Teledyne 6200T - Sulfides Analyzer - Manual (Page 290)

Principles of Operation Model 6200T Total Sulfur Analyzer
290 Teledyne Analytical Instruments
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 13.5.3 for more details regarding the electronic operation
of the PMT).
13.1.6. UV Lamp Shutter & PMT Offset
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 6200T 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
2
concentration calculation.
 The reference detector offset is stored as and viewable via the front panel as
the test function DRK LMP.
 The PMT offset is stored as and viewable via the front panel as the test
function DRK PMT.
13.1.7. Optical Filters
The 6200T analyzer uses two stages of optical filters to enhance performance. The first
stage conditions the UV light used to excite the SO
2
by removing frequencies of light
that are not needed to produce SO
2
*. The second stage protects the PMT detector from
reacting to light not produced by the SO
2
* returning to its ground state.
13.1.7.1. UV Source Optical Filter
Zinc-vapor lamps output light at other wavelengths beside the 214nm required for the SO
2
 SO
2
* transformation including a relatively bright light of the same wavelength at which
SO
2
* fluoresces as it returns to its SO
2
ground state (330 nm). In fact, the intensity of light
emitted by the UV lamp at 330nm is so bright, nearly five orders of magnitude brighter than
that resulting from the SO
2
* decay, it would drown out the SO
2
* fluorescence.