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4. Data Reduction ____________________________________________________
4.1 Determination of Water Elevation
Each type of weir, depending on its type, shape and size has an equation relating the
volume of fluid passing over it to the height of water, H, passing over the weir.
The change in height of the water is directly proportioned to the change in output of
the transducer. The following formula applies for the determination of the change in
water height.
ΔH = (R1 – R0) G
At the time that the initial reading R
0
is taken it is necessary to measure accurately as
possible the difference in elevation between the surface of the water in the weir box
and the tip of the weir. If the weir plate is graduated just record the initial water level
as indicated, Call this difference in elevation
Δ
E
. Then
H = (R1 – R0) G + Δ
E
inches or mm
Where H is the height of water above the tip of the weir,
R0 is the initial reading
R1 is the subsequent reading
G is the calibration factor in the height units /digit shown on the calibration sheet
Alternatively, from the initial reading R
I
taken
when the measured water height is Δ
E
inches, it is possible, using the gage factor, G, to calculate R
o
, the reading equivalent
to the water level at the tip of the V notch, For instance if R
I
= 6500 and Δ
E
= 5.2
inches, and G= - 0.001981 inches then R
o
= 6500 + 5.2/0.001981= 9125 and
H = G(R
I
-9125)
Another method, if at all possible, is to adjust the height of water so that water just
trickles over the V notch tip, and then take the R
o
reading.
