Thermo Scientific 146i, Instruction Manual

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System Description
Flow System
Thermo Fisher Scientific Model 146 i Instruction Manual 8-3
Precision Gas Dilution
Gas dilution is achieved by using two accurate mass flow controllers. One
governs high flow (typically 10 LPM fullscale) to control the diluting zero
air, the other governs low flow (typically 100 sccm) for controlling the gas
to be diluted. The two components are mixed in a Teflon mixing chamber.
Since the mass flow controllers used do not have a positive shutoff, a zero air
solenoid shuts off zero air when the system is in standby to reduce zero air
use.
The Model 146 i uses as many as six different independent gas inlets which
are individually controlled by gas solenoids A through F depending on the
mode being commanded.
Ozone Generation
The Model 146 i produces ozone by exposing air to light at 185 nm. The
ozone level can be changed by changing the flow through the ozone
generator or by changing the light intensity. In the Model 146
i , the selected
light intensity level is held constant by temperature controlling the lamp
and by using a highly stable lamp power supply. The flow is held constant
by the mass flow controller.
Gas Phase Titration
Gas phase titration in the Model 146 i is accomplished by combining the
outputs of the precision gas dilution system with that of the ozone
generating source. The gas dilution system is used to produce an accurately
known concentration of NO from a known concentration gas cylinder. The
amount of NO
2
formed is determined by mixing the NO concentration
with ozone and measuring the loss of NO, (it is equal to the NO loss).
The only caution that must be used in performing the gas phase titration is
to ensure the NO-O
3
reaction has been completed. Empirically, certain
dynamic parameter constraints have been derived to ensure completion of
the reaction. These conditions dictate that the residence time in the reaction
chamber, the gas flow rate and reaction chamber volumes meet the
following conditions: