Which of the following flowmeters exhibit the highest turndown?
From a practical standpoint, it does not matter much which of these technologies has the highest turndown. Of importance is whether a given flowmeter technology will accurately measure flow over the desired flow range for the application at hand.
Presuming the flowmeter will operate over a wide range of flows, it would be helpful to eliminate those technologies with limited turndown by using the magnetic flowmeter as a benchmark. A typical magnetic flowmeter typically operates accurately at velocities from approximately 0.1 meters to 10 meters per second. This corresponds to a turndown of approximately 100-to-1. Coriolis, turbine, and ultrasonic flowmeters can approach or exceed this turndown.
Vortex-shedding flowmeters typically operate at velocities from approximately 0.3 meters to six meters per second and turn off at low flowrates so turndown is typically less than 20-to-1. Differential-pressure primary flow elements can operate over wide ranges of flow; however, their differential-pressure transmitter(s) often limit turndown to approximately 10-to-1.
Needless to say, there are exceptions to the above numbers. Magnetic and ultrasonic flowmeters can increase their turndowns by measuring at velocities over 10 meters per second, turbine flowmeters can measure lubricating fluids at low flowrates and higher turndowns, and differential-pressure flowmeters can achieve over 10-to-1 turndown.
Which flowmeter technology exhibits the highest turndown boils down finding the specific flowmeter within the appropriate technologies that may be capable of achieving the desired turndown in a specific application.
Additional Complicating Factors
There are hundreds of flowmeters that can potentially be applied to achieve high turndown in a given application. Only one may meet all of the requirements. Investigating the possibilities can sometimes be a daunting task.
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.