurality of concentric rings through which special suction air is sucked. The web moved over the measuring head is thus sucked onto the edge of the measuring plate. In order to achieve a perfect seal between the moving web and the fixed measuring head, considerable suction pressures are necessary. The porous web, which is mostly produced for the most part, is therefore pressed against the edge of the measuring plate with a relatively high surface pressure and grinds over the measuring head. Here is a great risk that fibers loosen from the not yet fully settled tissue structure and either remain on the web itself liable and the quality of the finished Ba reduce hn or arrive as a result of Meßvakuums in the measuring chamber and cause blockages, thereby Porosity measurement.

p0004It is therefore the object of the present invention to improve a measuring head of the type mentioned at the outset in such a way that the disadvantages described above are avoided, in particular the web moved over the measuring plate is stressed as little as possible so as to achieve an improved seal between the measuring chamber and the web and thus achieve a higher measuring accuracy becomes.

p0005This object is achieved by a measuring head according to the preamble of the first patent claim, and the object is achieved in that the device for sucking the web has a circumferential blowing slot with an outlet opening directed tangentially to the underside of the web.

p0006In the measuring head according to the invention, instead of the suction air used in conventional measuring heads, blown air is blown tangentially against the underside of the moving web for sucking the web onto the edge of the measuring plate. The air, which flows in this way between the underside of the web and the outer edge of the measuring head, produces a vacuum which sucks the web onto the edge of the measuring plate and thus effects a lateral sealing of the measuring chamber. In this case, the porous web does not grind over the surface of the measuring head, but slides over the measuring plate on an annular air cushion serving at the same time as a seal. The mechanical stress on the sensitive web of porous material is therefore substantially lower than in the previously known measuring heads. An additional advantage is that fibers which are loosely adhered to the underside of the web as well as dirt particles are not sucked into the measuring head, but are blown off from the surface of the web. The interior of the measuring head, and in particular the measuring chamber, remain largely free from contamination and deposits. Since the moving web passes through a vacuum zone before reaching the actual measuring openings, pre-evacuation of the cavities in the porous material takes place, as a result of which a distortion of measurement due to entrained air entering the measuring chamber is prevented. The measuring head according to the invention therefore also permits an increase in the accuracy of the porosity measurement.

p0007Expediently, the blowing slot tapers towards the blow-out opening in order to achieve a sufficiently high exit velocity of the blow air for establishing a sufficient negative pressure.

p0008In a preferred embodiment of the measuring head according to the invention, an annular channel surrounding the measuring chamber, which passes into the circulating blowing slot, serves for uniform supply of the blowing slit with blown air, which is fed by a suitable blower via at least one connecting stub opening into this annular channel.

p0009In a further advantageous embodiment of the invention, the measuring plate is surrounded by a guide plate which extends as far as the discharge opening. This guide plate guides the moving web in the vertical direction, so that the guide plate is always completely covered. Expediently, the tops of the measuring plate and guide plate lie flush and thus form a common planar surface over which the web can slide freely. A slight chamfering of the guide plate at its circumference makes it easier for the moving web to reach the measuring head.

p0010In a preferred embodiment of the invention, the annular channel for the distribution of the blow air is essentially formed by a circulating blow ring. Advantageously, this blow ring has an approximately circular-section cross-section and, on its flat underside, has a semicircular groove which forms the annular channel. The rounded contour of the upper side of the blow ring promotes the formation of the air flow producing the desired negative pressure at the periphery of the measuring head. A particularly calm, plane air flow is formed when the blow ring has a flat flattening parallel to the web on its upper side.

p0011An embodiment in which the blowing slot is formed by the underside of the guide plate and the top of the blow ring is particularly preferred. Expediently, the guide plate covers the blow ring approximately halfway. In addition, if the blow ring is installed vertically displaceably, the thickness of the blow-through slot can be adjusted in a simple manner by changing the distance between the blow ring and the guide plate. With a measuring head of this kind, the force with which the moved web is sucked can be easily adapted to different measuring conditions and web materials.

p0012In an expedient development of the measuring head according to the invention, the measuring plate, the guide plate and the blow ring are mounted on a common carrier plate surrounding the measuring chamber. However, other constructive solutions are also conceivable in which, for example, the measuring plate and the guide plate are designed in one piece. If an elastic sealing ring is provided between the support plate and the installed blow ring, the distance between the blow ring and the guide plate and thus the thickness of the blow slot can be achieved by more or less strong tightening of the connecting elements, the sealing ring yielding elastically.

p0013In an advantageous further development of the invention, an additional annular measuring channel is provided on the upper side of the measuring head between the measuring openings and the blow-out opening, by means of which the negative pressure prevailing directly on the underside of the path can be measured. A negative pressure measurement at this point is a prerequisite for controlling the quantity of blowing air flowing out of the blow-out opening in such a way that the negative pressure acting thereby on the underside of the bottom corresponds approximately to the negative pressure prevailing in the measuring chamber. A pressure gradient along the path in the region of the measuring plate edge can then not occur, as a result of which measurement distortions due to the air flowing obliquely or even laterally through the porous path are avoided.

p0014A circular design of the measuring head according to the invention is particularly preferred so that the blow slot concentrically surrounds the measuring plate. The rotationally symmetrical design of the measuring head is characterized by particularly good aerodynamic properties and is simple and thus inexpensive to manufacture.

