Teledyne 6400EH - Sulfur Dioxide Analyzer User Manual
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Theory of Operation
Model 6400EH
Teledyne Analytical Instruments
38
9.1.1 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. The location of the 6400EH reference detector differs from that of
the 6400E.
On the 6400E this detector is located directly across the reaction cell
from the lamp where it can measure the output of the lamp
directly. Because the 6400E is designed to measure relatively
low concentrations of SO
2
, enough of the lamp’s 214 nm source
light makes it through the reaction cell to get a reliable reading.
On the 6400EH the detector is located between the UV lamp and the
reaction cell and to the side. A beam splitter reflects a portion of
the lamp output 90 degrees, through a window and onto the
detector. This arrangement is required because nearly all of 214
nm UV source light entering the reaction cell is absorbed by the
higher concentrations of SO
2
typically measured by the 6400EH.
A window transparent to UV light provides an air-proof seal that
prevents ambient gas from contaminating the sample chamber.
9.1.2 Direct Measurement Interferences
The most common source of interference when measuring SO
2
is
from other gases that fluoresce in a similar fashion to SO
2
when exposed
to UV Light. The most significant of these are:
A class of hydrocarbons called poly-nuclear aromatics (PNA) of
which xylene and naphthalene are two prominent examples.
Nitric oxide (NO), which fluoresces in a spectral range near to SO
2
.
For critical applications where high levels of NO are expected an
optional 360 nm optical filter is available that improves the rejection
of NO (contact customer service for more information).
The methods by which the Model 6400EH rejects interference for
these substances differs from the M1 00E as follows.
Since the typical application for which the 6400EH rarely includes
the presences of hydrocarbons or PNA’s, no hydrocarbon scrubber
(kicker) is included in the 6400EH’s base configuration. An
optional scrubber (see Section 5.4 of this manual is available).