An orifice meter is a conduit and a restriction to create a pressure
drop. An hour glass is a form of orifice. A nozzle, venturi or thin
sharp edged orifice can be used as the flow restriction. In order
to use any of these devices for measurement it is necessary
to empirically calibrate them. That is, pass a known volume
through the meter and note the reading in order to provide a
standard for measuring other quantities. Due to the ease of
duplicating and the simple construction, the thin sharp edged
orifice has been adopted as a standard and extensive calibration
work has been done so that it is widely accepted as a standard
means of measuring fluids. Provided the standard mechanics of
construction are followed no further calibration is required. An
orifice in a pipeline is shown in figure 1 with a manometer for
measuring the drop in pressure (differential) as the fluid passes
thru the orifice. The minimum cross sectional area of the jet is
known as the “vena contracta.”