Teledyne T400, Руководство пользователя, Страница: 142 / 177

140 Teledyne API – Model T400 O
3
Analyzer 083730400A DCN8130
6.1.1 C
ALCULATING
O
3
C
ONCENTRATION
The basic principle by which the analyzer works is called the Beer-Lambert Law or
Beer’s Law. It defines how light of a specific wavelength is absorbed by a particular gas
molecule over a certain distance. The mathematical relationship among these three
parameters is:
Equation 6-1
I = I
o
e
- α
L c
Where:
I
o is the intensity of the light if there was no absorption.
I
is the intensity with absorption.
e
is the mathematical constant (Euler’s number)
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 T400, Ozone (O
3
)).
α
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:
Equation 6-2
C =
X ln
( )
I
0
I
( )
1
α L
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.
In order to account for this effect the following addition is made to the equation:
Equation 6-3
C =
X
ln
( )
I
0
I
( )
1
α
L
X
( )
X
T
273K
29.92 inHg
P
Where:
T
= sample temperature in Kelvin
P
= sample pressure in inches of mercury