Part II: Open-Channel Flowmeter Problems

David W. Spitzer

Last month I asked what can be wrong with a flowmeter and referred to an article where an open-channel measurement (used to measure the wastewater effluent prior to the pumping station) was suspected to be “way off” (Flow Control, May 2012, page 14). A number of end-users and consultants provided responses, and their suggestions varied all over the map, including:

Installing an ultrasonic level transmitter that does not contact the water might be better than the existing transmitter. However, transmitter replacement implies that the existing (unidentified) level transmitter is not functioning properly. This is certainly possible, but I suggest that the operation of the existing level transmitter should be verified prior to its replacement. Investigation could also reveal that the existing level transmitter already uses ultrasonic technology.

Determining the type of flow conditions in the flowmeter (free or submerged) might seem reasonable. Most of these flowmeters are designed for submerged flow conditions so as to require only one level transmitter. Free flow conditions would require two level transmitters. It is certainly possible that the flowmeter is being operated under free flow conditions, so this would warrant checking—especially if the flowmeter is new or never operated properly.

Measuring the velocity of a liquid flow in an open channel with a Pitot tube will likely be technically difficult and exhibit poor accuracy. Not only is the differential pressure produced by a Pitot tube relatively low and often difficult to measure, but maintaining the impulse tubing full of liquid between the taps and the transmitter will likely be difficult. Beyond these issues, the Pitot tube measurement (even if accurate) does not solve the open-channel flow measurement problem.

More next month …

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. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.

David can be reached at 845 623-1830 or spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.