Model 100E Instruction Manual Theory Of Operation
045150102 Rev XB1 163
10. 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 (iDAS Section 6.10) and reported to
the user through a vacuum fluorescent display or several communication ports.
10.1. Measurement Principle
10.1.1. SO
2
Ultraviolet Fluorescence
The physical principle of the M100E’s measurement method 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 10-1) occurs when SO
2
molecules are struck by ultraviolet
photons of the appropriate wavelength (190 nm - 230 nm). The SO
2
retains some excess
energy that causes one of the electrons of the SO
2
molecule to move to a higher energy
orbital state. In the case of the Model 100E, a band pass filter between the source of the
UV light and the affected gas limits the wavelength of the UV light to approximately 214
nm.
*2SOhvSO
Ia
nm2142
→+
(Equation 10-1)
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