3.15
3.6.3 Calibration Limits
There are built in limits for both the zero and span calibration points. If the oxygen signal
exceeds these the analyser will not calibrate. This has been done to prevent the analyser
being calibrated when:
1. The mechanical zero is a long way out of adjustment.
2. There is a fault in the optical feed-back system of the cell which prevents a correct
span reading.
If the analyser will not calibrate because it is out of limits, check the mechanical zero and
perform a loop gain check. See Sections 4.3 and 4.6 respectively.
As the analyser is supplied the limits are 2% O
2
on the zero and 25% O
2
on the span.
It is recommended that the span gas limit is set to 25% of the oxygen content of the span gas
being used. For example for air (21% oxygen) the tolerance is 5%.
The limits are stored in parameter 75 for the zero and parameter 85 for the span. To change
these, first enter the password, press SET and then key-in the appropriate parameter number
followed by the value. Terminate the sequence with the enter key.
If the analyser is to operate at high altitudes, or the sample output does not vent to
atmospheric pressure, difficulty may be experienced in setting the span calibration. In this
case it will be necessary to adjust the coarse gain resistors. Refer to Section 5.3.6.10 for
details.
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