# Quiz Corner: Calculating flow error for pressure transmitter installation and calibration

Feb. 1, 2023
Do you have the right answer? Test your knowledge with the latest installment of Quiz Corner.

An orifice plate is sized to produce a differential pressure of 0 to 100 inches of water column (WC) corresponding to zero to full-scale flow respectively through a nominal 200# steam header where the flowmeter is located approximately 25 feet above grade. A pressure transmitter calibrated for 0 to 300 psig will be installed and used to pressure compensate the flow measurement. The pressure and differential pressure transmitters will be mounted on pipe stands for convenient access. What is the approximate flow error associated with the pressure transmitter installation and calibration?

1. 5% high
2. 2.5% high
3. No error
4. 2.5% low
5. 5 % low

The steam header is located approximately 25 feet above grade. Transmitters are typically mounted approximately 4.5 feet above grade. Therefore, the pressure transmitter calibration will introduce an error in the amount of its condensate leg of approximately 20 feet, or approximately 10 psi where 2 feet of water column roughly corresponds to 1 psi.

The pressure transmitter will measure high by approximately 5% (10/200) but slightly less so when considered in absolute pressure terms (10/215). The output of differential pressure flowmeters changes by approximately -0.5% per percent density change where increasing pressure increases density. In this application, the measured pressure is higher than the actual header pressure so the differential pressure flowmeter will tend to measure lower than the actual flow by approximately 2.5% (Answer D).

The density of the condensate is dependent on its temperature, which is usually assumed to be uniform along the length of the condensate leg. However, the temperature of the condensate (and thereby its density) can vary with the seasons in outdoor locations in some applications. In other applications, the condensate temperature can be significantly different in different parts of the condensate leg such as when the differential pressure transmitter is located outdoors, and the pressure transmitter is located indoors.

David W. Spitzer has written over 500 technical articles and 10 books on flow measurement, instrumentation, process control and variable speed drives. David offers consulting services, writes/edits white papers, presents seminars and provides expert witness services at Spitzer and Boyes LLC (spitzerandboyes.com or +1.845.623.1830).

### David W. Spitzer

David W Spitzer’s new book Global Warming (aka Climate Change): An Understandable Data-Driven Explanation and Pathway to Mitigation (Amazon.com) adds to his over 500 technical articles and 10 books on flow measurement, instrumentation, process control and variable speed drives. David offers consulting services and keynote speeches, writes/edits white papers, presents seminars, and provides expert witness services at Spitzer and Boyes LLC (spitzerandboyes.com or +1.845.623.1830).

### Micro Motion™ G-Series Compact, Drainable Coriolis Flow and Density Meters

Micro Motion G-Series Coriolis flow and density meters are ideally suited for Process Monitoring and Optimization applications, offering easy selection with pre-selected models...

### Learn about: Micro Motion G-Series Coriolis Flow and Density Meters

The Micro Motion G-Series is designed to help you access the benefits of Coriolis technology even when available space is limited.

### Micro Motion 4700 Coriolis Configurable Inputs and Outputs Transmitter

The Micro Motion 4700 Coriolis Transmitter offers a compact C1D1 (Zone 1) housing. Bluetooth and Smart Meter Verification are available.