Abstract: A device or method for monitoring the gas-permeability of a sheet material is disclosed. The device comprises a first gas-flow chamber having an inlet, which, during operation of the device is closed by the sheet material extending across the inlet. A second flow chamber has an outlet by which a gaseous medium can be directed towards the sheet material with a leakage path whereby a significantly greater proportion, suitably 90%, of the gas-flow in the second flow chamber is permitted to leak to the atmosphere. This leakage path is provided by spacing the gas outlet of the second chamber from the position in which the sheet material extends across the gas inlet of the first chamber. A measurement means, preferably a pressure transducer is connected across a low-impedance laminar-flow device.

Abstract: A device for the determination of the gas permeability of a material such as paper includes a holder 1 for holding a sample across a gas flow passage of predetermined cross section and a pump 2 for drawing air through the passageway via one or more constant flow device C.sub.1 -C.sub.6 arranged in parallel. Each constant flow device has a closable control valve V.sub.1 -V.sub.6 respectively. A first pressure transducer measures the pressure drop across the sample and a second pressure transducer, in cooperation with a lamina flow element measures the rate of air flow through the sample. A by-pass by-passes selectively three of the control valves of the constant flow devices. The by-pass has valves B.sub.1 -B.sub.3 arranged in parallel and operable to close the by-pass to gas flow therethrough to the respective constant flow devices C.sub.1 -C.sub.6.The by-pass also includes an atmospheric bleed 11 and a valve B.sub.4 closable to stop the flow of air into the by-pass.