EZine July 2005
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 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
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 (10mm) 
0% (empty) 
1100mm 
10mm 
25% 
850mm 
10mm 
50% 
600mm 
10mm 
75% 
350mm 
10mm 
100% (full) 
100mm 
10mm 
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 1001100 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 = 11mm 
25% 
1% of 1100 = 11mm 
50% 
1% of 1100 = 11mm 
75% 
1% of 1100 = 11mm 
100% (full) 
1% of 1100 = 11mm 
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 6000 mm. The absolute error at
different levels is:
Level 
Absolute Error (1% of maximum sensor distance) 
0% (empty) 
1% of 6000 = 60mm 
25% 
1% of 6000 = 60mm 
50% 
1% of 6000 = 60mm 
75% 
1% of 6000 = 60mm 
100% (full) 
1% of 6000 = 60mm 
Note the significant variation in absolute errors associated
with the different error statements above.
The remaining performance statements will be described in Part 2.
This article was excerpted from The Consumer Guide to NonContact Level Gauges
ISSN 15385280
