E-Zine September 2005
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To review --- 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 level measurement, parameters are commonly
described in terms of a:
- absolute (fixed) distance error
- percentage of the empty distance (farthest measurement in span)
- percentage of the maximum sensor distance
- percentage of measured distance
- percentage of set span
- a percentage of maximum span
Note that other terminology may be used to express these
concepts. Some variations actually used by suppliers include mm, cm and
percentages of:
Span |
Full span |
Span in air |
Rated span |
Maximum span |
Calibrated span |
Maximum measured span |
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Maximum span of the sensor |
Maximum measuring span |
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Span value |
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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 |
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Full scale |
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Maximum distance |
Target distance |
Measured distance |
URL |
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Distance |
Tank height |
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An undefined parameter (for example, 0.25%) |
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Many of the above terms do not have clear meanings. In addition,
discussions with suppliers 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 understand them correctly so
as to correctly evaluate performance.
More importantly, the performance specifications may not describe
performance. Consider some examples that were actually encountered.
Stated Accuracy |
Meaning (after discussion with supplier) |
0.25% Range |
0.25% of empty distance (farthest measurement) |
1.2% of range |
1.2% of maximum sensor range |
0.25% of measuring range |
0.25% of maximum sensor range |
0.25% of span |
0.25% of maximum sensor range |
0.25% |
0.25% of maximum sensor range |
0.3% |
0.3% of measured distance |
These examples illustrate the difference between published
specifications and their actual meaning. From the above data set, it would be
conservative to assume that statements expressed as percentages are percentages
of the maximum sensor range until they are confirmed otherwise by the supplier.
This article was excerpted from The Consumer Guide to Non-Contact Level Gauges
ISSN 1538-5280
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