E-Zine September 2009

Non-Contact Level Gauge Performance
(Part 1 of 4)

By David W. Spitzer

 

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. In non-contact level measurement, parameters are commonly described in terms of a:

 

  • absolute (fixed) distance error
  • percentage of measured distance
  • percentage of set span
  • percentage of maximum span
  • percentage of the empty distance (farthest measurement in span)
  • percentage of the maximum sensor distance

 

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. Some variations include mm, cm and percentages of:

Span Full span Span in air
Rated span Maximum span Calibrated span
Maximum measured span   Maximum span of the sensor
     
Range Full range Detected range
Measured range Target range Measuring range
Maximum range Range distance Maximum target range (in air)
Set measuring range Range with no temperature gradient  
     
Full Scale    
     
Maximum distance Target distance Measured distance
URL    
     
Distance Tank height  
     
An undefined parameter (for example, 0.25%)

Many of the above terms do not have clear meanings. In addition, discussions with suppliers during investigation for this report revealed different meanings for specifications that otherwise seemed to be clear and well defined. 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.

 

Excerpted from The Consumer Guide to Non-Contact Level Gauges

 

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