Xylem H-3311 - Manual (Page 6)

The H-3301/3311 comes from the factory with the following settings:
SDI Address: 0
Slope: 0.0050
Offset: 0.000
With these values Stage will be in units of feet when used with a pulley having a circumference
of 12 inches. The slope value can be changed to accommodate other pulley circumferences or to
change the data to other engineering units such as inches or Meters. The Address, Slope and
Offset setup is stored in EEPROM within the H-3301/3311 and will not be lost if the power is
disconnected. The extended commands for changing the slope, offset and sensor address are
described in detail in Chapter 3. If more than one sensor is to be connected to the SDI-12 bus,
make certain each sensor has a different sensor address.
Many applications use a shaft encoder to monitor water in a stilling well. A float and pulley
translate the water level to rotation of the encoder’s shaft. When the H-3301/3311 is installed,
the encoder will be at a position determined by the current water level. When the H-3301/3311 is
powered up it resets its internal position count to 0. All subsequent measurements will be
relative to this initial position. If a measurement in absolute units or stage is wanted, you will
need to change the OFFSET value to match the current stage as measured by a staff gauge or
other datum.
Incremental encoders such as a quadrature encoders, monitor CHANGES in shaft position. With
an incremental encoder, if the power is lost the encoder reference or zero position is also lost.
The position count is maintained in RAM (not EEPROM) within the H-3301/3311. Storing the
position in non-volatile (EEPROM) memory would not protect from power loss because encoder
position changes would be missed during the loss of power. The H-3301/3311 protects its
4
INTRODUCTION
You must prepare your data recorder to receive and record the H-3301/3311 data. Since data
recorders differ widely, refer to your recorder manufacturer’s directions. The H-3301/3311
maintains two values or parameters about the shaft position: the Stage in the selected engineering
units and the Absolute Shaft Position in single unit steps. In general, program your data recorder
to record one or two values from the H-3301/3311 via the SDI-12 port. Usually only the first value
is recorded. The data recorder must issue an “aM!” command, then collect the data with a “aD0”
command, as explained in Chapter 3. The H-3301/3311 places two data values in its data buffer:
a±XX.XX±NNNN
Where:
a Is the the SDI-12 address 0-9, A-Z
XX.XX Is the current encoder position (Stage) in user units (Feet, Meters etc.)
NNNN Is the current absolute encoder position in units of steps (raw counts)
Programming The Data Recorder
Programming The H-3301/3311