The answer to this question is surprisingly simple. Turbulent and swirling media within a pipe is hard to measure in a reliable and accurate manner. There are many obstructions and details within a piping system that can cause a flow profile to become turbulent in the pipe. If this occurs within a relative distance to instrumentation, it can lead to significant errors, up to 50% of the reading.

There are many types of obstructions and configurations that can lead to an undesirable flow profile. Some common examples are: bends in the pipes from elbow joints, control valves, pressure taps, tees, other instrumentation, thermowells, reducers, and more. Anything that could cause the flow to act in an erratic or nonlinear way can be considered an obstruction. These need to be accounted for in creating an ideal flow profile to ensure the accuracy of your meter readings.

Different flow meter technologies have different straight run requirements, based on the inherent character of how each technology works. The requirements are usually expressed in terms of distance away from the instrument in a multiplication factor of the pipe diameter. They are also expressed as either being upstream (from the instrument) or downstream (from the instrument). It is important to consider straight run requirements to ensure that the meter you select will fit in the space and system you already have in place or to help design the process layout.