I have not used the ONSET Hobos or other models but investigated them for
ecological research.  They seem perfect for museum use.  The only
drawback is that you can't casually walk by a case and look at the
record.  The ideal situation is to have a portable computer to download
the data to and then you can go to all your loggers and collect the data
without moving them.
  Another thing useful to know is that ONSET siad that one of their
biggest groups of customers ar trucking companies and food manufacturers
who ship frozen food.  You can include a HOBO (or their current model, I
forget the name) with a shipment and find out if it was exposed to
extreme temperatures or humidity.  If a shipper is told that this in the
crate they may be more likely to be sure not to leave that box out on the
loading dock in the cold rain.
   Dan Freidus
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P.S. accuracy is good enough for conservation uses that it should not be
a concern.