When the primary element is used in conjunction with a differential pressure transmitter, the volumetric flow rate may be calculated as a function of the differential pressure across the obstruction; hence the term “differential pressure flow meter”. 

DP flow measurement in one of the oldest flow measurement technologies and dates back to at least the 17th century — though the measurement of flow using nozzles actually goes all the way back to Roman times. In 1738, Daniel Bernoulli developed his famous equation for flow rate calculation which shows that increasing velocity by constricting a flow path increases fluid pressure. Restricting the flow path creates an increased pressure and if you can measure the upstream and downstream pressure, you can calculate the flow rate.