What does the flow measurement typically require from the Venturi tube?

The correct choice pertains to the operational principle of a Venturi tube, which is designed to measure flow by utilizing the concept of differential pressure.

In a Venturi tube, fluid flows through a constricted section, which leads to a decrease in pressure and an increase in flow velocity according to Bernoulli's principle. The primary requirement for accurately measuring flow with this device is to determine the differential pressure between two points: before the constriction and at the throat of the Venturi. This pressure difference is directly related to the flow rate of the fluid through the tube.

By applying Bernoulli's equation, one can derive the flow rate from the measured differential pressure, along with the fluid density and specific geometry of the Venturi tube. Therefore, the measurement of differential pressure is crucial for determining the flow rate accurately, making this parameter essential for the functionality of the Venturi tube in flow measurement applications.

Other factors like flow velocity and temperature may influence certain calculations or conditions but are not the primary requirements for the operation of the Venturi tube itself in terms of flow measurement. The notion of a constant flow rate is also not a prerequisite, as the Venturi tube can function with varying flow rates, producing a corresponding change in differential pressure.