When checking the accuracy of a flow meter using a V-notch weir, the head should be measured at:

• A. Downstream at a distance equal to the maximum weir head
• B. Upstream at a distance equal to the maximum weir head
• C. Upstream at a distance 3-4 times the maximum weir head
• D. Downstream at a distance 3-4 times the maximum weir head

Answer: (C)

For accurate flow over a V-notch, measure the head well upstream of the notch where the flow has stabilized and is not disturbed by the weir’s presence. Taking the upstream head at about three to four times the maximum head above the crest ensures a hydrostatic, uniform water surface and a true representation of the driving head behind the discharge equation. If you measure too close to the notch or downstream, local disturbances, jet contraction, or tailwater effects can skew the head reading and give an incorrect discharge estimate. So the best practice is to measure upstream at a distance of 3–4 times the maximum weir head.