As I understand it, the Manometer is read, the difference between the
two sides, i.e., if the water or mercury column, is 3" from zero on one
side, the other side is "down" 3", then the diff. = 6", therefore the
vacuum is 6".  When calibrating the scale, mark 1" as 1/2" and so on,
then it makes it easier to read. Please someone correct me if I am
wrong...