A differential pressure transmitter with an accuracy specification of ±0.05% of span calibrated in the shop and set for 0-100 differential pressure units will be used to measure the flow through an orifice plate. What is the approximate performance of the orifice plate flow measurement system at 20% of flow?
A. ±0.7%
B. ±1%
C. ±1.5%
D. ±2%
Despite apparent detail in the question, the information presented could be more precise. For example, over what range of differential pressures was the transmitter calibrated? Is transmitter performance predicated on maximum, calibrated or set span? Conditions under which the transmitter will operate can significantly affect performance and are not stated. How well the orifice plate installation conforms to applicable published standards will affect flow measurement system performance. Is the orifice plate operating properly?
Ignoring these potentially significant issues and other details, orifice plate performance of approximately 0.7% of rate should be statistically combined with the transmitter performance at 20% of flow, which is approximately 1.25% of rate (assuming performance is based on set span). Answer C would be an appropriate estimate.
Additional complicating factors
The aforementioned (and other) significant issues plus the reality that most noncustody orifice plate installations do not conform to published standards suggest that the actual installation will likely exhibit significant additional performance degradation.
David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.
