Thermo Scientific 49i, Instruction Manual, Page: 24 / 314

Introduction
Principle of Operation
1-2 Model 49
i
Instruction Manual Thermo Fisher Scientific
• “Specifications” on page 1-3 is a list of the analyzer’s performance
specifications.
Thermo Fisher Scientific is pleased to supply this UV photometric ozone
analyzer. We are committed to the manufacture of instruments exhibiting
high standards of quality, performance, and workmanship. Service personnel
are available for assistance with any questions or problems that may arise in
the use of this analyzer. For more information on servicing, see Chapter 7,
“Servicing”.
Principle of Operation
The Model 49 i 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:
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)
The sample is drawn into the Model 49 i through the sample bulkhead and is
split into two gas streams, as shown in Figure 1-1. One gas stream flows
through an ozone scrubber to become the reference gas (I
o
). The reference
gas then flows to the reference solenoid valve. The sample gas (I) flows
directly to the sample solenoid valve. The solenoid valves alternate the
reference and sample gas streams between cells A and B every 10 seconds.
When cell A contains reference gas, cell B contains sample gas and vice
versa.
The UV light intensities of each cell are measured by detectors A and B.
When the solenoid valves switch the reference and sample gas streams to
opposite cells, the light intensities are ignored for several seconds to allow
the cells to be flushed. The Model 49
i calculates the ozone concentration for
KLC
−
=
e
o
I
I