{HEAD ~ a pair} of clamping devices, each said clamping device being disposed in {TAIL ~ a respective flow system} to hold a porous sheet like material sample across said respective system for a flow of fluid therethrough; and 
{ HEAD ~ a pair } of clamping devices , each said clamping device being disposed in { TAIL ~ a respective flow system } to hold a porous sheet like material sample across said respective system for a flow of fluid therethrough ; and
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH