Service Manual 71
© 2018 Nortek AS
of new magnetic materials. The compass calibration procedure requires a single, slow rotation
around the instrument's tilt axis. A rotation taking at least 60 seconds is sufficient. For the
calibration procedure to work, the compass and all magnetic materials must remain fixed relative to
each other. As long as this is the case, the calibration procedure can correct for magnetic field
disturbances that are greater than the earth's magnetic field. You should conduct this procedure
outdoors, away from other possible magnetic elements that will not be part of the mounting structure
during deployment.
The procedure:
1) Assemble the frame (or similar) with the instrument, battery canister, and extra ballast, etc. and
connect the system to the computer.
2) Make sure the frame is level when calibrating the compass, and that it is possible to rotate the
entire system 360° horizontally.
3) Click On-line > Compass Calibration.
4) Click Start and rotate the entire system slowly around the Z-axis of the instrument. An example of
a successful rotation is shown below. Note that when doing this in the field, you cannot expect to
end up with a circle as perfect as this one. However, we recommend you to do this slowly in an
attempt to come as close to the ideal circle as possible.
5) To utilize the obtained values, click Done. You will be prompted to confirm that the new values
shall be transferred to the instrument to serve as the new compass setting.
Figure: Successful rotation of a compass calibration.
As a side note, the compass is not used when measuring velocity in XYZ or beam coordinates, but if
you plan on using the compass heading at a later point (for instance to orient the XYZ velocities
relative to the lake axes), it is probably worth calibrating the compass in advance.
3.2.2 Set Pressure Offset (Vector)
The pressure sensor offset is not calibrated to, nor corrected by the atmospheric pressure. This
means that the pressure sensor reading will follow the atmospheric pressure, thus the sensor signal
may have an offset. Atmospheric changes can vary the depth reading as much as 0.3 m. Note that
all data on the recorder will be erased during this procedure. To set the Pressure Offset
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