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 Measurements
are used to monitor and ultimately, to control and optimise processes.
Automatic control and optimisation procedures enable and facilitate consistent
production. However, they require regular and reliable measurements at
the appropriate frequency. Difficulties in measuring quality (primary)
variables inevitably mean poor or no control at all. Measurement difficulties
can be due to a variety of reasons, inclduing:
- lack
of appropriate on-line instrumentation
Process operation then has to depend on laboratory assays,
which means that results can be infrequent and irregular, in addition
to long analysis delays. Depending on how the laboratory analyses are
carried out, the results may also be subject to reliability problems.
- reliability
of on-line instruments
On-line sensors may be available but they may suffer from long
measurement delays (e.g. gas chromatographs) or may be subject to factors
that affect the reliability of the sensor (e.g. drifts and fouling).
In either
case, automatic control and optimisation schemes cannot be implemented
and as a result, process performance is degraded.
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