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Calibration
Equipment Required
If a gas titration, or dilution, system will be used, the dilution air should
contain <0.01 ppm CO. In addition, the dilution air should be dry (Dew
point < 10
o
C) and free of oil mist and dust particles. Since the Model 48i
Trace Level-Enhanced is virtually interference free, it is not necessary to
include special scrubbers for removal of SO
NO
, CO
or volatile organic
2,
x
2
compounds.
Zero air cylinders from scientific and commercial suppliers typically
contain CO concentrations in the 0.1 - 0.3 ppm range. Thus, cylinder zero
air may need to be scrubbed of the residual CO prior to its use as a dilution
gas or as a zero standard in multi-point calibration.
If dilution air will be generated on-site, a commercial system such as the
Thermo Scientific Model 1160 Zero Air Supply is highly recommended. A
dilution air system can also be built using the air-drying and CO removal
techniques discussed below.
Compression
The zero air should be supplied at an elevated pressure to allow accurate
and reproducible flow control and to aid in subsequent drying, oxidation,
and scrubbing. An air compressor that gives an output of 30 to 40 psig is
usually sufficient. In addition to supplying high-pressure air, a compressor
equipped with condensation coils and a water trap can remove some water.
Drying
Several drying methods are available. Passing the compressed air through a
bed of silica gel, using a heatless air dryer, or removing water vapor with a
permeation dryer are three possible approaches to achieving a lower dew
point. If a large volume or continuous flow of dry air is needed, silica gel or
other similar drying agents will require frequent replacement. In those
cases, a permeation dryer or heatless air dryer will usually be a better
solution.
A platinum on alumina catalyst, operated at 250 °C, has been found to be
CO Removal
a convenient oxidizer to convert CO to CO
.
2
Gas Titration System
If the analyzer is being calibrated with zero air and a single span gas, the
internal scrubber can be used to provide the zero, and the span gas should
(Required for multi-point
be purchased at the appropriate concentration so that no dilution will be
calibration only)
necessary. However, if the unit will be operated under regulations that
require an external multi-point calibration, or if a multi-point accuracy test
is planned, a gas titration system will be required. A high quality gas
titration system, such as the Thermo Scientific Model 146 Series Multigas
Calibration System, is suggested for these applications. If a titration system
Thermo Fisher Scientific
Model 48i Trace Level-Enhanced Instruction Manual
4-3
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Troubleshooting
