This document discusses various methods of flow measurement using differential pressure, including orifice plates, venturi tubes, flow nozzles, pitot tubes, and annubar devices. It explains that flow measurement is inferential, measuring a pressure drop to infer flow rate, and is affected by fluid density which depends on temperature, pressure, and other process conditions. Methods seek to compensate for these factors to provide an accurate flow indication. Key aspects covered are the basic Bernoulli's equation, installation and operation of differential pressure measurement elements, factors that influence accuracy such as erosion, and density compensation techniques.