When using a weir, where should an operator check for accuracy of flow measurement?

To ensure accurate flow measurement when using a weir, the operator must check upstream of the weir crest. This is because the flow profile and head level upstream are critical for determining the accurate flow rate over the weir. The weir works by measuring the height of water (head) before it flows over the crest, and this height is used in flow equations to calculate the flow rate.

The head must be stable, and measuring it upstream allows the operator to account for any disturbances or variations in flow conditions that may affect accuracy. If the flow measurement is taken elsewhere, such as downstream of the crest or at the outlet, it will not provide reliable data because the effects of turbulence and other factors could lead to incorrect flow calculations. Thus, checking upstream ensures that the measurements reflect the actual conditions of the water that is entering the weir.