ThermoFisher Scientific 146iQ, Instruction Manual

Page: 12 / 302

Introduction
Principle of Operation
1-4 146iQ Instruction Manual Thermo Scientific
In the 146iQ, GPT is accomplished by routing zero air, at approximately
150 cc per minute, through the internal ozone generator to create ozone
and then precisely mixing this with NO from a gas cylinder connected to
one of the gas inlets. From there, the NO/O
3
goes to the reaction chamber
whose volume meets the dynamic parameter specification requirements of
the U.S. EPA. The gas is then diluted with zero air to provide the desired
NO
2
concentration out the manifold at the rear of the instrument.
The 146iQ photometer operates on the principle that ozone (O
3
)
molecules absorb UV light at a wavelength of 254 nm. The degree to which
the UV light is absorbed is directly related to the ozone concentration as
described by the Beer-Lambert Law:
KLC


e
o
I
I
where:
K = molecular absorption coefficient, 308 cm -1 (at 0°C and 1
atmosphere)
L = length of cell, 38 cm
C = ozone concentration in parts per million (ppm)
I = UV light intensity of sample with ozone (sample gas)
I
o
= UV light intensity of sample without ozone (reference gas)
Zero air is supplied to the 146iQ through the zero air bulkhead and is split
into two gas streams, as shown in Figure 1–4 . One gas stream flows
through the zero air solenoid, then through the ozonator to the manifold
and to the external/internal solenoid to become the sample gas (I). The
other stream flows from the zero air bulkhead to the sample/reference
solenoid in the photometer and is used as the reference gas (Io).
Alternately, the sample gas may be pulled from an external source through
the external/internal solenoid. The sample/reference solenoid valve
alternates the reference and sample gas streams between the cell every 5
seconds. Ozone measured by the photometer is dumped out the exhaust
bulkhead.
The UV light intensities of the zero and sample gases are measured by the
detector. When the solenoid valves switch the reference and sample gas
streams, the light intensity is ignored for several seconds to allow the cell to
Photometer