GB980058A - Improvements in or relating to devices and methods for measuring variations in the air permeability and/or surface roughness of a moving web of material - Google Patents

Abstract

980,058. Measuring variations in air permeability and/or surface roughness. WIGGINS TEAPE RESEARCH & DEVELOPMENT Ltd. Oct. 8, 1962 [July 7, 1961], No. 24764/61. Headings G1M and G1S. A method of measuring variations in the air permeability and/or the surface roughness of a continuously moving web of paper or like porous and/or rough material to improve or control the manufacture thereof, comprises applying a measuring head having an open measuring aperture to the moving web so that the web covers the aperture, reducing the pressure in the aperture relative to ambient pressure, maintaining in fixed or substantially fixed relation to a datum parameter one of two parameters i.e. the air pressure in or the air flow through the aperture, and measuring variations in the other of the two parameters. In Fig. 1a of the Provisional drawings a measuring head P is connected by a duct H to be exhausted by a vacuum pump A. An orifice plate 0 is located between the head P and pump A. The head P is applied to a moving web of paper so that air is drawn through the web. The air permeability of the paper can be measured either by operating a control valve E to maintain a constant flow through the orifice plate 0, so that variations of pressure in the head P give an indication of variations of the porosity of the web, or by operating the valve E to maintain the pressure in the head P constant, in which case the variations in flow through the orifice plate 0 follow the variations in porosity of the web. The valve E can be placed as shown, or between the orifice plate 0 and the pump A, or in a side tapping leading to the atmosphere. In Fig. 2 of the Provisional drawings a vacuum pump A is again provided with the valve E in a side tapping U. The duct H leads to the measuring head P through an orifice Q and also to an adjustable simulator restrictor S through an orifice R. The sizes of the orifices Q, R are either the same or in fixed ratio and a pressure detector D is connected between the points L and N to detect any pressure unbalance between these two points. The head P is applied to a moving web as before and variations in the air flow through the head cause pressure variations at the point L causing the detector D to give an output which is fed to a controller C which in turn adjusts the restrictor S to re-balance the pressures at the points L and N. The output of the controller C may be recorded on a chart recorder G and this gives an indication of the variations in air permeability of the web. A pressure controller B is connected at L to sense deviations from the desired pressure in the head P and to open or close the valve E to correct for this. The orifice plates may be eliminated and the detector D replaced by a pressure transmitter acting on the controller C in an alternative arrangement (Fig. 3, not shown). In further alternative arrangements (not shown) the air passing through the head may be filtered, means may be provided for raising and lowering the head on to the web, and this may be coupled to a compressed air supply for blowing out the head, which supply is automatically connected for a short time interval on raising the head. Arrangements for effecting this, for automatically disconnecting the chart recorder whilst the flow is taking place, and for checking the calibration of various parts of the apparatus are described in the Specification. A suitable form of head 18 is shown in Fig. 6b of the Provisional drawings, the head having a top plain surface with an aperture consisting of three parallel slots 18a, 18b, 18c to prevent the web being drawn into the aperture. Guard slots 52, 53 which may be in the form of a ring surrounding the aperture are provided and are connected to the vacuum pumps separately from the aperture. The guard slots serve to hold the web on the head 18. Another form of head is shown in Fig. 8a of the Provisional drawings and the air permeability of the web 77 is computed in this case by the amount of air passing into the slot 74 through the web. Another slot 73 is provided and apart from the air passing through the web 77 air also enters this slot over a blunt knife edge 75 from an open slot 72. By comparing the measurements at the slots 73 and 74 a value for the surface roughness of the web 77 may be computed.