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7 CALIBRATION
7.1 Calibration Sequence
Calibration of the monitor is carried out by replacing the
sample solution sequentially with two standard solutions of
known concentration. This calibration sequence (see Table
7.1) can be initiated automatically at preset times, or manually
on demand.
The sensor outputs obtained during calibration are used to
calculate the calibration graph for the monitor and the
accuracy of the standard solutions will, therefore, have a direct
effect upon the overall accuracy of the monitor. Given accurate
standards, it is clear that the best accuracy would be expected
at the two calibration points. Ideally, the concentrations of the
two standards should bracket the expected sample
concentration closely, but the latter often varies quite widely. In
practice it is convenient, and often the best compromise, to
use standards whose concentrations are one decade apart,
i.e. differ by a factor of 10, and bracket the expected range of
sample concentrations. For example, if the range of sample
concentrations is expected to be 5 to 10 ppm, then standards
of 2 and 20 ppm could be used – see Section 8.1.2 concerning
low concentration ammonia standards.
Note. The concentrations of the two standard solutions
must differ from each other by a factor of at least four.
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Table 7.1 Calibration Sequence Summary
On initiation of the calibration sequence, either manual or
automatic, the 'Cal' l.e.d. is illuminated and the Calibration
Mode relay is energised. Two solenoid valves, SV1 and SV2
operate sequentially to shut off the sample and admit standard
solutions of known concentration, one low and one high (STD1
and STD2), to the sample path.
Once a valve is energised, sufficient time (preset to suit the
sensor response time) is allowed for the previous solution to
be displaced, and for the sensor to respond to the new
solution, before the sensor output is evaluated for stability by
the microprocessor. When a stable output is achieved the next
stage in the sequence is initiated.
After calibration the probe outputs corresponding to the two
standard solutions are used to calculate a new calibration
graph for the monitor, thus compensating for any drift in probe
or liquid handling performance characteristics since the last
calibration, and the new slope value can be displayed in
Operating Page 1 (100% slope is the theoretical value).
If the values are within limits, 'CALIBRATION PASSED' is
displayed.
'CAL FAILED (SLOPE)' is displayed if the slope value is
outside acceptable limits and 'CAL FAILED (SLOW)' is
displayed if the sensor output failed to stabilise within 15
minutes after the end of Cal Time 1 or Cal Time 2.
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