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Model T101 Instruction Manual Principles Of Operation
247
10.2. THE UV LIGHT PATH
The optical design of the Model T101’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
SO2
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 circuit constantly measures lamp intensity (see Equation 10-
2). A Photo Multiplier Tube (PMT) detects the UV given off by the SO
2
* decay (214
nm) and outputs an analog signal. Several focusing lenses and optical filters make sure
that both detectors are exposed to an optimum amount of only the right wavelengths of
UV. To further assure that the PMT only detects light given off by decaying SO
2
* the
pathway of the excitation UV and field of view of the PMT are perpendicular to each
other and the inside surfaces of the sample chamber are coated with a layer of black
Teflon® that absorbs stray light.
UV
Lamp
PMT
UV Source
Optical Filter
(214 nm)
Fluorescent UV
Optical Filter
(330 nm)
UV Source
Lens
PMT Lens
Reference
Detector
Window / Seal
Sa
m
p
l
e
Gas
IN
Sample Gas OUT
Unabsorbed Excitation UV
Broadband
UV From
Lamp
Filtered
Excitation UV
Fluorescent
UV
Only
Focused
Fluorescent
UV
Collimated
Excitation UV
Reflected
Excitation UV
and
Fluorescent U
V
SO
2
Figure 10-2. UV Light Path
10.2.1. UV SOURCE LAMP
The source of excitation UV light for the Model T101 is a low pressure zinc-vapor lamp.
An AC voltage heats up and vaporizes zinc contained in the lamp element creating a
light-producing plasma arc. Zinc-vapor lamps are preferred over the more common
mercury-vapor lamps for this application because they produce very strong emission
levels at the wavelength required to convert SO
2
to SO
2
*, 214.3 nm (see Figure 10-4).
07266B DCN6485