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