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P/N 06160D DCN6409 Teledyne API Model 465M O
3
Monitor Instruction Manual - Page 13
3. PRINCIPLE OF OPERATION
The detection of ozone molecules is based on absorption of 254 nm UV light due to an
internal electronic resonance of the O
3
molecule. The M465M uses a mercury lamp
constructed so that a large majority of the light emitted is at the 254nm wavelength. Light
from the lamp shines through an absorption cell through which the sample gas is passed.
The ratio of the intensity of light passing through the absorption cell and the intensity of
light measured by a reference detector forms a ratio I/I
o
. This ratio forms the basis for
the calculation of the ozone concentration.
The Beer-Lambert equation, shown below, calculates the concentration of ozone from the
ratio of light intensities.
o
O
n
psi
L
C
695.14
273
10
6
3
Where:
I = Intensity of light passed through the sample
I
o
= Intensity of light through sample free of ozone
= absorption coefficient
L = path length
3
O
C = concentration of ozone in parts per million
= sample temperature in Kelvin
= pressure in pounds per square inch (absolute)
As can be seen the concentration of ozone depends on more than the intensity ratio.
Temperature and pressure influence the density of the sample. The density changes the
number of ozone molecules in the absorption path which impacts the amount of light
removed from the light beam. These effects are addressed by directly measuring
temperature and pressure and including their actual values in the calculation. The
absorption coefficient is a number that reflects the inherent ability of ozone to absorb 254
nm light. Most current measurements place this value at 308 cm
-1
atm
-1
at STP. The
value of this number reflects the fact that ozone is a very efficient absorber of UV
radiation which is why stratospheric ozone protects the life forms lower in the
atmosphere from the harmful effects from solar UV radiation. Lastly, the absorption path