I was recently asked by a customer to propose a method where he could check the contents of a barrier foil bag to make sure they were safe from corrosion during long term storage, without opening the bag. I advised the Customer that we can add a clear window to the foil bag, a humidity indicator card can be fitted behind the window. Alternatively we can fit a screw in humidity indicator to the bag. A blue colouration to the card indicates the relative humidity is below the critical level, if the card changes colour to pink, this indicates that the humidity within the bag has increased, by adding more desiccant to the bag and hermetically sealing, the humidity will reduce and the card will revert back to blue.
This allows the customer to check the contents of the foil bag without the need to continually open.