Teledyne 6200E - Sulfides Analyzer - Manual (Page 204)

Theory Of Operation Model 6200E Instruction Manual
204 M6200E Rev: A1
warning message, CONV TEMP WARNING (see Section 6.2.2) will be issued by the CPU if the
converter’s temperature is below 310°C or above 320°C.
When the converter is operating at peak efficiency there is a nearly 1:1 relationship between the
amount of H
2
S entering the catalytic converter and the amount of SO
2
leaving it. Therefore, by
measuring the amount of SO
2
in the gas after it leaves the converter, the amount of H
2
S originally
present on the sample gas can be directly inferred. This is accomplished by measuring the
ultraviolet fluorescence of the SO
2
in the sample chamber.
10.1.2. SO
2
Ultraviolet Fluorescence
The physical principle of the M6200E’s measurement method relies on the fluorescence that
occurs when Sulfur dioxide (SO
2
) is excited by ultraviolet light with wavelengths in the range of
190 nm - 230 nm. This reaction is a two-step process.
The first stage (Equation 10-1) occurs when SO
2
molecules are struck by ultraviolet photons of the
appropriate wavelength (19 nm - 230 nm). The SO
2
retains some excess energy that causes one
of the electrons of the SO
2
molecule to move to a higher energy orbital state. In the case of the
Model 6200E, a band pass filter between the source of the UV light and the affected gas limits the
wavelength of the UV light to approximately 214 nm.
* 2 SO hv SO Ia
nm 214 2
⎯ ⎯ → ⎯ +
(Equation 10-1)
The second stage of this reaction occurs after the SO
2
reaches its excited state (SO
2
*). Because
the system will seek the lowest available stable energy state, the SO
2
* molecule quickly returns to
its ground state (Equation 10-3) by giving off the excess energy in the form of a photon (hν). The
wavelength of this fluoresced light is also in the ultraviolet band but at a longer (lower energy)
wavelength centered at 330nm.
nm 330 2
kF
2
hv SO * SO + ⎯ ⎯ → ⎯
(Equation 10-2)
Obviously the more SO
2
that is present in the sample gas the more fluorescence will be detected
in the sample chamber. Unfortunately there are several other factors that also can affect the
amount of fluorescence detected by the analyzer.
For instance, the amount of SO
2
* is dependent on the average intensity of the UV light and not its
peak intensity because some of the photons are absorbed by the SO
2
as the light travels through
the sample gas.