Model 100E Instruction Manual Theory Of Operation
045150102 Rev XB1 165
Another factor affecting the amount of detectable UV present is the rate at which this
reaction occurs (k). which varies depending on the temperature of the SO
2
*. The warmer
the gas, the faster the individual molecules decay back into their ground state and the more
photons of UV light are given off per unit of time.
()
*SOkF
2
=
(Equation 10-4)
Where:
•
F = the amount of fluorescent light given off.
•
k = The rate at which the SO
2
* decays into SO
2.
•
SO
2
= Amount of excited SO
2
in the sample chamber.
In summary, the amount of fluorescence is
• Indirectly affected by the temperature of the gas and the factors from equation
10-2 above (concentration of SO
2
; intensity of UV light; path length of the UV
light) and;
• Directly affected by two factors (equation 10-4): the rate of the reaction and the
amount of SO
2
* present.
So, when the path length of exciting light is short; no interfering gases are present; no
interfering light is present and; both the temperature of the gas and the intensity of the
light are known and compensated for; the amount of fluorescent light emitted (F) is
directly related to the concentration of the SO
2
in the Sample Chamber.
The Model 100 E UV Fluorescence SO
2
Analyzer is specifically designed to create these
circumstances.
• The light path is very short.
• The optical design reduces the effects of stray light geometrically and spectrally.
• A special hydrocarbon scrubber removes the most common interfering gases from
the sample gas.
• A reference detector measures the intensity of the available excitation UV light and
is used to remove effects of lamp drift.
• Finally, the temperature of the sample gas is measured and controlled via heaters
attached to the sample chamber.
The net result is that any variation in UV fluorescence can be directly attributed to
changes in the concentration of SO
2
in the sample gas.
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