Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 43
as the receiver optics tube of the Vaisala Transmissometer LT31 are
mounted in a cardanic frame so that the optics tube can be moved around
the lens position.
When the auto-alignment feature is activated, the optics tubes are
automatically moved in horizontal or vertical directions with gear motor
driven eccentric elements, while a spring is used to pull the optics tube
toward the eccentric elements. The mechanical position is monitored
using variable resistors. See Figure 9 below.
0401-026
Figure 9
Alignment Mechanism
The user must manually initiate the auto-alignment sequence after the
installation, when the realignment is requested or after maintenance
activities. For more information, see section Alignment on page 159.
Alignment Quality Control
The Transmissometer LT31 includes an integrated alignment quality
monitoring mechanism which is based on auto-calibration and the
window contamination measurement.
In general, a reduction of the alignment quality as well as increased
window contamination lead to the same effect, a decreased
transmissometer receiver signal strength. In the Transmissometer LT31,
the auto-calibration feature delivers the information about the total signal
loss since the initial calibration and in parallel, the window
contamination measurement indicates the signal reduction due to
contamination.