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|
Non-Contact Level Gauges
We set out to investigate non-contact level measurement systems. These systems have non-mechanical sensors and do not contact the material. They generally include laser, non-contact radar, and ultrasonic level measurement technologies, use level sensors that are located above the material, and are not directly exposed to the material. We did not include nuclear level gauges in our investigation because of the niche nature of the devices. Most non-contact sensors are exposed to the environment of the material in normal operation and can come in contact with the material when the level of the material is excessively high.
Non-contact level sensors generally emit a laser, radar or ultrasonic energy beam that is reflected back to the transmitter by the material. In many designs, the level measurement is derived from the amount of time measured for the laser beam to travel to and return from the material. However, some radar designs use modified energy waveforms to improve measurement.
Non-contact level measurement sensors direct an energy beam towards the surface to be measured within a total beam angle determined by energy intensity. Sensors with smaller beam angles generally emit more concentrated beams (as a percentage of total beam energy) that can produce a better and more reliable level measurement.
In general, larger ultrasonic and radar level sensors produce stronger beams that tend to measure longer distances with narrower beam angles than smaller sensors. In addition, increasing the sensor excitation frequency tends to make the beam angle smaller and allow measurement of shorter distances in addition to increasing resolution.
Level Measurement System Performance
The purpose of installing a level measurement system is to measure level accurately in a reliable manner. 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:
- absolute (fixed) distance error
- percentage of measured distance
- percentage of set span
- a percentage of maximum span
- percentage of the empty distance (farthest measurement in span)
- percentage of the maximum sensor distance
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.
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