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Pressure differences in hydraulic circuits

Consider water in a tank having a long horizontal pipe for an outlet (Figure 1). The hydraulic circuit goes from the top of the water in the tank (Point A), through the water in the tank (Points B and C), through the horizontal pipe from C to G, and finally back through the air to point A. The pressure of the liquid can be determined at points above which a water column is established (Points B, D, E and F) and at A and G (equal to air pressure p_a, which is equal to 97900 Pa for the time the data was taken; see Figure 1, right).

Interpretation

Pressure can be interpreted as the hydraulic level quantity. Pressures along a path can be viewed as levels in a landscape, going up and down. When we walk along a closed path, we return to the same level. This means that all level differences add up to zero along a closed path. Pressure goes up when we go down in the liquid in the tank. It goes down in the pipe as a consequence of fluid flow friction. The drop from B to C is a consequence of the speed change of the liquid going from the wide tank to the narrow pipe.