Even disturbances located downstream of the flow element may affect measurement accuracy if
the disturbances are severe enough and/or close enough to the flow element. Unfortunately, both
upstream and downstream flow disturbances are unavoidable on all but the simplest fluid systems.
This means we must devise ways to stabilize a flowstream’s velocity profile near the flow element in
order to achieve accurate measurements of flow rate. A very simple and effective way to stabilize a
flow profile is to provide adequate lengths of straight pipe ahead of (and behind) the flow element.
Given enough time, even the most chaotic flowstream will “settle down” to a symmetrical profile all
on its own. The following illustration shows the effect of a pipe elbow on a flowstream, and how the
velocity profile returns to a normal (symmetrical) form after traveling through a sufficient length of
straight pipe: