Remember that RO membranes filter based upon very small partical sizes - down at the molecular level. So although two membranes may look similar from the outside - the quality of the materials and manufacturing process, and qa/qc throughout is crucial. Two major criteria are:
Better membranes for this hobby produce purified water at lower pressures (e.g., Filmtec membranes are spec'ed at 50 psi while the reset of the industry is at 60 or 65 psi).
Better membranes have higher rejection rates (they reject/don't allow to pass more impurities).
If you see a membrane advertised where the rejection rate isn't specified - think twice about buying it.
How can a buyer know the quality that went into production of the membrane? Good luck. Here is one instance where buying a name brand product makes sense. There is a reason Filmtec residential membranes are sold all over the world and are the standard by which other residential membranes are compared.
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