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Introduction
Principle of Operation
Gas Phase Titration
Permeation Oven
1-2
Model 146i Instruction Manual
produces ozone by exposing air to light at 185nm. The ozone level is
changed by varying the intensity of the lamp and varying the zero air flow.
With all of the gas standard solenoids deactivated, the Model 146i can be
used as a transfer standard ozone-generating source.
Gas Phase Titration (GPT) is achieved by combining ozone with a known
NO concentration, and measuring the loss of NO using the NO channel of
a chemiluminescence analyzer. The amount of NO
measured decrease in the NO level, as shown in the following equation:
In the Model 146i, GPT is accomplished by routing zero air, at
approximately 150 cc per minute, through the internal ozone generator and
then precisely mixing this with NO from a gas cylinder connected to one of
the gas inlets. From there, the NO/O
volume meets the dynamic parameter specification requirements of the
U.S. EPA. The gas is then fed into the mixing chamber and out the
manifold at the rear of the instrument.
The Model 146i can also be used as a permeation gas source when
equipped with an optional permeation tube oven, as shown in
Accuracy is achieved since both the release rate of the permeation tube gas
and the flow of zero air through the zero-air mass flow controller are
known, and the temperature of the permeation tube is stable. Temperature
stability of the permeation tube is accomplished by allowing only a small
amount of zero air to enter the permeation tube oven to be heated. The
major portion of zero air bypasses the oven and is routed to the mixing
chamber. The solenoids on either side of the permeation tube oven, under
microprocessor control, are used to route the flow of gas going through the
oven to either the main air stream or to exhaust. It should be noted that
zero air must always flow through the permeation tube oven, regardless of
the 146i output flow, so the oven may be kept at a stable temperature. To
accomplish this, a flow of zero air set by a capillary at approximately 150 cc
per minute is continuously fed through the permeation tube oven.
When only the permeation oven is activated, up to five permeation dilution
gas levels (ppm) can be set. When the permeation gas source is used in
conjunction with the gas dilution system, a permeation dilution
concentration cannot be specified because the gas dilution system controls
the flow rate of zero air. In that case, only the permeation concentration,
and not a permeation level, is displayed.
+
→
+
NO
O
NO
O
3
2
goes to the reaction chamber whose
3
formed is equal to the
2
2
Figure
1–3.
Thermo Fisher Scientific
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