Velocity-based flow meters assume laminar flow through the meter and require minimum straight pipe distance on which sides of the meter?

• A. Upstream and downstream sides
• B. Upstream side only
• C. Downstream side only
• D. Either side

Answer: (D)

Velocity-based flow meters rely on the flow entering the meter being stable and fully developed. Any swirling or turbulence created by upstream fittings, valves, or bends can distort the velocity profile and cause errors in the reading. Providing a straight length of pipe allows those disturbances to decay and the flow to straighten before it reaches the meter’s sensing element. Because you just need a sufficient straight-run length to achieve that stable profile, the installation can satisfy the requirement with a long straight section on either side of the meter—depending on what space and piping layout allow. The key idea is having that straight, undisturbed flow adjacent to the meter so the velocity measurement reflects a uniform cross-section flow, not a distorted one.