ANS:A - pitot tube

The water flow rate in a pipe of 3.5 meters diameter can be most economically and conveniently measured by a venturimeter. Here's why:

• Venturimeter: A venturi meter is a device used to measure the flow rate of fluid in a pipe. It consists of a converging inlet section, a throat, and a diverging outlet section. As fluid flows through the venturi meter, the pressure difference between the inlet and throat is measured. This pressure difference is related to the flow rate according to Bernoulli's principle.
• Suitability for Large Pipes: Venturimeters are suitable for large pipes, including those with diameters as large as 3.5 meters. They provide accurate measurements of flow rate and are less prone to wear and tear compared to other devices like orifice meters, which require precise machining and installation.
• Economic and Convenient: Venturimeters are generally considered economic and convenient because they do not significantly obstruct the flow, require less maintenance, and provide reliable flow rate measurements over a wide range of flow conditions.

Pitot tube and rotameter are typically used for smaller diameter pipes or specific applications where their operational characteristics are advantageous. Orificemeters are also used for flow measurement but require more precise installation and are more sensitive to upstream conditions. Therefore, based on the diameter of the pipe (3.5 meters) and considering economic and convenience factors, the most suitable choice for measuring water flow rate would be a venturimeter.