Vaisala FD12P - Manual (Page 48)

User's Guide _______________________________________________________________________
48 ______________________________________________________ M210296en-A
Connecting a Background Luminance Sensor or a
Day/Night Switch to FD12P
The FD12P Weather Sensor supports two different methods for
ambient light sensing. The Background Luminance Meter LM11 can
be connected to the FD12P for accurate ambient light measurement.
The LM11 sensor and necessary wiring are included in option
FD12PLM11 (see Figure 15 on page 49 for the wiring details). The
background luminance measurement is typically used in the RVR
systems.
The LM11 output frequency is measured with the DRI21 interface
board and then converted into background luminance by the FD12P
software. The conversion uses a scaling factor, which needs to be
configured by the user. For details, see section BLSC Command on
page 84.
In certain applications it is necessary to calculate night visibility
separately using a formula that differs from MOR. In these cases a
simple day/night photo switch is sufficient for discerning between day
and night ambient light conditions. The switch can be connected to the
serial line control input on the FDP12 processor board. For wiring
details, see Figure 16 on page 50.
Positive voltage is interpreted as a night condition and the background
luminance value in the FD12P output message is set to 0. Negative
voltage or an open circuit is interpreted as a day condition and the
luminance value is set to 1. For details, see section BLSC Command
on page 84.