Chapter 4 Calibration
4-2
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.
Oxidation. NO is usually oxidized to NO
2
in order to ease its scrubbing. Oxidation can
be accomplished by either ozonation or chemical contact. During ozonation, the air is
passed through an ozone generator. The O
3
that is produced reacts with the NO to form
NO
2
. Care must be taken to allow sufficient residence time for the ozonation reaction to
go to completion.
Chemical oxidation is accomplished by passing the air stream through a reacting bed.
Such agents as CrO
3
on a alumina support or Purafil
are very efficient at oxidizing NO
to NO
2
. The chemical contact approach has the advantage of needing no electrical power
input for its application.
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. Table 4-1 shows effective scrubbing materials.
To Remove Use
NO
2
Soda-Lime (6-12 mesh), Purafil
Hydrocarbons Molecular Sieve (4A), Activated Charcoal
O
3
and SO
2
Activated Charcoal
Table 4-1. Scrubbing Materials
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
o
C and 760 mm
Hg, and that care should be exercised in correcting for water vapor content.
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