The differential pressure Δp is the differential pressure measured across the restriction, which is converted to a flow rate by a differential pressure transmitter (pressure drop inside the meter section). The difference between the upstream pressure and the recovered downstream pressure is the permanent pressure loss. Behind the restriction, the pressure is lower than the original upstream pressure in front of the restriction. Permanent pressure loss must be taken into consideration as it makes the pump, compressor or boiler work harder for generating the same flow in the system, i.e. higher energy consumption.