Hach-Lange POLYMETRON 9240 Operator Manual - Manual (Page 30)

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Analyzer Overview
3.6.2.2 Search stability set to YES - Automatic measurement mode
In this configuration, a maximum cycle time not to be exceeded has to be configured. This is the
sum of the maximum rinse time plus the fixed on-line measurement time.
As with the fixed measurement mode, the cycle and on-line measurement times need to be long
enough to meet the needs of the configuration in order to achieve accurate measurements.
In automatic measurement mode, the analyzer will minimize the rinsing time while searching for
a stable measurement. Then the cycle time is minimized by determining the shortest rinsing
time needed to reach accuracy (5% or 0.1ppb) for each channel measurement. The on-line
measurement time is fixed and remains unchanged.
If, at the end of the longest possible cycle time given by the user, the analyzer has still not
reached the stability criteria required for accuracy, the analyzer will give a measurement value
but also issues a stabiliz. errors warning. This warning alarm will stop if the stability criteria are
found the next time the analyzer is at the same point in the sequence.
If the analyzer never finds the stability criteria, a longer cycle time needs to be defined in order
to achieve accurate measurements. The recommended strategy is to give a very long time (e.g.
90 min) and the analyzer will minimize all rinsing times between channels.
Example:
Channel 1: 200 ppb
Channel 2: 0.1 ppb
Channel 3: 5 ppb
Channel 4: 50 ppb
Sequence: 1 2 3 4 * * * * * * * *
In this example, with an on-line measurement time fixed at 5 minutes and maximum cycle time
set at 60 minutes (much more than is necessary), then assuming stability for an accurate
measurement is reached, the analyzer will use the following timings:
The analyzer is assumed to be measuring channel 4:
It switches to channel 1.
Channel 1: to rinse from 50 ppb to 200 ppb, rinsing time < 15 minutes
Actual cycle time < 20 minutes (rinsing time < 15 minutes + on-line measurement time of
5 minutes)
Channel 2: to rinse from 200 ppb to 0.1 ppb, rinsing time ~ 45 minutes
Actual cycle time ~ 50 minutes (rinsing time ~ 45 minutes + on-line measurement time of
5 minutes)
Channel 3: to rinse from 0.1 ppb to 5 ppb, rinsing time ~ 20 minutes
Actual cycle time ~ 25 minutes (rinsing time ~ 20 minutes + on-line measurement time of
5 minutes)
Channel 4: to rinse from 5 ppb to 50 ppb, rinsing time < 15 minutes
Actual cycle time < 20 minutes (rinsing time < 15 minutes + on-line measurement time of
5 minutes)