Envent 330S-Ex, User Manual

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Envent Engineering Ltd. Page 9 Revision 1
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). The first part of the RRA Analysis (2) to (3)
completes when the mV value drops 100 mV. Once it drops 100 mV, stage (3) to
(4) "Concentration Calculation" starts.
On this stage, the H
2
S slope is optimal for calculating the final H
2
S value for that
Cycle. Algorithms are used by the controller board to calculate as accurately as