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By David W Spitzer, Spitzer and Boyes, LLC
LEVEL MEASUREMENT systems are used to sense the location of the interface between two or more materials in a fluid containment system. As such, level measurement systems are important and widely implemented throughout industry. The most common application of level measurement systems is to determine the inventory of a contained material. Although this may be desirable for the efficient operation and safety of the process, level measurements are also used for accounting purposes.
Level measurements can be used to keep tanks in the plant within reasonable operating limits. Tanks that become empty can potentially upset production due to the inability to feed the downstream process. They can also cause a lack of flowing material that can damage equipment, such as pumps. On the other hand, tanks that overflow can potentially create environmental hazards. Level measurements may also be used to manipulate the flow of material through the plant, such as in a sewage treatment plant where a high level in the primary collection tank would increase the flow through the entire treatment plant.
There are lots of ways to measure level. Various types of level measurement systems that are generally available are described in Table 1.
TABLE 1: LEVEL GAUGES
|Mechanical||Float, Magnetostrictive, Retracting|
|Non-Mechanical with Fluid Contact||Capacitance, Hydrostatic, Displacer, Contact Radar, Submerged Ultrasonic|
|Non-Mechanical without Fluid Contact||Laser, Non-Contact Radar, Ultrasonic|
|Externally-Mounted||Nuclear, Non-Contact Radar with Isolator, “Bottom-Up” Ultrasonic, Laser with Sight Glass|
|Open Channel Flow Measurement||Various technologies|
These terms are mathematically related so it is possible to convert one to another. Note that other terminology may be used to express these concepts. Some variations include mm, cm and percentages (See Table 2).
TABLE 2: SOME DIFFERENT WAYS OF SAYING THE SAME THING (WE THINK)
|SPAN||FULL SPAN||SPAN IN AIR|
|Rated span||Maximum span||Calibrated span|
|Maximum measured span||Maximum span of the sensor|
|Maximum measuring span||Span value|
|Range||Full range||Detached range|
|Measured range||Target range||Measuring range|
|Maximum target range (in air)|
|Set measuring range||Range with no temperature gradient|
|Maximum distance||Target distance||Measured distance|
Many of these terms do not have clear meanings. In addition, discussions with suppliers during investigation for our report revealed different meanings for specifications that otherwise seemed to be clear and well defined.
In particular, the published performance specifications may not describe performance. Consider examples tabulated in Table 3.
TABLE 3: WHAT DO SUPPLIERS MEAN?
|WHAT SUPPLIERS SAID||WHAT SUPPLIERS MEANT|
|0.25% of range||0.25% of empty distance (farthest measurement|
|1.2% of range||1.2% of maximum sensor range|
|0.25% of measuring range||0.25% of maximum sensor range|
|0.25% of span||0.25% of maximum sensor range|
|0.25%||0.25% of maximum sensor range|
|0.3%||0.3% of measured distance|
The examples in Table 3 illustrate the difference between published specifications and their intended meaning. Users are prudent to assume that statements expressed as percentages are percentages of the maximum sensor range until they are confirmed otherwise by the supplier.
Once the meanings of the published specifications were clarified, we compared the reference accuracy of non-contact level gauges for vessels that were nominally 2, 5 and 10 meters high. Performance was found to vary significantly between suppliers and models. My book, The Consumer Guide to Non-Contact Level Gauges tabulates the calculated reference accuracy of approximately 150 non-contact level gauges with nominal set spans of 5 meters at 0, 25, 50, 75 and 100 percent level. The calculated errors associated with measuring this vessel at 0 percent level (empty) was calculated to be between approximately 0.4 and 120 mm (depending upon model).
Further, the published reference accuracy specifications for non-contact level gauges could not be trusted because they often did not represent the performance statements that the supplier intended. This was found to be true even when the published accuracy specifications were seemingly technically clear. For example, one supplier who repeatedly attested to the validity of his gauges’ published specification called back the next day with a different (corrected) specification. When this type of discrepancy became apparent among a number of suppliers, we contacted many suppliers a second time to confirm that the published specifications reflected the suppliers’ intentions.
Having dealt with about 60 suppliers of non-contact level gauges, it became apparent that the published specifications often did not accurately represent performance. Users may well be surprised by the differences. It is a good idea to contact suppliers prior to purchasing the gauges to ask suppliers to clearly explain what their accuracy specification actually means as compared to what the specification says.
The discrepancies associated with level gauge specifications seem shocking at first glance. These discrepancies were not a total surprise, because similar discrepancies were found (to a lesser extent) while researching five flowmeter consumer guides. What was a surprise was the number of discrepancies that we found. Preliminary research for the upcoming consumer guide to pH and ORP measurements indicates that we will be uncovering more of the same.
|About the Author|
ControlGlobal.com is exclusively dedicated to the global process automation market. We report on developing industry trends, illustrate successful industry applications, and update the basic skills and knowledge base that provide the profession's foundation.