Xylem FC220 OPEN CHANNEL FLOW MONITOR - Manual (Page 15)

Global Water
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width of the rectangular channel. Use the RIGHT arrow to advance to the units until
they flash, use UP and DOWN to select the unit and press SAVE to store the settings.
Standard Flow Equation:
The water flow through most or all flumes and weirs can be described by the equation
shown above. Use the MENU, arrow buttons and SAVE to set the parameters A - D.
Not all flow equations require all 4 coefficients. If B is not used enter 0.000, if C is
not used enter 1.000 as shown above. A and D can also be set to 1.000.
3
rd
Order Polynomial:
A set of data points such as from a lookup table can be approximated by a polynomial.
The 3
rd
order polynomial shown above can give a very good approximation of flow in
many cases. The accuracy of the equation will vary depending on the accuracy and
number of the data points. Errors can increase toward the upper and lower ends of the
curve. If the normal range of flow is known, the equation coefficients can be adjusted
to minimize errors within the measurement range. Near zero flow the approximation
can result in negative numbers. While these negative numbers are generally very
small, they must be ignored for the purposes of displaying flow and updating the
totalizer and 4-20mA output. Any negative number calculated by the polynomial will
be considered zero flow. Using the arrow buttons, SAVE and MENU; set the
parameters A – D observing the +/- signs. The exponent is one digit in the range of -9
to +9.
Look-Up Table:
A single lookup table can be selected in cases where an application can’t be modeled
by an single equation. The lookup table and table name are factory programmable
only. A name of up to 16-characters, and lookup table data must be supplied. For
more information contact the Technical Support department at Global Water.