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Model 9110T NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 333
the air stream to prevent reaction with the O
3
. As the capacitor charges and discharges, electrons are
created and accelerated across the air gap and collide with the O
2
molecules in the air stream splitting
them into elemental oxygen.
Some of these oxygen atoms recombine with O
2
to O
3
. The quantity of ozone produced is dependent on
factors such as the voltage and frequency of the alternating current applied to the CD cells. When
enough high-energy electrons are produced to ionize the O
2
molecules, a light emitting, gaseous plasma
is formed, which is commonly referred to as a corona, hence the name corona discharge generator.
Figure 13-10: Ozone Generator Principle
13.2.3.2. Ozone Generator Dry Air Supply
Ambient air usually contains enough water vapor to greatly diminishes the yield of ozone produced by
the ozone generator. Water also reacts with chemicals inside the O
3
Generator to produce caustic
substances such as ammonium sulfate or highly corrosive nitric acid that will damage the optical filter
located between the reaction cell and the PMT.
To prevent this, the air supply for the O
3
generator is dried using a special Perma Pure
®
single tube
permeation dryer. The dryer consists of a single tube of Nafion
®
that is mounted within an outer,
flexible plastic tube. Nafion
®
is a co-polymer that absorbs water very well but not most other chemicals.
As gas flows through the inner Nafion
®
tube, water vapor is absorbed into the membrane walls. The
absorbed water is transported through the membrane wall and evaporated into the dry purge gas flowing
through the outer tube, countercurrent to the gas in the inner tube.