Chapter 6 _______________________________________________________________ Maintenance
VAISALA _______________________________________________________________________ 127
FS11 Calibration
The FS11 sensor has been calibrated at the factory. Normally FS11 needs
no recalibration as long as the circuit boards are not replaced or there are
no warnings or alarms. The circuit boards need no hardware calibration.
A calibration check should be made every year with the FSA11 and
PWA11 calibration kits. If the checks show less than +3% change, no
recalibration is recommended because the change is within the
repeatability range of the calibration procedure.
Recalibration is needed if any of the receiver modules or the transmitter
modules is replaced. In such a case both forward scatter and
contamination measurements need recalibration.
FSM102 Visibility Calibration
When the visibility measurement is calibrated, the visibility should
ideally be better than 500 m, and there should be no precipitation. The
calibration is checked and adjusted with the FSA11 calibration kit. The
kit consists of zero plugs used to block the receiver and transmitter
optics, mask plates, and two opaque glass plates with known scatter
properties.
The calibration procedure checks two points: a zero scatter signal and a
very high scatter signal. The zero signal is obtained using the zero plugs
and the high signal using the opaque glass scatter plates.
The visibility corresponding to the opaque glass calibrator signal is
approximately 3 to 4 meters. If the calibrator is used in precipitation, the
error will be proportional to the area of the scatter plates that is covered
with droplets. Make sure that this area is negligible compared to the total
area.
NOTE
Before the check and calibration, clean the windows and check the
condition of the calibrator glass plates cleaning them also if needed.
NOTE
Avoid scratching the glass plates since scratches will have a negative
affect on the results of the calibration.
