Thermo Scientific 146i, Instruction Manual

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Introduction
Principle of Operation
Thermo Fisher Scientific Model 146 i Instruction Manual 1-3
Figure 1–4 shows the hardware/plumbing configuration with both the
optional ozone generator and permeation oven installed. This figure is for
illustration purposes only, since there is no calibration gas requirement that
would necessitate activating both hardware components simultaneously.
The Model 146 i 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:
e
=
I
I
KLC -
o
where:
K = molecular absorption coefficient, 308 cm
-1
(at 0
°
C and 1 atmosphere)
L = length of cell, 17.90 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 Model 146 i through the zero air bulkhead and is
split into two gas streams, as shown in Figure 1–5 . One gas stream flows
through the zero air solenoid and through the zero air mass flow controller,
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 (I
o
). 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.
Figure 1–5 and Figure 1–6 show the hardware/plumbing configuration
with optional ozone generator, internal permeation oven, and photometer.
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
be flushed. The Model 146
i calculates the ozone concentration from the
ratio of zero and sample gas intensities and outputs the concentration to
the front panel display, the analog outputs, and also makes the data
available over the serial or Ethernet connection.
Photometer