The Weather Measure E801 Evaporimeter records accumulated evaporation by measuring the water loss of a wetted filter paper.  The filter paper is mounted at one end of a hollow probe that is connected to a water chamber.  The water chamber contains a floating piston that is connected through a linkage to the pen arm, so that water level in the chamber indicates evaporation..  The clock driven chart drum can be set for 24 hour or 7 day rotation by a change of drive gear.  The E801 is shown in the 1974 Weather Measure catalog available on this website.

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