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Teledyne API T703 Calibrator Operation Manual Theory of Operation
169
9.6. PHOTOMETER OPERATION
The Model T703 calibrator’s optional photometer determines the concentration of Ozone (O
3
) in a sample gas
drawn through it. Sample and calibration gasses must be supplied at ambient atmospheric pressure in order to
establish a stable gas flow through the absorption tube where the gas’ ability to absorb ultraviolet (UV) radiation
of a certain wavelength (in this case 254 nm) is measured.
Gas bearing O
3
and zero air are alternately routed through the photometer’s absorption tube. Measurements of
the UV light passing through the sample gas with
and without O
3
present are made and recorded.
Calibration of the photometer is performed in software and does not require physical adjustments. Two internal
variables, a slope and offset are used to adjust the calibration of the photometer.
The CPU uses these calibration values, the UV absorption measurements made on the sample gas in the
absorption tube along with data regarding the current temperature and pressure of the gas to calculate a final O
3
concentration.
9.6.1. MEASUREMENT METHOD
9.6.1.1. Calculating O
3
Concentration
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:
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
07223B DCN6378