What Is the Overriding Consideration in Flow Measurement?

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Rugged, Proven Technologies Have a Real Advantage

Endress+Hauser views high precision as a key basic requirement in flow measurement, but there are other major considerations as well. “Repeatability and stability are also important for process instrumentation. In billing applications, good traceability providing proof of meter accuracy is important,” reported Christian Rützel who is in charge of the Flow Marketing Department at Endress+Hauser.

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Endress+Hauser points to surveys which show that high meter reliability and quality along with durability, ease of use and IEC 61508 SIL compliance are among the key criteria which influence purchase decisions. “This is what really separates the wheat from the chaff.” Rützel’s impression is that “extremely rugged, proven technology has a definite advantage”. “Standardized operation and development of IEC 61508 compliant devices have been high up on our development agenda for many years.”

Digital Is Best when the Going Gets Tough

A number of factors have an influence on the total measurement error including the temperature and pressure of the medium, zero stability, current output (resolution) and signal processing (power supply isolator, I/O card).

According to figures put together by the Namur Working Group, the combined error caused by these factors on analogue meters is significantly higher than 0.25 percent. A high-end meter with a measurement accuracy quoted at 0.1 percent on the spec sheet is not absolutely necessary under these circumstances. On the other hand, this also means that when high precision is essential, the only viable option is a digital meter.

Reliability is definitely crucial from the user perspective. What good is ultra-high precision if the equipment is susceptible to repeated failure causing system downtime and creating more work for the maintenance team? Also, in real world industrial applications, reproducibility of the measurement results is more important than absolute precision.

Jahn draws the comparison with a clock that keeps irregular time, running sometimes fast and sometimes slow. “In contrast to that situation, you can easily live with a clock that always runs 90 seconds slow. You just go to the 10 o’clock meeting when 9:58:30 is displayed on the clock. Also, you can easily adjust the clock once and for all so that it is set to the correct time. That is exactly what we do with field calibration.” Jahn also mentioned the problem of measurement accuracy/inaccuracy over the full dynamic range.

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