David W. Spitzer

A differential-pressure transmitter is used to measure the level in a five-meter high atmospheric tank in water service. The transmitter high-pressure tap is located one meter from the bottom of the tank. The transmitter is located at the same elevation as the bottom of the tank (0 meters). The impulse tube constantly slopes downwards from the tap to the transmitter. What is the calibration of the differential-pressure transmitter?

A. 0 to 4 meters of water column
B. 0 to 5 meters of water column
C. 1 to 4 meters of water column
D. 1 to 5 meters of water column
E. None of the above

The low side of the differential-pressure transmitter is open to atmosphere at zero pressure (gauge). When the water level is at the tap at one meter, the impulse tube is full of water and produces one meter of water column at the differential-pressure transmitter. When the tank is full, the water level is at five meters and produces a pressure of five meters of water column on the transmitter. Therefore, Answer D is correct because the calibration is 1-5 meters of water column.
However, the specified calibration for an analog differential-pressure transmitter could be 0-4 meters of water column. This is because the one-meter offset would be adjusted out electronically. Microprocessor-based instruments allow the user to set the actual zero and span pressures in actual pressure units.

Additional Complicating Factors
This transmitter cannot measure levels below the tap located one meter above the tank bottom.

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.