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M400E Ozone Analyzer Operator’s Manual Theory of Operation
11. THEORY OF OPERATION
The Model 400E ozone analyzer is a microprocessor-controlled analyzer that determines the concentration of
Ozone (O
3
) in a sample gas drawn through the instrument. It requires that sample and calibration gasses 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.
Calibration of the instrument is performed in software and does not require physical adjustments to the
instrument. During calibration, the microprocessor measures the current state of the UV Sensor output and
various other physical parameters of the instrument and stores them in memory.
The microprocessor 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.
This concentration value and the original information from which it was calculated are stored in one of the unit’s
Internal Data Acquisition System (iDAS - see Sections 7.1) as well as reported to the user via a Front Panel
Display or a variety of digital and an
alog signal outputs.
11.1. MEASUREMENT METHOD
11.1.1. CALCULATING O3 CONCENTRATION
The basic principle by which the Model 400E Ozone Analyzer 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 11-1
I=I
O
e
-
LC
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 400E, 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 11-2
04315 Rev. C1 189