3A MOLECULAR SIEVE  is an alkali metal alumino-silicate; it is the potassium form of the type A crystal structure. 3A molecular sieve 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.