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