Did you ever notice that some flowmeters measure with an accuracy expressed as a percentage of actual flow rate while other flowmeters measure with an accuracy expressed as a percentage of full scale? Did you ever investigate the difference? Table 1 illustrates the performance difference between these statements.
| Table 1: Performance Within 1 Percent of Full Scale vs. 1 Percent of Rate | ||
| Pressure | Flowmeter – 1% Full Scale | Gauge – 1% of Actual |
| 100 units | ±1 unit (1% of Rate) | 1% of Actual (±1.00 unit) |
| 50 units | ±1 unit (2% of Rate) | 1% of Actual (±0.50 unit) |
| 25 units | ±1 unit (4% of Rate) | 1% of Actual (±0.25 unit) |
| 10 units | ±1 unit (10% of Rate) | 1% of Actual (±0.10 unit) |
The basis for comparison for most flowmeters is their performance as a percentage of rate. The above table illustrates that the same numerical specification is superior when it represents a percentage of rate. Did you ever wonder why some flowmeters exhibit performance as a percentage of rate while others exhibit performance as a percentage of full scale? The major difference between these groups of flowmeters is often the ability to effectively calibrate the zero of the flowmeter.
Some flowmeters have a well-defined zero, so they do not have a zero calibration adjustment. Table 2 lists some examples of flowmeters with inherently well-defined zero calibration adjustments.
| Table 2: Flowmeters with Well-Defined Zero Calibration Adjustments | ||
| Flowmeter Technology | Zero Adjustment | Span Adjustment |
| Orifice Plate | Zero Differential Pressure | Bore Size |
| Positive Displacement | Zero Rotation | Speed of Rotation |
| Turbine | Zero Rotation | Speed of Rotation |
| Vortex Shedder | Zero Vortices Shed | Flowmeter Size |
These flowmeters exhibit percentage of rate performance because a span calibration error results in calibration errors that are proportionately lower at lower flow rates.
Some flowmeters have zero and span calibration adjustments. If the zero adjustment is in error, it will shift the flowmeter measurement in the entire flow range by the amount of the calibration error. As a result, the potential measurement error is constant, so it can be expressed as a percentage of full scale, and it tends to dominate the performance specification.
Nevertheless, electronic techniques have allowed some technologies with zero calibration adjustments to effectively eliminate the need for zero calibration. For example, AC magnetic flowmeters had zero adjustments and were subject to zero calibration drift from various sources. DC magnetic flowmeters sense the flow signal plus noise when the magnetic field is on, and noise when it is off. Subtraction of these sensed quantities results in a measurement of the flow signal. The net result is a magnetic flowmeter with no zero adjustment and a percentage of rate performance specification. In essence, the flowmeter automatically zeros itself during operation.
Similar measurement techniques have been developed for other flowmeter technologies, including ultrasonic flowmeters and thermal flowmeters. One ultrasonic flowmeter design uses the ultrasonic propagation times in the fluid and in the pipe wall to automatically zero the flowmeter. One thermal flowmeter design uses an in-situ introduction of gases to allow periodic updating of the zero calibration.
Flowmeters with percentage of rate statements usually perform better than flowmeters with percentage of full scale statements. However, this should be investigated in detail because it is sometimes not the case. Eliminating zero adjustments allows flowmeters using the technology to make significant improvements in flowmeter performance over previous designs. This can result in an increase in market share for the technology when the price premium is reasonable and the improvement is well communicated to people who use and specify flowmeters.
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. Mr. Spitzer is a member of ISA and belongs to ASME MFC and ISO TC30 committees. He 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.
For More Information: www.spitzerandboyes.com
