Contact Level Measurement System Performance (Part 3 of 3)

E-Zine September 2007

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A percentage of set range statement describes a parameter that is in error by a constant percentage of the set range of the transmitter. The transmitter set range can be thought of as the coarse span of the transmitter. For a level measurement using a transmitter measuring a 1000 mm level with an accuracy of 1 percent of set range where the range setting is equivalent to 2000 mm, the absolute error can be calculated as:

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 might be ±10 mm. The absolute error at different levels is:

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 of the level measurement system 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.

The table below illustrates the significant variation in absolute errors (mm) associated with the different “1 percent” 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 can be 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 active length, measured distance, set span, and set range can have significantly different absolute errors.
• Capacitance and radar level measurements are bottom-up and top-down measurements respectively.

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 Capacitance and Radar Level Gauges

ISSN 1538-5280