Envent 331S - PRINCIPLE OF OPERATION; Physical Reaction

Envent Engineering Ltd. Page 8 Revision 2.4
PRINCIPLE OF OPERATION
Physical Reaction
Envent's models 330S and 331S H
2
S analyzers use ASTM D4084 – 07: Standard
Test Method for analysis of hydrogen sulfide in gaseous fuels (Lead Acetate reaction
rate method). This method uses lead acetate impregnated paper. Throughout this
document the term lead acetate tape will be written as "H
2
S sensing tape".
The H
2
S sensing tape reacts when in contact with hydrogen sulfide by the compound
relationship shown below. This tape does not react to any other sulfur compounds in
the gas stream. This makes it free from interference when more than one sulfur
compound is present in the sample stream. The H
2
S reaction is visibly evident by a
brown stain directly on the H
2
S sensing tape.
COOHCHPbSCOOCHPbSH
OH
3232
2)(
2
+ →+
The electronics built into the models 330S and 331S have been programmed to
measure the rate of darkening over time which, in turn, gives the hydrogen sulfide
concentration level. When no H
2
S is in contact with the H
2
S sensing tape, the
analyzer sensor reads 1000 mV (+/- 100 mV).
The sensor block has a LED and a photodiode detector. The LED emits a red beam of
light which is reflected off of the H
2
S sensing tape to the photodiode which detects the
light intensity. The darker the H
2
S sensing tape becomes when in contact to H
2
S, the
less light the photodiode detector receives reducing the millivolt value, which in turn,
increases the H
2
S value. The "SDS" models uniquely measures rate of change on
both sides of the H
2
S sensing tape, allowing for simultaneous readings of two
separate samples.