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Teledyne API Model 401 O
3
Photometric Calibrator Instruction Manual, 01124, Rev. J2
NOTE
Model 401 Photometric Calibrator is designed to be a dedicated
calibrator. The M401 Photometer should not be used as a monitor.
The Beer-Lambert equation, shown below, calculates the concentration of ozone from the ratio
of light intensities.
0
0
9
I
I
n
P
inHg92.29
K273
T
a
10
O
C
3
l
l
×××
×
−=
Where:
I
= Intensity of light passed through the sample
0
I = Intensity of light through sample free of ozone
a
= absorption coefficient
l
= path length
C
= concentration of ozone in ppm
T
= sample temperature in Kelvin
P
= pressure in inches of mercury
nl
= natural logarithm
As you can see 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 pathlength
determines how many molecules are present in the column of gas in the absorption tube.
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