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Principles Of Operation Model T101 Instruction Manual
244
The chemical process is:
2222
SO2OH2O3SH2
The converter is a heated stainless steel core containing a catalyst across which the
sample gas passes just before induction into the reaction cell. The temperature of the
converter is maintained by a heater controlled by the CPU via the I
2
C bus and the relay
card. The converter is enclosed in high-temperature insulation and encased in a stainless
steel housing.
The converter is most efficient when it operates at 315°C, converting 95% of the H
2
S
into SO
2
. Converter temperature is viewable via the front panel as the test function
CONV TEMP (see Section 4.2.1) and can also be output via the test channel analog
output (see Section 4.6.9). A
warning message, CONV
TEMP WARNING (see Section
4.2.2) 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 n
early 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 upon which the T101’s measurement method is based is the
fluorescence that occurs when Sulfur dioxide (SO
2
) is changed to excited state (SO
2
*) 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 (hv) of the appropriate wavelength. (In the case of the Model T101, 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.) The SO
2
absorbs some of the energy from the UV
light, causing one of the electrons of each affected SO
2
molecule to move to a higher
energy orbital state (SO
2
*).
*
22142
SOhvSO
Ia
nm
(Equation 10-1)
The amount of SO
2
converted to SO
2
* in the sample chamber is dependent on the
average intensity of the UV light (Ia) and not its peak intensity
because the intensity of
UV light is not constant in every part of the sample chamber. Some of the photons are
absorbed by the SO
2
as the light travels through the sample gas.
07266B DCN6485