What is the specification of the AP4 Porometer blotting pads?
In general the paper pad material for the AP4 calibration plate is not critical, as long as it is completely clean and grease free.
Filter or blotting paper is good. It is absorbent and will not contaminate the water used. Make sure it is a paper-based filter, not plastic. The finer the fibres in the paper the more water it will hold and the longer it will last before becoming dry.
We currently use blotting pads 32x60mm in size.