Mel Vinton

"This bears no resemblance to defect size, which is the object of the test," says Mel Vinton, design engineer at Ion Medicare."In addition, standards tend to be comparative and usually based on ‘known-OK' parts with little or no consideration given to the package contents or conditions of storage."VLD provides a means to perform adequate testing while not rejecting packages that are fit for their purpose, adds Vinton.

Sealing integrity can be related to defect size by applying the Poiseuilles-Hagan equation, notes Vinton.The operative standard can then be based on gas laws and physics.This is achieved by calculating the maximum allowable hole for a given package and using that value as your standard.

"For example, take a product that will be stored and sold in a location where the humidity is high and where freight conditions produce rapid changes in barometric pressure and temperature," explains Vinton."Under those circumstances, a blister pack containing a hydroscopic material would have a maximum 5-µm VLD, a sterile-sealed device would be 10 µm, and hand lotion would be about 20 µm.It is not correct or cost-effective to test these products to the same standard," says Vinton.
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Basing a standard on the maximum allowable hole for a given package represents a realistic and cost-effective alternative to current techniques, Vinton adds.