DAVID W SPITZER'S E-ZINE (September 2023)

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. Nonetheless, 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 reader must understand not only which parameter is being described, but also the manner in which the statement is expressed. In non-contact level measurement, parameters are commonly described in terms of a:

• absolute (fixed) distance error
• percentage of measured distance
• percentage of set span
• 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. In general, performance comparisons of different equipment are predicated on the fixed level error.

Note that other terminology may be used to express these concepts. Some variations include mm, cm and percentages of span, full span, span in air, rated span, maximum span, calibrated span, maximum measured span, maximum span of the sensor, range, full range, detected range, measured range, target range, measuring range, maximum range, range distance, maximum target range (in air), set measuring range, range with no temperature gradient, full scale, maximum distance, target distance, measured distance, URL and an unidentified parameter (for example 0.25%). 

Many of the above terms do not have clear meanings. In addition, discussions with suppliers during investigation for this report revealed different meanings for specifications that otherwise seemed to be clear and well defined. Regardless of the terminology used by the supplier, the reader is advised to confirm exactly what the meaning of the terms used in the specification in order to correctly relate them to the terms used in this report and to correctly evaluate performance.

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

Differential pressure flowmeters are a case in point. Primary flow elements such as orifice plates, Venturi tubes, and flow nozzles, are typically designed and manufactured by one supplier. Another supplier (or a different division of the first supplier) typically designs and manufactures the associated differential pressure transmitter. The flow measurement system consists of these two components plus other components such as flanges, taps, impulse tubing, and the like. This is in contrast to a thermal flowmeter that is designed and manufactured to be a flow measurement system with only one component.

The primary element and transmitter have specifications that describe their performance. Specifications for the primary element tend to be expressed as a percentage of the actual flow measurement while the transmitter specifications tend to be expressed as a percentage of full scale. In general, the primary element specifications tend to dominate system performance when the flow is high in the flow range while the differential pressure transmitter tends to dominate system performance when the flow is low in the flow range. Overall flow measurement system performance should take both the primary element and the transmitter specifications into account at the flow rate in question. Therefore, system performance is not the same at all flow rates.

Flow element and differential pressure specifications can give the user a false sense of the expected performance of the flow measurement system. Flow element performance is often better than one percent of the actual flow rate, but it may be predicated on flowmeter operation above a certain Reynolds number. Failure to comply with this requirement can result in a measurement error that might exceed a few percent depending upon the technology and flowmeter operation. A differential pressure transmitter touted as having 0.05 percent accuracy can have a measurement error that exceeds two percent of the actual flow rate when the flow measurement system is operated at 10 percent of flow.

There is nothing wrong with purchasing and installing flow measurement systems that consist of a number of components that may be designed, manufactured and purchased from a number of sources. However, the designer should be aware of the technology and performance issues surrounding the performance of each component as it relates to the entire flow measurement system.

1. Target flowmeter
2. Variable area flowmeter
3. Target and variable area flowmeters
4. None of the above

Differential pressure flowmeter primaries such as the orifice plate, Venturi flow tube, and flow nozzle present an obstruction to the flow. When the fluid is flowing, this obstruction generates a differential pressure across the flowmeter that is related to the flow rate through the flowmeter. The differential pressure is measured and the flow rate is calculated.

Target flowmeters are similar in that they constructed to present an obstruction to the flow. Whereas an orifice plate allows flow in its hole and restricts flow in its annular area, target flowmeters restrict flow in its target and allows flow in its annular area. Instead of measuring differential pressure (as does an orifice plate), target flowmeters sense the force produced on the target by the differential pressure produced across the target. Therefore, the equations used for target flowmeters are similar to those used for differential pressure flowmeters.

Variable area flowmeters also present an obstruction to flow. Similar to target flowmeters, variable area flowmeters restrict flow at the float and allow flow in the annular area. The differential pressure across the float produces an upward force that is counterbalanced by gravity such that the float reaches an equilibrium position that reflects the fluid flow. Therefore, the equations used for variable area flowmeters are similar to those used for differential pressure flowmeters.

Answer C is correct.

Additional Complicating Factors

There are other flowmeters and variants that are essentially differential flowmeters in disguise. Be aware of this possibility when flowmeter equations are similar to those for differential pressure primary elements. One clue is when the flow rate in the turbulent flow regime is related to the square root of its measurement.

This article originally appeared in P. I. Process Instrumentation magazine.