Square-edged orifice plates may be installed in either direction, since the orifice plate “appears”
exactly the same from either direction of fluid approach. In fact, this allows square-edged orifice
plates to be used for measuring bidirectional flow rates (where the fluid flow direction reverses itself
from time to time). A text label printed on the “paddle” of any orifice plate customarily identifies
the upstream side of that plate, but in the case of the square-edged orifice plate it does not matter