Converts a raw ultrasonic level-element reading into live flow over a weir, given the crest's reading (“true zero”). Runs entirely offline; inputs persist in this browser via localStorage.
Head on weir: —
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How the Numbers Are Derived
Head from the raw reading
Level mode: H = live − true zero. Distance-down mode (sensor above the water reading air gap): H = true zero − live. If H ≤ 0 the water is below the crest and flow is zero — the tool says so instead of printing a negative number.
Weir equations (Q in cfs, H and L in ft)
V-notch 90°: Q = 2.5 · H^2.5 (standard field form; USBR's precise fit is 2.49·H^2.48 — within ~1% over the working range)
Rectangular, suppressed: Q = 3.33 · L · H^1.5. If the notch has end contractions (narrower than the channel), the Francis correction L − 0.2H applies — use the contracted length as L for a quick approximation.
Conversions: 1 cfs = 448.83 GPM = 0.6463 MGD.
Accuracy fine print
Sharp-crested formulas are rated for roughly 0.2–2.0 ft of head; the tool warns outside that band. Below ~0.2 ft the nappe clings and the reading is mush.
The weir must run free — air under the nappe, no submergence from downstream.
Ultrasonic LEs average over a footprint and dislike foam, steam, and standing waves. If the reading is bouncing, the flow number bounces with it — read the trend, not one sample.
This translates an instrument reading; it is not a billing meter. See the site's Weir & Orifice Flow Calculator for the fuller device set.
References
USBR, Water Measurement Manual — weir formulas and installation conditions. https://www.usbr.gov/tsc/techreferences/mands/wmm/