As you can see by the pressure gauge’s indication in the photograph, the static line pressure of the natural gas inside the pipe is over 300 PSI. The amount of pressure drop generated by the orifice plate at full flow, however, is likely only a few PSI (100 inches water column is typical for many orifice plate installations). This is why we must use a differential pressure transmitter to measure the orifice plate’s pressure drop: only a DP transmitter will sense the difference in pressure across
the orifice while rejecting the static (common-mode) line pressure inside the pipe