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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.
✶
Note. The concentrations of the two standard solutions
must differ from each other by a factor of at least four.
On initiation of the calibration sequence, either manual or
automatic, the 'Cal' l.e.d. is illuminated and the Calibration Mode
relay is energized. Two solenoid valves, SV1 and SV2 operate
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Table 7.1 Calibration Sequence Summary
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 energized, 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 sensor outputs corresponding to the two
standard solutions are used to calculate a new calibration graph
for the monitor, thus compensating for any drift in sensor 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 stabilize within 15 minutes after the end of
Cal Time 1 or Cal Time 2.
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