Teledyne 6200E - Sulfides Analyzer - Manual (Page 206)

Theory Of Operation Model 6200E Instruction Manual
206 M6200E Rev: A1
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.
( )
* SO k F
2
=
Where: (Equation 10-4)
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 6200E 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.
10.2. The UV Light Path
The optical design of the Model 6200E’s sample chamber optimizes the fluorescent reaction
between SO
2
and UV Light (Figure 10-2) and assures that only UV light resulting from the decay
of SO
2
* into SO
2
is sensed by the instrument’s fluorescence detector.
UV radiation is generated by a lamp specifically designed to produce a maximum amount of light
of the wavelength needed to excite SO
2
into SO
2
* (330 nm) and a special reference detector