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Theory Of Operation
9.1.4. THE REFERENCE DETECTOR
9.1.5. THE PMT
9.1.6. UV LAMP SHUTTER & PMT OFFSET
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Vacuum
Jacket
Dual Bore
Element
Figure 9-3: Source UV Lamp Construction
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.
The amount of fluoresced UV produced in the sample chamber is much less than the
intensity of excitation UV source lamp (refer to Figure 9-4). Therefore a much more
sensitive device is needed to detect this light with enough resolution to be meaningful.
The T100 uses a Photo Multiplier Tube or PMT for this purpose.
A PMT is typically a vacuum tube containing a variety of specially designed electrodes.
Photons enter the PMT and strike a negatively charged photo cathode causing it to emit
electrons. These electrons are accelerated by a high voltage applied across a series of
special electrodes called dynodes that multiply the amount of electrons until a useable
current signal is generated. This current increases or decreases with the amount of
detected light (refer to Section 9.5.3 for more details regarding the electronic operation
of the PMT).
Inherent in the operation of both the reference detector and the PMT are a minor
electronic offsets. The degree of offset differs from detector to detector and from PMT
to PMT and can change over time as these components age.
To account for these offsets the T100 includes a shutter, located between the UV Lamp
and the source filter that periodically cuts off the UV light from the sample chamber.
This happens every 30 minutes. The analyzer records the outputs of both the reference
detector and the PMT during this dark period and factors them into the SO
concentration calculation.
06807A(DCNxxxx)
Teledyne ML - T100 UV Fluorescence SO2 Analyzer
Light Output
Portal
Zinc-Vapor
Plasma Arc
2
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