Teledyne 6200T - Sulfides Analyzer - Manual (Page 289)

6200T Total Sulfur Analyzer Principles of Operation
Teledyen Analytical Instruments 289
Figure 13-2: UV Light Path
13.1.3. UV Source Lamp
The source of excitation UV light for the 6200T 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 (refer to
Figure 13-4).
The lamp used in the 6200T is constructed with a vacuum jacket surrounding a double-
bore lamp element (refer to
Figure 13-3). The vacuum jacket isolates the plasma arc from most external temperature
fluctuations. The jacket also contains thermal energy created by the lamp’s operation
therefore helping the lamp to heat up and maintain proper vaporization temperature.
Light is emitted through a 20 mm x 5 mm portal.
Light Output
Portal
Zinc-Vapor
Plasma Arc
Vacuum
Jacket
Dual Bore
Figure 13-3: Source UV Lamp Construction
13.1.4. The Reference Detector
A vacuum diode, UV detector that converts UV light to a DC current is used to measure
the intensity of the excitation UV source lamp. It is located directly across from the
source lamp at the back of a narrow tube-shaped light trap, which places it directly in the
path of the excitation UV light. A window transparent to UV light provides an air-proof
seal that prevents ambient gas from contaminating the sample chamber. The shape of the
light trap and the fact that the detector is blind to wavelengths other than UV no extra
optical filtering is needed.
13.1.5. The PMT
The amount of fluoresced UV produced in the sample chamber is much less than the
intensity of excitation UV source lamp (refer to
Figure 13-4). Therefore a much more sensitive device is needed to detect this light with
enough resolution to be meaningful. The 6200T uses a Photo Multiplier Tube or PMT
for this purpose.