It would be fair to say that the overwhelming majority of instrument suppliers make valid claims about the instruments they sell. For example, a supplier may claim that its flowmeters perform within a specified accuracy, such as 0.5 percent of flowrate. The supplier has presumably tested the flowmeter and found that its performance falls within this specification. The user can then decide whether this particular flowmeter meets the needs of the application.
Farther along in the conversation, the supplier may mention that the flowmeter has a turndown of 1000-to-1. Again, the claim may be factual and supported by test data. But, think about what 1000-to-1 turndown really means. For example, magnetic flowmeters typically have a nominal maximum velocity of 10 meters per second. A turndown of 1000-to-1 describes flowmeter operation between velocities of 0.01 and 10 meters per second.
Consider the following questions about typical applications:
- How many flowmeter applications operate at velocities above (say) three meters per second?
- How many flowmeter applications operate at velocities below (say) 0.10 meters per second?
- Given your answers to the above questions, how many flowmeter applications operate at velocities above (say) three meters per second and below 0.10 meters per second? In other words, how many flowmeter applications have greater than a 30-to-1 turndown?
- As an afterthought … How many flowmeter applications operate with a maximum flow of 10 meters per second? In other words, how many applications are there where 1000-to-1 turndown is possible?
I suggest that most flowmeter applications require less than 10-to-1 turndown – although it would be nice to have more. Applications operating at 10 meters per second are not common; so 1000-to-1 turndown is not possible in the overwhelming majority of applications. On a practical note, I often perform calculations and specify flowmeters assuming a minimum of 10-to-1 turndown unless there is specific process information indicating that a higher turndown is required.
Playing out the mathematics, it becomes clear that many flow measurement applications can be reasonably designed for a 10-to-1 turndown. The flowmeter described above has a 1000-to-1 turndown. Is this flowmeter 100 times better?
David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular "Consumer Guide" series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845-623-1830.
