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OCF Research Center Montana Flumes Flow Characteristics
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Flow Characteristics

Properly installed and under free-flow conditions, the Montana Flume should have laboratory accuracy of +/-2%. However, practical considerations such as approach flow, installation, and dimensional tolerances mean that it's free-flow discharge precision is likely to be closer to +/-5% (per ASTM D1941).  

If a Montana Flume becomes submerged, the indicated flow rates will be substantially different than what is actually flowing through the flume.  While some studies have been conducted to develop corrections for submerged flows, Openchannelflow always recommends designing Montana Flume installations, in particular for free-flow discharge.

Montana Flumes use the same free-flow equations as Parshall Flumes.  The level-to-flow equation can be expressed as:

Q = KHan

Q = free flow rate (cfs / m3/s)
K = flume discharge constant (varies by flume size / units)
Ha = depth at the point of measurement (feet / meters)
n = discharge exponent (depends upon flume size)

Per ASTM D1941

Throat Width C (Imperial) C (SI) n
1” 0.338 0.0479 1.55
2” 0.676 0.0959 1.55
3” 0.992 0.141 1.55
6” 2.06 0.264 1.58
9” 3.07 0.393 1.53
1’ 4 0.624 1.522
1’-6” 6 0.887 1.538
2’ 8 1.135 1.55
3’ 12 1.612 1.566
4’ 16 2.062 1.578
5’ 20 2.5 1.587
6’ 24 2.919 1.595
7’ 28 3.337 1.601
8’ 32 3.736 1.607
10’ 39.38 4.709 1.6
12’ 46.75 5.590 1.6
15’ 57.81 6.912 1.6
20’ 76.25 9.117 1.6
25’ 94.69 11.32 1.6
30’ 113.13 13.53 1.6
40’ 150 17.94 1.6
50’ 186.88 22.35 1.6

Free-flow discharge tables for Montana Flumes can be found in the Discharge Tables section.

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