Teledyne 6200T - Sulfides Analyzer - Manual (Page 285)

6200T Total Sulfur Analyzer Principles of Operation
Teledyen Analytical Instruments 285
13. PRINCIPLES OF OPERATION
This section describes the principles of operation for the 6200T SO
2
analyzer (Section
13.1), for the optional O
2
sensor (Section 13.2) and for the optional CO
2
sensor (Section
13.3). It also describes the principles of operation for pneumatics (Section 13.4),
electronics (Section 13.5), communication interfaces (13.6) and software (13.7).
13.1. Sulfur Dioxide (SO
2
) Sensor Principles of operation
The 6200T 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; this exposure causes the
SO
2
molecules to change to an excited state (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 6.6) and reported to
the user through a vacuum fluorescent display or several communication ports.
13.1.1. SO
2
Ultraviolet Fluorescence Measurement Principle
The physical principle upon which the 6200T’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 13-1) occurs when SO
2
molecules are struck by photons of the
appropriate ultraviolet wavelength. In the case of the 6200T, 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.
* 2 SO hv SO Ia
nm 214 2
    
(Equation 13-1)