hygromètre à humus brut

CONT: The moisture content of the top layer of coniferous needles and twigs covering the forest floor is one of the factors of forest fire danger which must be determined accurately if fire danger in such timber types is to be measured. ... For daily use at numerous field stations, where duff is a forest fire fuel of material significance, some instrumental method giving an immediate indication of duff moisture is necessary. To meet this need [a member] of the U.S. [United States] Forest Products Laboratory, in 1923, invented an instrument called a duff hygrometer.