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Contact Level Measurement System Performance (Part 1 of 3)

By David W. Spitzer

E-Zine July 2007

The purpose of installing a level measurement system is to measure level accurately in a reliable manner. Whereas issues dealing with physical properties, process parameters, electronic features, and interconnections are often considered extensively, the quantification of the expected measurement quality of the installed level measurement system can be virtually neglected. Often, relatively little emphasis is given as to how well the level measurement system will perform its intended purpose. Adding to the confusion are the differences in the manner in which performance is expressed and the incomplete nature of the available information. Nonetheless, the quality of level measurement should be a prime concern.

The performance of a level measurement system is quantified by means of its accuracy statements. The reader must understand not only which parameter is being described, but also the manner in which the statement is expressed. Specifications include a:

  • percentage of active length of the sensor
  • percentage of the measured distance
  • percentage of the set span
  • percentage of the set range
  • absolute (fixed) distance error

Some of these terms are mathematically related so it is possible to convert one to another. In general, performance comparisons of different equipment are predicated on the fixed level error.

Note that other terminology may be used to express these concepts. Many of these terms do not have clear meanings and may be incomplete. Regardless of the terminology used by the supplier, the reader is advised to confirm exactly what the meaning of the terms used in the specification in order to correctly relate them to the terms used in this report and to correctly evaluate performance.

A percentage of active length error statement for a sensor describes an error that is constant. For example, the stated accuracy of a 1200 mm probe with a 1000 mm active length might be 1 percent of the active length of the probe. The absolute error at different levels is:

Level Probe Coverage Absolute Error (1% of active length)
0% (empty) 0 mm 1% of 1000 = 10 mm
25% 250 mm 1% of 1000 = 10 mm
50% 500 mm 1% of 1000 = 10 mm
75% 750 mm 1% of 1000 = 10 mm
100% 1000 mm 1% of 1000 = 10 mm

Click here to review Part 2

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

ISSN 1538-5280

Spitzer and Boyes, LLC
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