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Currently, there are at least 30 different methods for stating the performance of noncontact gauges, making it nearly impossible for users to compare and contrast level measurement devices against each other.
Level measurement systems perform a relatively basic function — they determine the inventory of a contained material. But selecting a level measurement instrument is far from simple, especially when one considers the wide variety of performance statement methodologies users must decipher when evaluating noncontact level gauges.
While there are many level measurement systems that can gauge the inventory of liquid in a vessel, the number of viable solutions decreases significantly when the vessel is agitated and/or operates at high temperatures. Further, foaming and the effects of filling and emptying the vessel can result in added complication.
Therefore, level measurement applications require careful engineering, which demands that users have a clear understanding of the actual meaning of different supplier performance specifications.
|Figure 1. In this ultrasonic level gauge design, material can accumulate on the sensors and stick to the reflector.
Noncontact Sensor Design
One of the most important factors involved with specifying a level measurement application is sensor design. Some noncontact sensor designs inherently tend to maintain the sensor cleanliness, while others tend to inherently accumulate material. For example, the ultrasonic sensor system in Figure 1 emits ultrasonic energy upward to a reflector that redirects the energy down to the material. In this design, material can accumulate on the sensors and stick to the reflector. Both of these phenomena can attenuate ultrasonic energy and cause the level measurement system to fail.
2. The ultrasonic level gauge here emits and receives ultrasonic energy
from the same surface on the bottom of the sensor, reducing the
tendency to accumulate material by eliminating the reflector.
Total Beam Angle
Laser, ultrasonic, and radar level measurement sensors direct an energy beam towards the surface to be measured within a total beam angle determined by energy intensity. As the energy beam moves away from the sensor, the beam progressively increases its diameter while containing progressively less energy per unit area. The beam angle is determined by the boundary formed where its energy level is reduced by 50 percent (3 dB). Sensors with smaller beam angles generally emit more concentrated beams, (as a percentage of total beam energy), which can produce a better and more reliable level measurement.
The total beam angle is illustrated in Figure 2. Some suppliers specify one-half of the total beam angle (that is the angle from the centerline of the beam). Other suppliers state the beam angle without explaining whether it is the total beam angle or the angle measured from the centerline.
The strongest reflections will generally occur due to objects or material within the cone formed by the total beam angle. To avoid additional (erroneous) reflections, the sensor should be located such that there are no obstructions within or near the cone. When this is not possible, a level transmitter that contains features that ignore these additional reflections should be considered.
Sensor Size & Excitation Frequency
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.
There is at least one transmitter currently on the market that can be described as self-adjusting in the sense that it automatically adjusts itself to modify the beam power, pulse duration, beam width, and sensitivity in order to improve reliability and performance at different operating conditions and material levels.
Level Measurement System Performance
The purpose of installing a level measurement system is to measure level accurately in a reliable manner. Whereas issues dealing with physical properties, process parameters, electronic features, and interconnections are often given extensive consideration, the quantification of the expected measurement quality of the installed level measurement system can be virtually neglected. Often, relatively little emphasis is given as to how well the level measurement system will perform its intended purpose. Adding to the confusion are the differences in the manner in which performance is expressed and the incomplete nature of the available information. Regardless, the quality of level measurement should be a prime concern.
The performance of a level measurement system is quantified by means of its accuracy statements. The user 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
- Percentage of Maximum Span
- Percentage of Empty Distance (Farthest Measurement in Span)
- Percentage of Maximum Sensor Distance
- An Undefined Parameter (e.g., 0.25%)
- Calibrated Span
- Detected Range
- Full Range
- Full Scale
- Full Span
- Maximum Distance
- Maximum Measured Span
- Maximum Measuring Span
- Maximum Range
- Maximum Span
- Maximum Span of the Sensor
- Maximum Target Range (In Air)
- Measured Distance
- Measured Range
- Measuring Range
- Range Distance
- Range with No Temperature Gradient
- Rated Span
- Set Measuring Range
- Span in Air
- Span Value
- Tank Height
- Target Distance
- Target Range
Having examined the mechanics of performance statements, note the following observations:
- Terminology used to specify level measurement system performance can be confusing, so the meaning of specifications should be verified with the supplier — even when the meaning seems clear.
- Statements expressed as percentages of different parameters, such as Empty Distance or Maximum Sensor Distance, can have significantly different absolute errors.
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.
Level measurement system claims often feature high reference accuracy. What is often not stated is that in some applications the errors associated with operating effects, such as the temperature, pressure, and composition, can be larger than the reference accuracy.
More importantly, the performance specifications may not describe performance. Consider some examples encountered while performing research for this article:
|0.25% of range
1.20% of range
0.25% of measuring range
0.25% of span
|.0.25% of empty distance (farthest measurement)
1.20% of maximum sensor range
0.25% of maximum sensor range
0.25% of maximum sensor range
0.25% of maximum sensor range
0.30% of measured distance
The reference accuracy of noncontact level gauges can vary significantly between suppliers and models. For example, The Consumer Guide to Non-Contact Level Gauges (the publication from which this article is derived) tabulates the calculated reference accuracy of approximately 150 noncontact level gauges with set spans of 5,000 millimeters at zero, 25, 50, 75, and 100 percent level. The errors associated with zero level (empty) ranged from 0.4 to 120 mm.
Further, the published reference accuracy specifications for noncontact level gauges were found untrustworthy 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 seemed technically clear. For example, one supplier who repeatedly attested to the validity of his gauges'' published specification, called back a day after being questioned about his performance statement with a different (corrected) specification. When the quantity of these discrepancies became apparent, many suppliers had to be contacted a second time to confirm that the published specifications reflected the suppliers'' intentions.
Having dealt with about 60 suppliers of noncontact level gauges in the course of researching The Consumer Guide to Non-Contact Level Gauges , it became apparent that users should be advised to contact suppliers prior to purchasing gauges. Further, users should pointedly ask suppliers to clearly explain what their accuracy specification actually means as compared to what the specification says. You may well be surprised by the differences you find.
About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control . He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at email@example.com or 845 623-1830.
For More Information: www.spitzerandboyes.com
This article is based on The Consumer Guide to Non-Contact Level Gauges (available at www.spitzerandboyes.com). The publication categorizes level measurement technologies by tank height, performance, supplier, and model. It calculates and ranks level gauge performance on a common basis — despite discrepancies in the published specifications.