E-Zine December 2009
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In order to fairly compare performance, the same type of accuracy statement should be used for each level measurement system. For level measurement, the best measure of performance is usually the absolute (fixed) level error statement because it quantifies the amount of error expected to be present. Therefore, in most cases, statements should be expressed or converted to an absolute (fixed) level error statement before using the information for comparison purposes.
Note the significant variation in absolute errors associated with the different error statements above.
Performance statements apply over a range of operation or, stated differently, between minimum and maximum measurement values. It is important to identify the range in which the statement applies, because performance can be significantly degraded or undefined when equipment is operated outside of this range.
Having examined the mechanics of performance statements, note the following observations.
Terminology used to specify level measurement system performance is confusing and should be verified with the supplier as to its meaning -- even when the meaning seems to be clear.
- Statement expressed as percentages of different parameters such as empty distance or maximum sensor distance can have significantly different absolute errors.
Error statements can influence level measurement system sizing. For example, the absolute (fixed) error associated with a 0 to 50 meter level measurement system used to implement a 0 to 10 meter level measurement may be ±30 mm. If a 0 to 10 meter level measurement system were selected, the error might be ±20 mm. Improved performance is often achieved by selecting the lowest range measurement system for the application.
Performance statements can be manipulated because their meaning may not be clearly understood and improperly expressed. In other instances, the performance specifications can become so intricate that technical assistance may be necessary to ascertain their meaning.
Excerpted from The Consumer Guide to Non-Contact Level Gauges