Xylem FL16 WATER FLOW LOGGERS - Manual (Page 31)

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Level Offset:
This setting lets you add or subtract a specified amount to or from the measured
water level to compensate for errors in installation and other factors that affect the
accuracy of your flow measurements. A positive number is added to measured level
and a negative number is subtracted from it. Note that this offset is used when
making flow calculations only. It does not affect the water level data displayed or
recorded by channel one of the data logger.
The offset number is used to adjust any positive non-zero level. When the sensor is
out of water, it may not read exactly zero. If this small error is a positive number,
regardless of how small, the offset will be added to it resulting in a small flow being
shown. If the offset value is a negative number, it will be subtracted from the
measured level but the result will never be less than zero. The offset number is
intended to be used in a condition where the sensor is submerged most or all of the
time so that incorrect readings near zero do not introduce errors.
Sensor Installed Horizontally:
The level sensor supplied with the FL16
has an outer diameter of one inch and
the sensing element itself is in the center.
When the sensor is installed horizontally
on the bottom of a pipe, it will not begin
to read water level until the level is already one half inch deep. Adding a 0.5 inch
offset will compensate for this and allows the flow to be calculated based on actual
level, not measured.
Sensor Installed Vertically:
When the sensor is installed vertically in an open channel, it does not need to be
installed at the critical measurement point but rather, can be installed at any point
and the offset number can be used to adjust the height of the sensor in software. It
may be desirable to install the sensor off of the bottom of a stream or river to prevent
silting or damage from rocks. If the sensor is installed too high, use a positive offset
number to add the distance to the bottom and “lower” it. If the sensor is installed too
low, such as below the sill in a Palmer-Bowlus flume, use a negative offset to
subtract off the error from the actual measured level and “raise” the sensor.