The following correction is in order:

The offered item is a Electrohygrometer--not a hydrothermograph.  The
difference being the hygrometer measures irh at a specific time and place.  The
hydrothermograph measures irh over a period of tiem and records the data.  (It
is also a far more expensive item)

The item advertised is STILL very useful in monitoring irh in areas for
exhibition and storage.

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