Flow Measurement What Is the Overriding Consideration in Flow Measurement?
... the answers are unequivocal. Reliability is the top priority in the process industry.
Flow metering creates the basis for accurate billing and input/output tracking of fuel, ultrapure water, steam, etc. High-precision flowmeters are also used to enhance quality in the pharmaceutical industry as well as for emissions trading and in safety instrumented systems. Accuracy, versatility and suitability for direct mass flow data acquisition are advantages which frequently make Coriolis flow meters the solution of choice. In addition, Coriolis mass flow detection systems are compact and have good immunity to contamination. It is little wonder that these devices are gaining market share.
A survey conducted by PROCESS two years ago revealed that one-quarter of companies surveyed were using this type of device. Volker Kramer, Product Manager Flow at Emerson Process Management, reports that there has been considerable movement in the Coriolis flowmeter supplier market in recent years, and he believes that this has unsettled users. He recommends that the prime consideration should be the dependability and experience of the manufacturer rather than the meter itself.
What Level of Precision is Sufficient?
The thing about measurement precision is that you need to take a closer look. How accurate is a meter really? The measurement uncertainty quoted by manufacturers continues to decrease. At the last Namur Annual General Meeting, Namur Working Group 3.2 “Flow Measurement Technology” quoted data published by several manufacturers of Coriolis mass flowmeters which shows that this is indeed the case.
However, it is important to realize that when a brochure lists a measurement error <0.1 percent, this refers to digital devices operating under reference conditions.
These numbers cannot be used as a yardstick in real world applications, because the meters are often analogue versions used outdoors. The measurement error in these situations is actually around one percent.
Variations in the references used are an additional problem. The error percentages can be based on the measurement value or the measurement range or instrument range, or they can be listed as an absolute μA value.
“Of course measurement accuracy says a lot about the quality of a meter or a technique,” said Thomas Jahn, Head of Product Management at Flexim in Berlin. “However, we have real concerns when some manufacturers specify extremely high measurement precision which is difficult to deliver even under laboratory conditions and is unrealistic in actual field use. So we were not at all surprised by the discrepancies between the data sheets and real world performance reported at the Namur meeting.”