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DAVID W SPITZER'S E-ZINE (September 2020)


Differential Pressure Transmitter Performance (Part 2 of 3) by David W Spitzer

Microprocessor-based differential pressure flow transmitters are generally more accurate and exhibit a larger turndown than analog differential pressure flow transmitters. As a result, fewer models (capsule sizes) are needed to measure the applicable differential pressure ranges. For example, 2 or 3 microprocessor-based transmitter models may be needed to measure over the same differential pressure ranges handled by 5 or 6 (older) analog transmitter models. As result, users could reduce their spare parts inventory costs while suppliers benefited by the savings resulting from the design and support of fewer models. From a performance perspective, transmitter accuracy specifications for each model generally apply over a range of set spans.

Part 2: Measuring Under Vacuum: The Phenomenon of Reverse Flow by David W Spitzer

Last month, I mentioned three situations that one should carefully consider: operation under vacuum, measuring downward liquid flow and gravity flow. Let's continue to discuss operating under vacuum.

Centrifugal Pump Speed Reduction by David W Spitzer

Various reference materials contain information describing the Affinity Laws associated with reducing the speed of a centrifugal pump. Mathematically, they show that the:

  • Pump flow is proportional to pump speed

  • Pump discharge pressure is proportional to the square of pump speed

  • Mechanical load on the pump is proportional to the cube of pump speed

What are the approximate pump speed, pressure and mechanical load at 50 percent flow?


In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 450 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.


Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.


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