Intelligent Instruments in Use
About the intelligent instruments in use
The intelligent instrument behaves as a
black box as far as the user is concerned, and no knowledge of its
internal mode of operation is required in normal measurement
situations.
Intelligent instruments offer many advantages over their
non-intelligent counterparts, principally because of the improvement
in accuracy achieved by processing the output of transducers to
correct for errors inherent in the measurement process. Proper
procedures must always be followed in their use to avoid the
possibility of introducing extra sources of measurement error.
One such example of the benefit that intelligence can bring to
instruments is in volume flow rate measurement, where the flow rate is
inferred by measuring the differential pressure across an orifice
plate placed in a fluid-carrying pipe. The flow rate is proportional
to the square root of the difference in pressure across the orifice
plate. For a given flow rate, this relationship is affected both by
the temperature and by the mean pressure in the pipe, and changes in
the ambient value of either of these cause measurement errors.
A typical intelligent flow rate measuring instrument contains three
transducers: a primary one measuring the pressure difference across an
orifice plate and secondary ones measuring absolute pressure and
temperature. The instrument is programmed to correct the output of the
primary differential pressure transducer according to the values
measured by the secondary transducers, using appropriate physical laws
which quantify the effect of ambient temperature and pressure changes
on the fundamental relationship between flow and differential
pressure. The instrument is also normally programmed to convert the
square root relationship between flow and signal output into a direct
one, making the output much easier to interpret. Typical accuracy
levels of such intelligent flow measuring instruments are ±0.1%,
compared with ±0.5% for their non-intelligent equivalents, showing an
improvement by a factor of five.
Intelligent instruments usually provide many other facilities in
addition to those mentioned above, such as the following:
-
Signal damping with selectable time
constants.
-
Switchable ranges (using several primary
transducers within the instrument which each measure over a different
range).
-
Switchable output units (e.g. display in
Imperial or SI units).
-
Diagnostic facilities.
-
Remote adjustment and control of
instrument options from up to 1500 meters away via four-way, 20 mA
signal lines.

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