A differential pressure transmitter with an upper range limit of 500 mbar and a set span of 0 to 250 mbar (approximately 0 to 100 inches) is installed outdoors. The transmitter has an ambient temperature effect specification of 0.001 percent upper range limit/°C plus 0.002 percent set span/°C. Approximately how is the flow accuracy of the transmitter affected by ambient temperature fluctuations of ±20°C?
A. 0.04 percent of flow rate
B. 0.08 percent of flow rate
C. 0.1 mbar
D. 0.2 mbar
E. None of the above
Ambient temperature fluctuations can affect the differential pressure measurement by 0.01 • 0.001 • 500 plus 0.01 • 0.002 • 250, or 0.01 mbar/°C. This corresponds to a 0.2 mbar potential error for the ± 20°C temperature fluctuation. Answer D might appear to be correct, but it is an absolute (fixed) differential pressure error and not a flow error.
The 0.2 mbar potential error represents 0.08 percent of the set span of 250 mbar and would introduce approximately 0.04 percent of flow rate error when the flow is at 100 percent. The flow error at other flow rates will be different, so Answer A and Answer B are not correct.
Answer E is correct.
Additional complicating factors
Not only will the flow error be different at different flow rates, but the differential pressure transmitter can also be significantly affected by the static pressure at which it is measuring. In many applications, both temperature and pressure effects should be considered.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC offering engineering; seminars; strategic marketing consulting; distribution consulting; and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. Spitzer has more than 40 years of experience and has written more than 10 books and 300 articles about flow measurement, instrumentation and process control. He may be reached at 845-623-1830 or at spitzerandboyes.com.
