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BS3680 Rectangular Broad Crested Weir
Point of Measurement
The transducer must be above the maximum head P704 by at least the near
blanking distance P107.
The head is measured at a point 3 to 4 times the maximum head upstream
from the weir crest, to ensure the surface of the liquid is not affected by
turbulence or drawdown.
Calculations
ABSOLUTE
If the flow calculation is to be absolute P702 = 1 the flow will be calculated
using the formula: q = (2/3)
1.5
C
e
b(gh
3
)
0.5
Where: q = flowrate
Ce = discharge coefficient calculated by Ultra Twin P723
b = approach width P710
g = gravitational acceleration (nominal value = 980.66 cm/s
2
)
h = head
RATIOMETRIC
If the flow calculation is to be ratiometric P702 = 2 the flow will be
calculated using the formula: q= q
cal
C
e
/C
ecal
(h
e
/h
ecal
)
1.5
Where: q = flowrate
q
cal
= flowrate at maximum head P705
Ce = discharge coefficient calculated by Ultra twin P723
Ce
cal
= discharge coefficient at maximum head
he = effective head
he
cal
= effective head at maximum head
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