THEORY OF OPERATION
Model 400E Ozone Analyzer
Instruction Manual
218
P/N 04316 Rev: B
10.1.
Measurement Method
10.1.1.
Calculating O
3
Concentration
The basic principle by which the Model 400E Ozone Analyzer works is called Beer’s
Law (also referred to as the Beer-Lambert equation). It defines the how light of a
specific wavelength is absorbed by a particular gas molecule over a certain distance
at a given temperature and pressure. The mathematical relationship between these
three parameters for gasses at Standard Temperature and Pressure (STP) is:
I=I
O
e
-
α
LC
at STP
Where
:
I
o
is the intensity of the light if there was no absorption.
I
is the intensity with absorption.
L
is the absorption path, or the distance the light travels as it is being
absorbed.
C
is the concentration of the absorbing gas. In the case of the Model 400E,
Ozone (O
3
).
a
is the absorption coefficient that tells how well O
3
absorbs light at the specific
wavelength of interest.
To solve this equation for
C
, the concentration of the absorbing Gas (in this case
O
3
), the application of a little algebra is required to rearrange the equation as
follows:
×
=
L
I
I
C
o
α
1
ln
at STP
Unfortunately, both ambient temperature and pressure influence the density of the
sample gas and therefore the number of ozone molecules present in the absorption
tube thus changing the amount of light absorbed.