How to Choose right ppi of ceramic foam filter

Ceramic foam filter is just developed as a new type molten metal filters to decrease casting flaw and also as a catalyst carrier for Air Treatment in recent years, play a vital role in the metal casting and air treatment, where they are used to filter out impurities in molten metal to improve the quality and performance of the end product or decompose organic pollutants into pollution-free water (H2O) and carbon dioxide (CO2) to meet high environmental requirements.

You can see some ppi shown in the below picture: