59
Teledyne API Model 480L O
3
Monitor User Manual
09419A DCN8342
9.
PRINCIPLES OF OPERATION
The detection of ozone molecules is based on absorption of 254 nm UV light due to
an internal electronic resonance of the O3 molecule. The monitor uses a UV LED
constructed so that a large majority of the light emitted is at the 254nm wavelength.
Light from the LED 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/Io.
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.
Where:
I = Intensity of light passed through the sample
I
o
= Intensity of light through sample free of ozone
α
= absorption coefficient
= path length
3
O
C
= concentration of ozone in ppb
T = sample temperature in 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.
