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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|
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