The ori-flow meter measures flow by inserting an orifice at part of the piping, generating differential pressure before and behind the orifice by means of the flow, and extracting this differential pressure by a suitable method.

Differential pressure (p1-p2) of main orifice & the flow q have shown in equation given below. The flow is proportional to square root of the differential pressure.

Q=cf [2g/ץ(p1-p2)]^1/2

Q :volumetric flow
c :flow co-efficient
f :cross sectional area of orifice hole
g :acceleration of gravity
p1-p2 :differential pressure
ץ : specific weight of fluid

The generated differential pressure is sent to the manometer or other differential pressure gauge & a differential flow meter can be formed by attaching a flow scale to this gauge. However,since flow indication is square root scale reading is difficult. Moreover, in case of remote transmission it is inconvenient because the evolution operation is performed to obtain linear relation.

Therefore if a bypass pipe connecting before and behind point of the main orifice is installed and the flow is branched to that pipe to overcome these disadvantage, that flow becomes proportional to the differential.