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Principles Of Operation Model T101 Instruction Manual
252
10.2.6.2. UV Absorption by Ozone
Because ozone absorbs UV Light over a relatively broad spectrum it could cause a
measurement offset by absorbing some of the UV given off by the decaying SO
2
* in the
sample chamber. The Model T101 prevents this from occurring by having a very short
light path between the area where the SO
2
* fluorescence occurs and the PMT detector.
Because the light path is so short, the amount of O
3
needed to cause a noticeable effect
would be much higher than could be reasonably expected in any application for which
this instrument is intended.
10.2.6.3. Dilution
Certain gases with higher viscosities can lower the flow rate though the critical flow
orifice that controls the movement of sample gas though the analyzer reducing the
amount of sample gas in the sample chamber and thus the amount of SO
2
available to
react with the to the UV light. While this can be a significant problem for some
analyzers, the design of the Model T101 is very tolerant of variations in sample gas flow
rate and therefore does not suffer from this type of interference.
10.2.6.4. Third Body Quenching
While the decay of SO
2
* to SO
2
happens quickly, it is not instantaneous. Because it is
not instantaneous it is possible for the extra energy possessed by the excited electron of
the SO
2
* molecule to be given off as kinetic energy during a collision with another
molecule. This in effect heats the other molecule slightly and allows the excited electron
to move into a lower energy orbit without emitting a photon.
The most significant interferents in this regard are nitric oxide (NO), carbon dioxide
(CO
2
), water vapor (H
2
O) and molecular oxygen (O
2
). In ambient applications the
quenching effect of these gasses is negligible. For stack applications where the
concentrations of some or all of these may be very high, specific steps MUST be taken
to remove them from the sample gas before it enters the analyzer.
10.2.6.5. Light Pollution
Because T101 measures light as a means of calculating the amount of SO
2
present,
obviously stray light can be a significant interfering factor. The Model T101 removes
this interference source in several ways.
The sample chamber is designed to be completely light tight to light from sources other
than the excitation UV source lamp.
All pneumatic tubing leading into the sample chamber is completely opaque in order to
prevent light from being piped into the chamber by the tubing walls.
The optical filters discussed in section 10.2.4; remove UV with wavelengths extraneous
to the excitation and
decay
of SO
2
/SO
2
*.
During instrument calibration, when the analyzer is sampling zero air (calibration gas
devoid of H
2
S) a measurement of the background light that is still present in the sample
chamber is recorded and used to offset the value of the PMT output used to calculate the
H
2
S concentration.
07266B DCN6485