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Non-Contact Level Gauge Performance
(Part 2 of 4)

By David W. Spitzer

E-Zine October 2009

Click here to review Part 1

An absolute (fixed) distance error statement describes an error that is constant. This error is independent of the calibration range and the actual level in the vessel. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be 10 mm. The absolute error at different levels is:

Level Distance Absolute Error (10 mm)
0 % (empty) 1100 mm 10 mm
25 % 850 mm 10 mm
50 % 600 mm 10 mm
75 % 350 mm 10 mm
100 % (full) 1% of 100 10 mm

A percentage of measured distance statement describes a parameter that is in error by a constant percentage of the actual distance measurement. In the measurement of a 1000 mm high vessel (100% level located 100 mm below the sensor) measured with an accuracy of 1 percent of measured distance, the absolute error can be calculated as:

Level Absolute Error (1% of measured distance)
0 % (empty) 1% of 1100 = 11.0 mm
25 % 1% of 850 = 8.5 mm
50 % 1% of 600 = 6.0 mm
75 % 1% of 350 = 3.5 mm
100 % (full) 1% of 100 = 1.0 mm

A percentage of set span statement describes the error in terms of the full scale range. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be 1 percent of set span. The set span is 1100-100 or 1000mm, so the absolute error at different levels is:

Level Absolute Error (1% of set span)
0 % (empty) 1% of 1000 = 10 mm
25 % 1% of 1000 = 10 mm
50 % 1% of 1000 = 10 mm
75 % 1% of 1000 = 10 mm
100 % (full) 1% of 1000 = 10 mm

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.

Click here to read Part 3

Excerpted from The Consumer Guide to Non-Contact Level Gauges

ISSN 1538-5280

Spitzer and Boyes, LLC
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