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A weir can be used to measure flow rate, from the known relationship between height of the water level above the weir crest, weir length and flow rate (assuming the water level downstream is below the weir crest). However, the flow coeffiecent depends on the conditions, among other things on wether the flow is aerated from below. In low flow conditions, the water flow may cling to the weir resulting in another flow coefficient.

DWA-M 181 - flow conditions at different flowrates

According to the DWA-M 181 straight weirs can achieve a decent accuracy for flow ranges $Q_{max}/Q_{min}$ = 25 to 30, if the weir is built according to the ISO 1438. The crucial element is areation of the water jet, according to the DWA-M 181. What does ISO 1438 (or other similarly authoritative voices) say about straight weirs and how to build to achieve decent accuracy over a wide range?

the weir will be mounted into a rectangular concrete channel.

(Image source is the DWA-M 181 that mostly points to the ISO 1438)

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  • $\begingroup$ So you make the water go over either a rectangular or v-notch weir. Usually a good « getting wet » exercise to make sure all the water goes through the weir and does not leak. $\endgroup$
    – Solar Mike
    Jan 27, 2020 at 15:22

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Used to do this for students for a lab.

Once you have a nice flow of water handy, nice small stream is good. Then you make a suitable notch either v-notch or a rectangular notch and you can get the coefficients for 60degrees which is easy to make from some suitable wood.

Then with some waterproof sheet you can make a "seal" attached to the wood notch which can be pushed to the sides and floor of the river bed so that all the flow goes through the notch. We used to weight this down with rocks.

Then all you have to do is measure accurately - make sure the wood notch is upright otherwise that will introduce an extra error. Working out the velocity is inherent with error - playing "poo sticks" is one way to get an approximation of the velocity - you will need this when designing the notch as it cannot overflow, you want the flow to be mid-range on the notch really.

If you want to have a smaller and a larger weir to cope with low flow issues then that is possible - just design a suitable blanking plate for the notch not in use...

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  • $\begingroup$ this is pretty cool but not what I wanted to know, I further clarified my question. $\endgroup$
    – mart
    Jan 27, 2020 at 15:47
  • $\begingroup$ Well, we got good results especially as we were comparing to several other methods and driving pelton wheels and producing energy - accuracy of measurement is critical. $\endgroup$
    – Solar Mike
    Jan 27, 2020 at 15:53
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Documentation about weirs states that low flow will be problematic due to the problem you describe. Notched weirs try to compensate for this, but there are still limits. Could we design a complex system that managed flow across a large range? Sure. But the beauty of weir measurement is the simplicity. If you want accurate flow measurement from very low to very high, we need something other than a weir.

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