Xylem PC320 Process Controller - Manual (Page 14)

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identifiers for the user on the LCD display. Calibrate the sensor normally. You can
set the sensor type to Water Flow and the units to CFS (Cubic Feet per Second).
A flume or weir equation usually has the form of Q = A * B ^ C; where Q is flow in
CFS, A is a multiplier, B is water level in feet and C is an exponent applied to B.
However; in many cases a 3
rd
order polynomial can give a close approximation of
flow. Example: The flow equation for a 3” Parshall Flume is:
Water Flow (CFS) = 0.994 * [(Water Level in Feet) ^ 1.55]
A 3
rd
order polynomial can approximate this with:
CFS = -1.499E-2 + [3.746E-1 * Level] + [7.164E-1 * Level^2] – [8.771E-2 * Level^3]
The accuracy of the equation will vary depending on the application but the
polynomial function can usually calculate non-linear parameters accurately and can
correct for some sensor non-linearities. This mode will only calculate positive
numbers from positive data, any negative results or data will be considered zero.
Press the RIGHT Arrow, then use UP or DOWN to select the polynomial mode to On,
press SAVE. Press MENU and program the numbers for each coefficient A - D.
Unused parameters should be set to zero.
4-20mA Output Calibration:
To scale and calibrate the 4-20mA output press the RIGHT arrow key. The 4mA Set
Point defines the sensor reading that results in an output current of 4mA. Use the
arrow buttons to change the sign to + or - and set the value, then press SAVE. The
20mA Set Point is the sensor value that results in a 20mA output current. As before,
use the arrows to change the 20mA Set Point. Press SAVE to store this setting. Any
sensor reading between these two points will be scaled to the corresponding output
current. The next three menus are set at the factory and should not normally need to be
changed. The 4mA Output Cal menu allows the 4-20mA output to be set to exactly
4.000mA, or very close. Connect a DC current meter between the 4-20mA output and
ground. Use the UP and DOWN arrows to raise and lower the raw data number (and
output current) until the current meter reads as close to 4.000mA as possible. Press