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:

  1. Signal damping with selectable time constants.

  2. Switchable ranges (using several primary transducers within the instrument which each measure over a different range).

  3. Switchable output units (e.g. display in Imperial or SI units).

  4. Diagnostic facilities.

  5. Remote adjustment and control of instrument options from up to 1500 meters away via four-way, 20 mA signal lines.

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