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05509F DCN6409 19
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 Model 465 uses a mercury lamp constructed so
that a large majority of the light emitted is at the 254nm wavelength. Light from the lamp shines
down a hollow quartz tube that is alternately filled with sample gas, then filled with gas scrubbed
to remove ozone. The ratio of the intensity of light passing through the scrubbed gas to that of
the sample 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
O
inHg
C
ln
92.29
273
10
9
3
Where:
I = Intensity of light passed through the sample
I
o
= Intensity of light through sample free of ozone
= absorption coefficient
= path length
3
O
C = concentration of ozone in ppb
T = sample temperature in degrees Kelvin
P = pressure in inches of mercury
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 tube 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 length
determines how many molecules are present in the column of gas in the absorption tube.