275
13. THEORY OF OPERATION
The Model T400 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 (DAS - see Section 7) as
well as reported to the user via a Front
Panel Display or a variety of digital and analog
signal outputs.
13.1. MEASUREMENT METHOD
13.1.1. CALCULATING O
3
CONCENTRATION
The basic principle by which the Model T400 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 13-1
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.
