Xylem FC220 OPEN CHANNEL FLOW MONITOR - Manual (Page 6)

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III. Flow Calculation Modes
The FC220 determines flow from water level in several different ways. Standard flow
equations, Manning’s Equation, polynomials or a lookup table can all be used. The
following equation formats calculate flow in cubic feet per second based on the
measured water level in feet. A conversion is made by the microprocessor so that the
display and 4-20mA output work in the user defined flow units.
Flow Equation:
There are standard flow equation parameters for over 40 flumes and weirs stored in the
FC220. The user also has the ability to enter their own parameters for the flow
equation in the form of:
Q = A * (B+Ch)^D
Where:
Q = Flow in Cubic Feet Per Second
A - D = User Programmable Coefficients. If B is not used, enter zero
h = Water Level in Feet
If this mode is selected, the user is prompted to enter the four parameters in the
equation, A, B, C and D.
Manning’s Equation:
The Manning’s Equation is a widely accepted way of calculating the flow in any
gravity feed partially full channel of known geometry. Two applications are
supported, round pipes and rectangular channels. The equation is in the form of:
Q = ( 1.486 / n ) * WA * ( WA / WP ) ^ 0.6667 * ( Slope ) ^ 0.5
Where:
Q = Flow in cubic feet per second
n = Roughness Factor
WA = Wetted Area in Feet², cross sectional area of the water at any given depth
WP = Wetted Perimeter in ft, length of bottom and sides that is wet at any given depth
Slope = Slope of channel or pipe, drop in elevation divided by length of drop.