Teledyne 501 TRS, Руководство по эксплуатации, Страница: 58 / 111

THEORY OF OPERATION Teledyne API - T102/501 TRS, Addendum to T101 Operation Manual
56
10.1.2.
SO
2
Ultraviolet Fluorescence
The physical principle upon which the T102’s measurement method is based is the
fluorescence that occurs when sulfur dioxide (SO
2
) is changed to an 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 9-2) occurs when SO
2
molecules are struck by photons ( hv ) of
the appropriate ultraviolet wavelength. (In the case of the Model T102, a band pass filter
between the source of the UV light and the affected gas limits the wavelength of the light to
approximately 214 nm). The SO
2
absorbs some of energy from the UV light causing one
of the electrons of each of the affected SO
2
molecules to move to a higher energy orbital
state.
*
2 214 2
SO hv SO
Ia
nm
→  +
Equation 10-2
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.
Figure 10-1. UV Absorption in the T102 Reaction Cell
The equation for defining the average intensity of the UV light ( Ia ) is:
( ) ( )
[ ]
2 0
SO ax exp 1 I Ia
− − =
Equation 10-3
Where:
I
0
=
Intensity of the excitation UV light.
a =
The absorption coefficient of SO
2
(a constant).
SO
2
=
Concentration of SO
2
in the sample chamber.
x =
The distance between the UV source and the SO
2
molecule(s) being
affected (path length).
07267B DCN6485