Reynolds Number (Re) is a dimensionless unit which can affect the performance of flowmeters. It is defined as the ratio of the liquid’s inertial forces to its drag forces (viscous forces). The flow rate and the specific gravity are inertia forces, and the pipe diameter and viscosity are viscous forces. The pipe diameter and the specific gravity remain constant for the majority of liquid applications.

According to the definition, if Re number is high i.e. inertia forces are superior as compared to viscosity forces then the flow is turbulent and if Re number is low i.e. viscosity forces are superior then the flow is laminar. With R values more than 3000, nearly all applications involve turbulent flow whereas with R values less than 2000, liquids typically show laminar flow. Between these two levels, there is a transition zone which may be either laminar or turbulent depending upon the piping configuration and other installation conditions.

While selecting a good flowmeter, one of the initial steps is to find out both the minimum and the maximum Reynolds numbers for the application.