Australasian Hydrographer, March 2012
+/-% Measurement Uncertainty of Calculated Average Velocity (for discharge)
If we fit a power curve to these points and extrapolate backwards for lesser numbers of verticals (such as used in sewer pipe gauging), then we get even higher measurement uncertainties for less than 5 verticals, as shown in Figure 2: • ±82.5% for 1 vertical only • ±41.7% for 2 verticals • ±28.0% for 3 verticals • ±21.1% for 4 verticals 50 45 40 35 30 25 20 AS3778 Based Estimate
15 10 5 0 0
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45
50
Number of Verticals Used
+/-% Measurement Uncertainty of Calculated Average Velocity (for discharge)
Figure 1 Measurement uncertainty versus number of verticals (from AS3778 Part 3.1 table E.6)
50.0 45.0 40.0 35.0 30.0 25.0 Extrapolated Curve
20.0
AS3778 Based Estimate
15.0 10.0 5.0 0.0 0
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Number of Verticals Used Figure 2 Fitted curve and extrapolated portion for less than 5 verticals
45
50
07