p0015Embodiments of the invention will now be described with reference to the accompanying drawings. Show it:<ul><li>1 shows a measuring head with a central measuring air connection;</li><li>FIG. 2 shows a second embodiment of the measuring head according to the invention with a measuring air nozzle designed in the form of a ring and a likewise annular filter arranged in the measuring chamber;</li><li>FIG. 3 shows a third embodiment of the measuring head according to the invention with a measuring air nozzle designed in the form of a ring and a filter in the form of a hollow cylinder arranged in the measuring chamber;</li><li>FIG. 4 shows a fourth embodiment of the measuring head according to the invention with a central measuring air nozzle and an axially arranged filter covering the latter;</li><li>FIG. 5 shows a further embodiment of the measuring head according to the invention with an annularly formed measuring air nozzle, annular filter and additional suction connection for connecting a vacuum cleaner.</li></ul>

p0016In all figures, only one half of the rotationally symmetrical measuring head is shown in a vertical section.

p0017According to FIG. 1, the web of porous material whose porosity is to be measured is moved in the direction indicated by an arrow over the top of the measuring head. The measuring chamber 2 is covered upwards by a measuring plate 3 in which measuring openings 4 are provided. The measuring chamber 2 opens at the bottom into a central measuring air connection 5, to which a device (not shown) for generating a vacuum is connected. During the measuring process, air is sucked into the measuring chamber 2 through the porous web 1 and the measuring openings 4. The quantity of suctioned air which is measured with a suitable measuring device is a measure of the porosity of the web 1. The vacuum prevailing in the measuring chamber 2, which is kept as constant as possible during the measurement, can be measured separately via a separate measuring channel 6 . The circular measuring plate 3 is surrounded by a guide plate 7, which is also round. The tops of the measuring plate 3 and the guide plate 7 are flush, so that they form a common flat surface over which the web 1 slides. At its outer periphery, the guide plate 7 has a slight bevel 8. The guide plate 7 partially covers a blow ring 9 of approximately circular-section cross-section and a flat flattening 10 lying parallel to the web 1 on its upper side. On its flat underside, the blow ring 9 has a semicircular groove 11 which forms an annular channel 12 concentrically surrounding the measuring chamber 2. During the measuring operation, blowing air is introduced into the annular channel 12 via a connection piece 13. The annular channel 12 merges into a blowing slot 14, which surrounds the measuring plate 3 and which is formed by the curved underside of the guide plate 7 and the opposing upper side of the blowing ring 9. The blow-out opening 15 of this blow-slot 14 is directed outwards tangentially to the underside of the moving track 1. The blown air flowing through it generates a vacuum between the underside of the web 1 and the circularly curved outer contour of the blowing ring 9, as a result of which the web 1 is pressed onto the edge of the guide plate 7. Any particles adhering to the underside of the web 1 are thereby blown outwards. The continuous tapering of the blow slot 14 toward the blow-out opening 15 causes a sufficiently high blow-out speed of the flowing blow air. The measuring plate 3, the guide plate 7 and the blow ring 9 are common. Mounted on a round carrier plate 16. An elastic sealing ring 17 is provided between this carrier plate 16 and the blow ring 9. The blow ring 9 is mounted so as to be vertically displaceable such that the thickness of the blow slot 14 can be adjusted by changing the distance between the blow ring 9 and the guide plate 7, the sealing ring 17 being able to yield elastically.

p0018The measuring chamber 2 of the second exemplary embodiment of the measuring head according to the invention shown in FIG. 2 is characterized by an enlarged diameter with simultaneously reduced height. The measuring air connection 5 is annular and runs around the lower edge of the measuring chamber 2. The measuring plate 3 is integrally formed on the carrier plate 16. This results in a generally lower overall height of the measuring head. A ring-shaped filter 18 is provided in the measuring chamber 2 and is arranged between the measuring openings 4 and the measuring air connection 5. It serves to filter the measuring air flowing through the measuring chamber 2 in order to protect the sensitive measurement system, which is connected to the measuring air connection 5, from dust and dirt particles.

p0019FIG. 3 shows an embodiment of the measuring head according to the invention, which is distinguished by a substantially enlarged filter 18 arranged in the form of a hollow cylinder in front of the annular measuring air connection 5.

p0020FIG. 4 shows a further embodiment of the measuring head. The measuring chamber 2 is closed downwards by a cover 19, into which a central measuring air connection 5 with an annular circular outlet opening is inserted. The cylindrically designed filter 18 completely covers the measuring air connection 5. On the inside of the cover 19, the filter 18 is releasably mounted by means of a central support rod 20 and a fastening cap 21, so that it can be easily replaced.

p0021In the measuring head shown in FIG. 5, an additional suction connection 22 is provided on the side of the measuring chamber 2 opposite the measuring plate 3 for the connection of a vacuum cleaner (not shown). The filter 18 is located between this suction connection 22 and the annularly extending measuring air connection piece 5. To clean the filter 18 and the measuring chamber 2, scavenging air is introduced into the measuring chamber 2 through the filter 18 in the direction opposite to the direction of flow during the measuring operation and via the suction connection 22 Suctioned. If the quantity of the introduced scavenging air is metered in such a way that it does not exceed the amount of air sucked off by the suction connection 22, it is ensured that the measuring openings 4 are always flowed through from the outside to the inside and never vice versa so that no dirt or dirt is formed Dust on the inside of the measuring plate 3.