Vaisala PWD22 - Page 74

USER'S GUIDE ____________________________________________________________________
72 __________________________________________________________________ M210543EN-F
Mode 1
Mode 1 is the logarithmic mode and it uses ln (visibility) values for the
output. The calculation is the following:
( )
,
min coeffrange
out
XII
I
⋅+=
where
I
out
=
Current that flows to analog output current sink.
I
min
=
Specified lowest output current value (for example 4 mA)
I
range
=
Current range specified
(for example I
range
= 20 mA - 4 mA =16 mA)
,
ln
ln
)ln()ln(
)ln()ln(
min
max
min
minmax
min
















=
−
−
=
VIS
VIS
VIS
VIS
VISVIS
VISVIS
X
i
i
coeff
where
VIS
i
=
Initial visibility value in meters
VIS
min
=
Lowest visibility value specified in configuration
(for example 10 m or 100 m).
VIS
max
=
Corresponding maximum visibility value (for example
2000 m or 20000 m).
The corresponding DAC output LSB value is calculated from I
out
by
using DAC scale parameters.
Mode 2
Mode 2 is the extinction mode, which gives lower output for higher
visibility. The maximum visibility must be higher than the minimum.
The corresponding currents are automatically handled so that the lower
current corresponds to the maximum visibility. The calculation is the
following:
( )
,
max coeffrange
out
XII
I
⋅−=
where
I
out
=
Current that flows to analog output current sink.
I
max
=
Specified highest output current value (for example 20 mA).
I
range
=
Current range specified
(for example I
range
= 20 mA - 4 mA =16 mA)