TELEDYNE API T703U - Page 203

Teledyne API T703/T703U Calibrator Operation Manual Principles of Operation
201
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:
Equation 9-5
LC-
o
II

e
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 T703,
Ozone (O
3
).

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 9-6
















LI
I
C
o

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.
In order to account for this effect the following addition is made to the equation:
Equation 9-7

























Ρ
inHgΤ
LI
I
C
o
92.29
273
1
ln

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:
Equation 9-8
07223D DCN7107