Answered August 21st, 2012 by Expert:
The specified limits of 5% to 95% RH are pretty broad; however, with a pouch like yours that has a high MVTR, continuous exposure to an atmosphere outside this range would clearly lead to the interior of the pouch also being outside the limits eventually.
The low-level RH limit is likely to be encountered in winter within heated buildings in cold areas such as Finland. Cold dry outdoor air falls to extremely low RH when heated to a comfortable temperature for humans; this low RH has been known to result in brittle fracture of nylon bone straps. Recently in Ireland, we have had 100% RH outdoors (and indoors as air conditioning in our homes is rare); it must be a common situation in the tropics.
Personally, I have never attempted to measure the humidity within a package but have been concerned of its impact on the moisture content of the product contained. The approach is to weigh unpackaged product taken from the packaging area, condition the same product at the specified humidity limits in a humidity cabinet and compare the new weight with the original; this enables determination of the acceptable range of moisture content.
Sample-filled packages can then be weighed as packaged, stored at worst case low and high RH and weighed at daily intervals until weight change ceases. Separate samples are used for the high- and low-limit test because of the potential hysteresis effect.
ASTM D4332 – 01 (2006), Standard Practice for Conditioning Containers, Packages, or Packaging Components for Testing contains helpful guidance for proper package conditioning.
In the past I have found that the Rapid Regain Dryer (as used for wool moisture testing) is a useful tool for use in determining product moisture content.