Envent 331S - Figure 2. H 2 S Analysis Cycle

Envent Engineering Ltd. Page 10 Revision 2.4
Once an analysis cycle is complete, the motor moves the H
2
S sensing tape giving the
sensor block new tape surface area to start the analysis again. In normal operation, if
the analyzer is being exposed to H
2
S within its range, the analysis cycle should last
between 150 to 210 seconds (1.5 to 2.5 min); the cycle lasts 720 seconds if no H
2
S is
present.
Figure 2. H
2
S Analysis Cycle
Figure 2 shows a complete analysis cycle from when the motor has advanced the H
2
S
sensing tape from a previous analysis (1), to the end of the current analysis (4). Once
the H
2
S sensing tape has finished moving and new tape surface area is exposed, the
reflection of light from the LED to the photodiode detector is at its maximum and
results in a voltage output from the sensor block of 1000 mV (+/- 100 mV). This
voltage is captured by the analyzer and it is referred to as the "Zero Voltage". The
zero voltage will vary for each surface area of the H
2
S sensing tape and will represent
the starting point for the H
2
S reading for that cycle.
From stage (2) to (4) the H
2
S value starts increasing as the millivolt value drops from
exposure to H
2
S. This stage is called "RRA Analysis". The RRA stands for Rapid
Response Algorithm and it is the instantaneous H
2
S readings calculated every 67 ms.
As the H
2
S sensing tape darkens, the RRA value starts going up every second.
Although the RRA values are calculated almost instantaneously, they are not as
accurate as the final reading obtained at the "Concentration Calculation" stage (3) to
(4). However, RRA values can be used as a trigger alarms setpoint in case the
application requires a rapid response time (less than the RRA Analysis completion).