Measurement Method; Calculating O 3 Concentration - Teledyne T750 User Manual

M
EASUREMENT
Calculating O
The basic principle by which photometer 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:
Where:
I
o
I
L
C
α
To solve this equation for C, the concentration of the absorbing Gas (in this case O
application of a little algebra is required to rearrange the equation as follows:
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:
Where:
T
P
174
M
ETHOD
Concentration
3
- α LC
I = I
e
0
is the intensity of the light if there was no absorption.
is the intensity with absorption.
is the absorption path, or the distance the light travels as it is being
absorbed.
is the concentration of the absorbing gas. In the case of the Model T753U,
Ozone (O ).
3
is the absorption coefficient that tells how well O
wavelength of interest.
I
o
= ×
C ln
I α
I
o
= ×
C ln
I α
= sample ambient temperature in degrees Kelvin
= ambient pressure in inches of mercury
at STP
absorbs light at the specific
3
1
at STP
L
1 Τ 29
× ×
o
L
273 Κ
Equation 9-5
), the
3
Equation 9-6
Equation 9-7
. 92 inHg
Ρ
083730750B DCN8606