Will the Venturi measure accurately if the differential pressure transmitter can measure the differential pressure accurately between 10 to 100 percent of full scale? 


This is somewhat of a trick question because the information presented is not complete — even though it may appear to be complete. Note that the first sentence clearly addresses the entire system: Venturi and transmitter. The second sentence is a bit dubious because the question only refers to the Venturi but does not include information to evaluate the hydraulic operation of the Venturi.

If the question is whether the Venturi flow meter system will operate accurately, we should calculate the differential pressures at 10 and 40 percent of full scale, which are 2.5 and 40 mbar respectively. The transmitter will operate accurately above 10 percent of its full scale, or 25 mbar. Therefore, without examining the operation of the Venturi primary flow element, the Venturi flow meter system will not measure accurately over most of its anticipated operating flow rates. 

Additional complicating factors

The Venturi flow meter system may not operate accurately over most of its anticipated operating flow range, but it will nonetheless operate with reduced accuracy that might be acceptable for the application. You might say, especially in difficult application, that sometimes poor performance is not that poor.

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.