1.4 Background/Theory
Within the industrial sector, Ozone is not directly emitted to the atmosphere. Its formation is normally
due to the reaction of sunlight with air containing hydrocarbons and nitrogen oxides. Ozone in the
ambient air is also found to create other pollutants in the air, such as components of smog.
Ground-level Ozone has become a global air pollution problem. According to measurements at remote
sites, the Ozone
background concentration in the northern hemisphere has increased by about 2 ppb
(approximately 6%) per decade since 1980 and is expected to rise further.
Ozone has been found to affect human health (when close to ground level) by harming the respiratory
and immune system. People with pre-existing respiratory damage or diseases are more likely to suffer
from the effects of Ozone.
1.4.1 Measurement Theory
Ozone is measured by UV absorption analysis. The UV photometer determines the concentration of
Ozone (O
3
) in a sample gas at ambient pressure by detecting the absorption of UV radiation in a glass
absorption tube. The Serinus 10 follows these principles and measurement techniques:
Ozone shows strong absorption of UV light at 254 nm.
Sample air is passed into the glass absorption tube (measurement cell).
Within the measurement cell a single beam of UV radiation (from a mercury vapour lamp) passes
through the sample and is absorbed by the Ozone.
The solar blind vacuum photodiode detects any UV that is not absorbed.
The strength of the UV signal being detected is proportional to the amount of UV light being
absorbed by Ozone.
The Serinus 10 analyser uses the Beer-Lambert relationship (refer to Equation 1) to calculate the
Ozone concentration.
O
3
is not the only gas that absorbs UV (254 nm), SO
2
and aromatic compounds also absorb UV
radiation at this wavelength. To eliminate these interferences a second cycle is performed. sample
air is passed through an Ozone scrubber, removing but allowing all interfering gases through.
Therefore, accurately measuring the effect of interfering gases. This effect is removed from the
sample measurement signal. This enables the accurate measurement of Ozone without the
influence of interferent.
The main controller PCA contains electronics to, measure and correct for all the major external
variables to ensure stable and reliable operation.
1.4.2 Kalman Filter Theory
The digital Kalman filter provides an ideal compromise between response time and noise reduction for
the type of signal and noise present in ambient air analysers.
The Kalman filter enhances measurements by modifying the filter time base variable, depending on
the change rate of the measured value. If the signal is changing rapidly, the instrument is allowed to
respond quickly. When the signal is steady, a long integration time is used to reduce noise. The system
continuously analyses the signal and uses the appropriate filtering time.
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