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