GB1452273A - Monitoring web conductivity - Google Patents

Abstract

1452273 Measuring the thickness of a web coating: measuring conductance KODAK Ltd 1 Oct 1973 [26 Oct 1972] 49424/72 Headings G1N and G1U The conductance, and hence the average thickness, of a coating 13, Fig. I, on a web 12 is measured by positioning two electrodes 11, 11 adjacent to the coating, thereby forming with the portions of the coating 13 immediately opposite the electrodes two air-dielectric capacitors, which are connected together by the portion of the coating between the two electrodes, by applying a squarewave voltage pulse, Fig. 2a, between the electrodes, and by measuring the peak current that flows. Current will, of course, only flow, as shown in Fig. 2b, at the edges of the voltage pulse, and in the preferred embodiment the first current pulse x is gated to a peak follower and the second y is utilised to reset the peak follower, thereby producing a pulse, Fig. 2d, whose height is proportional to the conductance - and hence average thickness - of the coating 13. The arrangement is particu- larly useful for measuring the thickness of a coating on a moving web. A plurality of electrodes are arranged in a row transversely to the direction of movement of the web Fig. 3, (not shown) and switching circuitry is provided whereby voltage pulses are sequentially applied between each adjacent pair of electrodes. This results in a sequence of pulses, like that of Fig. 2d, which may, for example, be displayed on an oscilloscope, to provide a simple indication of coating thickness and its variation across the web.