Measurement Method; Calculating O Concentration - Teledyne T700 Operation Manual

Teledyne API – Model T700 Dynamic Dilution Calibrator

10.7.1. MEASUREMENT METHOD

10.7.1.1. Calculating O
06873E DCN7106
Concentration
3
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 gases at
Standard Temperature and Pressure (STP) is:
=
I I e
0
Where:
I
is the intensity of the light if there was no absorption.
o
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 T700, Ozone (O
α
is the absorption coefficient that tells how well O
wavelength of interest.
To solve this equation for C, the concentration of the absorbing Gas (in this case O
application of algebra is required to rearrange the equation as follows:
I
o
C = ln
I
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:
I
o
C = ln
I
Where:
T
= sample ambient temperature in degrees Kelvin
P
= ambient pressure in inches of mercury
Finally, to convert the result into Parts per Billion (PPB), the following change is made:
α
- LC
at STP
1
×
α
L
at STP
Τ
1
× ×
α
L
273
Principles of Operation
Equation 10-5
absorbs light at the specific
3
Equation 10-6
Equation 10-7
29 . 92 inHg
×
Ρ
o
Κ
Equation 10-8
).
3
), the
3
287