Teledyne 6200E - Sulfides Analyzer - Manual (Page 203)

Model 6200E Instruction Manual Theory Of Operation
M6200E Rev: A1 203
10. THEORY OF OPERATION
The M6200E UV Fluorescence H
2
S Analyzer is a microprocessor controlled analyzer that
determines the concentration of hydrogen sulfide (H
2
S ), 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 H
2
S in
the sample gas is converted into SO
2
which is then exposed to ultraviolet light causing the SO
2
to
become excited (SO
2
*). As these SO
2
* molecules decay back into SO
2
they fluoresce. The
instrument measures the amount of fluorescence to determine the amount of SO
2
is present in the
sample chamber and by inference therefore the amount of H
2
S 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 H
2
S 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 H
2
S 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 6.11) and reported to the user through a
vacuum fluorescent display or several communication ports.
10.1. Measurement Principle
10.1.1. H2S Conversion
The M6200E H
2
S analyzer is basically a SO
2
analyzer with a H
2
S Æ SO
2
conversion stage inserted
into the gas stream before the sample gas enters the sample chamber.
The H2S to SO2 converter, receives sample gas from which the SO2 has been removed by a
scrubber. Once the naturally occurring SO
2
is removed from the sample gas, the special converter
changes the H
2
S in the sample stream to SO
2
using a high-temperature catalytic oxidation.
The chemical process is:
2 2 2 2
SO 2 O H 2 O 3 S H 2 + ⎯ ⎯ → ⎯ +
The converter is a heated stainless steel core containing a catalyst across which the sample gas
passes just before induction into the reaction cell. The temperature of the converter is maintained
by a heater controlled by the CPU via the
I 2 C bus and the relay card. The converter is enclosed in
high-temperature insulation and encased in a stainless steel housing.
The converter is most efficient when it operates at 315°C, converting 95% of the H
2
S into SO
2
.
Converter temperature is viewable via the front panel as the test function CONV TEMP (see
Section 6.2.1) and can also be output via the test channel analog output (see Section 6.9.10). A