User Guide

219
11. THEORY OF OPERATION
The M100E UV Fluorescence SO
2
Analyzer is a microprocessor controlled analyzer that determines the
concentration of sulfur dioxide (SO
2
), in a sample gas drawn through the instrument. It requires that sample and
calibration gases be supplied at ambient atmospheric pressure in order to establish a constant gas flow through
the sample chamber where the sample gas is exposed to ultraviolet light causing the SO
2
become excited
(SO
2
*). As these SO
2
* molecules decay into SO
2
they fluoresce. The instrument measures the amount of
fluorescence to determine the amount of SO
2
present in the sample gas.
Calibration of the instrument is performed in software and usually does not require physical adjustments to the
instrument. During calibration, the microprocessor measures the sensor output signal when gases with known
amounts of SO
2
at various concentrations are supplied and stores these measurements in memory. The
microprocessor uses these calibration values along with other performance parameters such as the PMT dark
offset, UV lamp ratio and the amount of stray light present and measurements of the temperature and pressure
of the sample gas to compute the final SO
2
concentration.
This concentration value and the original information from which it was calculated are stored in the unit’s internal
data acquisition system and reported to the user through a vacuum fluorescent display or as electronic data via
several communication ports.
This concentration value and the original information from which it was calculated are stored in the unit’s internal
data acquisition system (refer to Section 7.2) and reported to the user through a vacuum fluorescent display or
several com
munication ports.
11.1. MEASUREMENT PRINCIPLE
11.1.1. SO
2
ULTRAVIOLET FLUORESCENCE
The physical principle upon which the M100E’s measurement method is based is the fluorescence that occurs
when sulfur dioxide (SO
2
) is excited by ultraviolet light with wavelengths in the range of 190 nm-230 nm. This
reaction is a two-step process.
The first stage (Equation 11-1) occurs when SO
2
molecules are struck by photons of the appropriate ultraviolet
wavelength. In the case of the M100E, a band pass filter between the source of the UV light and the affected gas
limits the wavelength of the light to approximately 214 nm. The SO
2
molecules absorbs some of energy from the
UV light causing one of the electrons of each of the affected molecules to move to a higher energy orbital state.
*2SOhvSO
Ia
nm2142

(Equation 11-1)
04515F DCN6048