Placing an obstruction in the flow path causes the velocity to increase and the pressure to drop. The difference between this pressure and the pressure in the pipe can be used to measure the flow rate of most liquids, gases, and vapors, including steam. In turbulent flow, the differential pressure is proportional to the square of flow rate.

An orifice plate is the most common type of obstruction, and, in fact, differential pressure across an orifice is used more than any other type of flow measurement. The installed base of orifice meters is probably as great as all other flow meters combined. The orifice plate is a metal disc with typically a round hole in it, placed between flanges in the pipe. Differential pressure can be measured at the pipe flanges directly upstream and downstream of the orifice or further upstream and downstream. The calculation formulas of differential pressure for a given orifice size and given location of the pressure taps are well developed, so no field calibration based on actual flow is needed (although the dP cell may have to be calibrated).

Orifice flow measurements are relatively cheap to purchase but have relatively high installation costs. They have high operating costs because they create a fairly large unrecoverable pressure loss. Also, they have low turndown, in part due to the squared relationship.

Orifices are suitable for high temperature and pressure, and are best for clean liquids, gases, and low velocity steam flows. They require long straight runs...