Reasons

• Due to inkbottle effect, saturation pressure required for emptying is smaller than filling because the bottom radius is larger than mouth radius.
• Contact angle of advancing (filling) is greater than contact angle of receding (emptying).

Fig:4.8: Saturation curves for non porous material(solid curve) and for material with almost constant pore radius (dotted)

Mercury Porosimeter

It works on the same principle. As contact angle for mercury is , it does not fill pores by capillary condensation. To fill the pores, external pressure has to beapplied.

All pores of radius would get filled.