A7.4
Once the calibration point has been established the zero gas flow is stopped and the span gas
flow is started. There is another DV lag and the analyser searches for the end of this DV lag
indicated by a change in oxygen signal. If a change is not detected within a certain time the
analyser will abort the auto-calibration sequence and will return to the sample gas without
automatically re-calibrating. The 90% point is searched for and if it is not reached within a pre-set
time the auto-calibration is aborted and the analyser returns to sample. If the 90% point is detected
the analyser will measure the oxygen reading every 15 second. When two consecutive readings
are identical the analyser will store that as the span calibration point. The span gas flow is stopped
and the sample gas flow is started.
Note: The analyser at this stage has not yet re-calibrated.
A sequence similar to that already described is established for the sample gas. The analyser
searches for an oxygen concentration in the sample gas which lies within a tolerance band of the
oxygen reading at the start of the auto-calibration sequence. If this is established, again within a
given time interval then the analyser assumes that the auto-calibration sequence is correct and
the analyser then re-calibrates.
These checks ensure that a gas flow is available on each of the three gas streams obviating the
need for flow alarms on the individual streams. This system does however, impose limitations on
the oxygen concentrations of the three gases. The concentration in the sample gas must be
greater (in order of 1.5% O
2
) than the concentration in the zero gas and less than the
concentration of the span gas. Similarly the oxygen concentration of the sample gas should not
change by more than 1.5% O
2
during the calibration sequence.
The accuracy from this system will be within approximately .02 - .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.
To Next Page