Model 106-H Ozone Monitor Manual Rev. D-2
1
1.
OZONE MONITOR INTRODUCTION
The 2B Technologies Model 106-H Ozone Monitor is designed to enable accurate
measurements of ozone over a wide dynamic range extending from 0.01 percent by
weight in oxygen (wt% O
2
) to an upper limit of 20 wt% O
2
based on the well-
established technique of absorption of ultraviolet light at 254 nm. The Ozone Monitor
is lightweight (4.4 lb, 2.0 kg) and has low power consumption (~3.9 watt) relative to
conventional instruments and is well suited for applications such as:
•
Monitoring of ozone generator output
•
Monitoring of residual ozone after a cleaning or oxidation process
•
Control of industrial ozone processes
The Model 106-H is a flow-through instrument with no air pump. The plumbing is
designed for high pressure (up to 50 psig) and partial vacuum.
1.1
Theory of Operation
Absorption of UV light has long been used for measurements of ozone with high
precision and accuracy. The ozone molecule has an absorption maximum at 254 nm,
coincident with the principal emission wavelength of a low-pressure mercury lamp.
Fortunately, few molecules found in ozone process streams absorb at this
wavelength.
Figure 1.1 is a schematic diagram of the Ozone Monitor. Ozone is measured based
on the attenuation of light passing through a 0.1 cm absorption cell fitted with sapphire
windows. A low-pressure mercury lamp is located on one side of the absorption cell,
and a sample photodiode is located on the opposite side of the absorption cell. The
photodiode has a built-in interference filter centered on 254 nm, the principal
wavelength of light emitted by the mercury lamp. A beamsplitter and reference
photodiode are used to monitor the lamp intensity without the sample in the beam
path. The intensity of light is measured from the reference photodiode (
I
o
) and from
the sample photodiode (
I
).
Ozone concentration is calculated from the measurements of
I
o
and
I
according to the
Beer-Lambert Law:
I
I
l
C
o
O
ln
1
3
where
l
is the path length (0.1 cm) and
is the absorption cross section for ozone at
254 nm (1.15 x 10
-17
cm
2
molecule
-1
or 308 atm
-1
cm
-1
), which is known with an
accuracy of approximately 1%. The 2B Technologies instrument uses the same
absorption cross section (extinction coefficient) as used in other commercial
instruments.
