The most important as well as most difficult to measure aspects of flow measurement are flow conditions within a pipe upstream of a meter. Flow conditions mainly refer to the flow velocity profile, irregularities in the profile, varying turbulence levels within the flow velocity or turbulence intensity profile, swirl and any other fluid flow characteristics which will cause the meter to register flow different than that expected. It will change the value from the original calibration state referred to as reference conditions that are free of installation effects.