temperature of a blood bag or other refrigerated package is continuously monitored throughout long periods of low temperature transportation and / or storage by an attached button . if the package contents exceed the predetermined safe upper temperature limit , the button produces an indication of having exceeded such limit sometime during transportation and / or storage . the indication is provided by a visual marker mounted on a metal part having a phase transition temperature corresponding to said limit and formed above the safe temperature to have a position which holds the indicator in a visible position corresponding to excessive temperature . the wire is shaped to a second position at freezing temperatures below said safe temperature by means which provide a visual indication of such setting .