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Non-Contact Level Gauge Performance
(Part 3 of 4)

By David W. Spitzer

E-Zine November 2009

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

A percentage of maximum span statement describes the error in terms of the maximum sensor distance minus the blocking distance. For example, the stated accuracy of a level measurement system with a sensor that can measure from 400 mm to 8000 mm might be 1 percent of the maximum span. The maximum span is 8000-400 or 7600 mm, so the absolute error at different levels is:

Level Absolute Error (1% of maximum span)
0 % (empty) 1% of 7600 = 76 mm
25 % 1% of 7600 = 76 mm
50 % 1% of 7600 = 76 mm
75 % 1% of 7600 = 76 mm
100 % (full) 1% of 7600 = 76 mm

A percentage of empty distance statement describes a parameter that is in error by a constant percentage of the farthest measurement distance in the span. In the measurement of 100-1100 mm high vessel (100% level located 100 mm below the sensor) measured with an accuracy of 1 percent of empty distance, the empty distance is 1100 mm, so the absolute error can be calculated as:

Level Absolute Error (1% of empty distance)
0 % (empty) 1% of 1100 = 11 mm
25 % 1% of 1100 = 11 mm
50 % 1% of 1100 = 11 mm
75 % 1% of 1100 = 11 mm
100 % (full) 1% of 1100 = 11 mm

The percentage of maximum sensor distance statement is similar to an absolute (fixed) distance error statement in that its absolute error is constant. However, the error is related to the maximum distance that can be measured by the sensor. 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 maximum sensor distance of 8000 mm. The absolute error at different levels is:

Level Absolute Error (1% of empty distance)
0 % (empty) 1% of 8000 = 80 mm
25 % 1% of 8000 = 80 mm
50 % 1% of 8000 = 80 mm
75 % 1% of 8000 = 80 mm
100 % (full) 1% of 8000 = 80 mm

Click here to read Part 4

Excerpted from The Consumer Guide to Non-Contact Level Gauges

ISSN 1538-5280

Spitzer and Boyes, LLC
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