Teledyne 6200E - Sulfides Analyzer - Manual (Page 207)

Model 6200E Instruction Manual Theory Of Operation
M6200E Rev: A1 207
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
le
Gas
I
N
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 UV
SO
2
Figure 10-2: UV Light Path
10.2.1. UV Source Lamp
The source of excitation UV light for the Model 6200E 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
*, 213.9 nm (see Figure 10-4).
The lamp used in the Model 6200E is constructed with a vacuum jacket surrounding a double-bore
lamp element (Figure 10-3). The vacuum jacket isolates the plasma arc from most external
temperature fluctuations. The jacket also contains the thermal energy created by the lamps
operation thereby helping the lamp heat up to and maintain proper vaporization temperature.
Light is emitted through a 20 mm x 5 mm portal.