David W. Spitzer

Which of the following performance statements is better?

A. 1 percent of actual
B. 1 percent of measured value
C. 1 percent of rate
D. 1 percent of reading

This question only begins to illustrate some of the instrument specification problems that exist in industry. All of the above specifications are well-defined, and all of these statements mean the same thing — only the wording is different. Answers A, B, C and D are correct.

Additional Complicating Factors
Instrumentation suppliers often specify performance in a vague manner, such as “1 percent.” This type of statement begs the question, “1 percent of what?” Answers often include 1 percent of rate, span, maximum span, or other semantic variations thereof. Some telephone inquiries actually reveal that the supplier does not know the answer to the question, but will check and call back. In one case, a supplier investigated, provided an answer, and later changed the answer after re-investigating.
Sometimes suppliers state instrument performance clearly, but actually mean something else. This became evident when investigating non-contact level gauges where accuracy specifications were expressed over 25 different ways by the different suppliers.

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.