Molecular Sieve type 3A is an alkali metal alumino-silicate; it is the potassium form of the typ A crystal structure. Type 3A has an effective pore opening of about 3 angstroms ( 0.3nm) . This is large enough to allow in moisture, but excludes molecules such as unsaturated hydrocarbons which can potentially form polymers; and this maximises lifetime when dehydrating such molecules.