On a venturi meter, the upstream or downstream pressure is compared to the pressure?

A venturi meter is designed to measure the flow rate of liquids by utilizing the principle of differential pressure. The flow of water through the venturi meter creates a change in velocity as it passes through a constricted area known as the throat. According to Bernoulli's principle, as the fluid velocity increases in the throat, the pressure decreases, creating a pressure differential.

To accurately determine the flow rate, pressure measurements are taken at both the upstream (inlet) and the throat of the venturi. The comparison of these pressures allows for the calculation of flow based on the differential pressure created by the geometry of the venturi. This relationship is critical for determining the flow rate accurately, as it relies on the change from higher pressure to lower pressure through the venturi's constriction.

In this context, measuring the pressure at the throat is vital because it directly correlates with the flow characteristics of the liquid, providing the necessary data for accurate calculations. The use of the throat pressure as a reference point is fundamental to the functioning of the venturi meter, which is why this answer is correct.