Hygrothermograph charts

I know this is a silly topic, but.... I am responsible for changing the
charts on the ten hygrothermographs in the library. I have a backfile of
these charts that cover more than the last six years. Needless to say,
they take of a fair amount of file space. Is there a suggested time
period for retaining them? I can't see a reason for keeping these things
ad infinitum. Thought you might have a suggestion. Thanks.

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                  Conservation DistList Instance 7:47
                  Distributed: Sunday, January 2, 1994
                        Message Id: cdl-7-47-006
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