Common features of porosimetry curves

All mercury intrusion-extrusion curves have certain characteristics in 

On porous powdered samples intrusion takes place at low pressures as mercury penetrates the large interparticle voids. Additional intrusion occurs at higher pressures into pores within the particles. 

All porosimetry curves exhibit hysteresis. That is, the path followed by the extrusion curve is not the same as the intrusion path. At a given pressure the volume indicated on the extrusion curve is greater than that on the intrusion curve and for a given volume the pressure indicated on the intrusion curve is greater than that on the extrusion curve. 

Upon completion of a first intrusion-extrusion cycle some mercury is always retained by the sample, thereby preventing the loop from closing. 

Intrusion-extrusion cycles after the first will continue showing hysteresis but eventually the loop will close, showing that entrapment of mercury eventually ceases. On most samples the loop closes after the second pressurization-depressurization cycle. 

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