Which device is primarily used to measure liquid flow rates in pipelines?

The Venturi tube is primarily used to measure liquid flow rates in pipelines due to its design that takes advantage of the principles of fluid dynamics. In a Venturi tube, the fluid flows through a pipe that has a constricted section, which causes the fluid velocity to increase while the pressure drops. By measuring the difference in pressure between the wider section of the pipe and the narrower constriction, the flow rate can be calculated using Bernoulli's principle. This method is effective and widely used in various applications, especially in systems where accurate flow measurements are critical.

The other devices listed do not serve the primary function of measuring liquid flow rates in the same manner. For example, a diaphragm type pressure gauge measures pressure, not flow. A pneumatic control system typically manages the operation of devices rather than measuring flow. Similarly, a strain gauge is used to measure strain or deformation in objects, rather than directly measuring flow rates in liquid systems. Thus, the Venturi tube stands out as the correct choice for measuring liquid flow rates in pipelines.