3.30
Similarly, the span stabilisation time aborts the auto-calibration if a stable calibration point is
not reached. This could be because of a failure in the sampling system or a malfunction of the
analyser.
3.10.4 Accuracy
The accuracy from this system will be within approximately 0.02 - 0.03% oxygen, for both the
zero and span gas calibrations.
The accuracy may be altered by changing parameter 05 (the asymptote timer, which is factory
set to 15 seconds), the time interval the analyser takes to determine two identical readings.
The auto-calibration accuracy will improve if this interval is increased to, say, 20 seconds, but
the auto-calibration period will be extended. Reducing this interval to, say, 7 seconds,
shortens the auto-calibration time but reduces the accuracy.
When an analyser with auto-calibration is installed it is recommended that a manual calibration
is first carried out. A chart recorder should then be connected to the analogue output of the
analyser and an auto-calibration done. If the calibration points are examined on the chart (at
1% span) then the accuracy of the calibration can be determined. This can then be optimised
by changing the value of parameter 05, the asymptote timer.
3.10.5 Analogue Output Freeze
During auto-calibration the analogue output may be frozen at the last sample gas value. This
prevents the analogue output following the oxygen content of the calibrating gases and
possibly tripping external alarms or control equipment which may be connected. The analogue
output becomes live again at the end of auto-calibration when the analyser re-calibrates (or
aborts if the auto-calibration sequence has failed).
Parameter no. 45 is used to set this. It is set to 0 if the analogue output is to be frozen or set
to 4 if it is to follow the calibrating gases. The status can be interrogated by displaying
parameter 45.
Whilst the analogue output is frozen alarm code 45 will be displayed. This alarm may be
allocated to a relay.
3.10.6 Automatic Calibration Status Indication
The status of the analyser during an auto-calibration sequence can be checked with the
displays listed in Table 3.3.
The oxygen mode may be displayed during the sequence by pressing
DISPLAY GAS SAMPLE
9
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