5. Which size to use and when?

5.1 Quantity transported Q, OTR, WVTR, permeance, geometry and time

It is difficult to rank materials, or rather packaging, according to their performance, because of difficulties in converting units, confusion linked to the quantity considered, or the unavailability of the necessary quantities (thickness in particular).

In general, the packaging supplier, food processing company or technical center juggles packaging geometry data and test conditions to calculate flows (OTR and WVTR) or cumulative quantities. $Q$ depending on demand.