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T400 Ozone Analyzer Operator’s Manual Theory of Operation
279
13.1.4. INTERFERENT REJECTION
The detection of O
3
is subject to interference from a number of sources including, SO
2
,
NO
2
, NO, H
2
O, aromatic hydrocarbons such as meta-xylene and mercury vapor. The
Model T400’s basic method or operation successfully rejects interference from most of
these Interferents.
The O
3
scrubber located on the reference path (see Figure 13-2) is specifically designed
ONLY to remove O
3
from the sample gas. Thus, the variation in intensities of the UV
light detected during the instrument’s measurement phase versus the reference phase is
ONLY due to the presence or absence of O
3
. Thus, the effect of interferents on the
detected UV light intensity is ignored by the instrument.
Even if the concentration of interfering gases were to fluctuate so wildly as to be
significantly different during consecutive reference and measurement phases, this would
only cause the O
3
concentration reported by the instrument to become noisy. The
average of such noisy readings would still be a relatively accurate representation of the
O
3
concentration in the sample gas.
Interference from SO
2
, NO
2
, NO and H
2
O are very effectively rejected by the Model
T400. The two types of Interferents that may cause problems for the Model T400 are
aromatic hydrocarbons and mercury vapor.
AROMATIC HYDROCARBONS
While the instrument effectively rejected interference from meta-xylene, it should be
noted that there are a very large number of volatile aromatic hydrocarbons that could
potentially interfere with ozone detection. This is particularly true of hydrocarbons with
higher molecular weights. If the Model T400 is installed in an environment where high
aromatic hydrocarbon concentrations are suspected, specific tests should be conducted
to reveal the amount of interference these compounds may be causing.
MERCURY VAPOR
Mercury vapor absorbs radiation in the 254nm wavelength so efficiently that its
presence, even in small amounts, will reduce the intensity of UV light to almost zero
during both the Measurement and Reference Phases rendering the analyzer useless for
detecting O
3
.
If the Model T400 is installed in an environment where the presence of mercury vapor is
suspected, specific steps MUST be taken to remove the mercury vapor from the sample
gas before it enters the analyzer.
06870C DCN6332