Calibration
Equipment Required
4-2 Model 48i Instruction Manual Thermo Electron Corporation
It is not necessary to remove SO
2
, NO, NO
2
, CO
2
, water vapor, or
hydrocarbons, since the Model 48i does not respond to these molecules. If
water vapor is not removed, it might be necessary to correct the flow
measurement data when calculating the dilution ratio of the span CO
reference.
A platinum on alumina catalyst, operated at 250 °C, has been found to be a
convenient oxidizer to convert CO to CO
2
.
If a zero air source is required, the following interferant removal methods are
recommended:
• Compression
•Drying
• Oxidation
• Scrubbing
Compression The zero air source should be at an elevated pressure to allow accurate and
reproducible flow control and to aid in subsequent operations such as
drying, oxidation, and scrubbing. An air compressor that gives an output of
10 psig is usually sufficient for most applications.
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.
Scrubbing The last step in the generation of the zero air is the removal of the remaining
contaminants by either further reaction or absorption. Fixed bed reactors are
usually employed.
Flow Meter(s) and
Controller(s)
In order to obtain an accurate dilution ratio in the dilution method used for
calibration, the flow rates must be regulated to 1%, and be measured to an
accuracy of at least 2%. The meter and controller can be two separate
devices, or combined in one device. The user's manual for the meter should
be consulted for calibration information.
Additional information on the calibration of flow devices can be found in
the Quality Assurance Handbook
1
. It should be noted that all flows should be
corrected to 25 °C and 760 mm Hg, and that care should be exercised in
correcting for water vapor content.
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