Level Gauge Performance (Part 3 of 3)

By David W. Spitzer

E-Zine September 2005

Click here to review Part 1
Click here to review Part 2

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		Maximum span of the sensor
Maximum measuring span		Span value

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 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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