Sensing lamp signal
S
1
=l
1
e
(-KCL
1
)
Reference lamp signal
S
2
=l
2
e
(-KCL
2
)
K= absorption constant
C= gas concentration
I= IR intensity emitted by the source
The ratio “R” (S
1
/S
2
) = (l
1
/l
2
)e
{-KC(L
1
-L
2
)}
Since the radiation from Lamp 2 travels a much shorter path through the gas, it will be absorbed less than
the radiation from Lamp 1. The ratio of these signals shows the same absorption characteristics with an
equivalent cell length of (L
1
-L
2
). However, the signal from Lamp 2 will be equally effected by
environmental factors, system aging, and other unwanted factors. The absorption calculated from this ratio
for the differential path system can be used to sense the gas concentration while referencing out
non-signal impacts. Lamp 2 acts as a “virtual reference” to the system at the absorption wave length.
A key factor in traditional IR sensors is the temperature dependence of the detector/filter pair. The use of a
single detector and single optical filter in calculating the absorption ratio in Alpha Omega Instrument’s
sensor provides excellent temperature immunity. Movements in output due to temperature dependence of
the detector/filter pair are effectively referenced out. Furthermore, this design is considered to be optically
efficient. IR radiation is provided by incandescent lamps that are operated at a fraction of their
recommended power. This provides the sensor with a long mean time between failure (MTBF). It also
adds stability since the heated elements age at an insignificant rate. Utilizing signal ratios with proprietary
signal processing schemes adds higher resolution and provides more stable output from the sensor.
ALPHA OMEGA INSTRUMENTS CORP.
SERIES 9510 CARBON DIOXIDE ANALYZER INSTRUCTION MANUAL
4
DET
Lamp 1
Sensing
Lamp 2
Reference
4.2um Optical Filter
Pathlength L
2
Pathlength L
1
