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Teledyne API Model 100AH SO
2
Analyzer Instruction Manual, 02417, Rev. D
4 THE M100AH SO
2
ANALYZER
4.1 Principle of Operation
The operation of Teledyne API Model 100AH Analyzer is based upon the well proven technology
from the measurement of fluorescence of SO
2
due to absorption of UV energy. Sulfur Dioxide
absorbs in the 190 nm - 230 nm region free of quenching by air and relatively free of other
interference.
The UV lamp emits ultraviolet radiation which passes through a 214 nm filter (allowing 214 nm
light through), exciting the SO
2
molecules and producing fluorescence which is measured by a PMT
with a secondary UV filter. The equations describing the above reactions are as follows:
*2SOhvSO
Ia
12
→+ (1)
The ultraviolet light at any point in the system is given by:
()()
[]
20
SOaxexp1IIa −−= (2)
Where I
0
is the UV light intensity, a is the absorption coefficient of SO
2
, x the path length, and (SO
2
)
the concentration of SO
2
. The excited SO
2
decays back to the ground state emitting a characteristic
fluorescence:
22
KF
2
hvSO*SO +→ (3)
The block diagram in Figure 4-1 illustrates the general operation principle of the Model 100AH.
Ultraviolet light is focused through a narrow 214 nm bandpass filter into the reaction chamber. Here
it excites the SO
2
molecules, which give off their characteristic decay radiation. The sample is under
vacuum to minimize quenching effect from CO
2
and O
2
. A second filter allows only the decay
radiation to fall on the PMT. The PMT transfers the light energy into the electrical signal in the
sample stream being analyzed. The preamp board converts this signal into a voltage which is further
conditioned by the signal processing electronics.
The UV light source is measured by a UV detector. Software calculates the ratio of the PMT output
and the UV detector in order to compensate for variations in the UV light energy. Stray light is the
background light produced with zero PPM SO
2
. Once this background light is subtracted, the CPU
will convert this electrical signal into the SO
2
concentration.
4-1