Complex sentence: In those instances where velocity is additionally used as a surrogate, called an index velocity determination uses a velocity sensor, often either magnetic or acoustic, to measure the velocity of the flow at a particular location in the stream cross section.
Split into simpler sentences: In those instances where a velocity measurement is additionally used as a surrogate, an index velocity determination is conducted. This analysis uses a velocity sensor, often either magnetic or acoustic, to measure the velocity of the flow at a particular location in the stream cross section.