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Calibrating a Differential-Pressure Transmitter (Part II)

September 26, 2010
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David W. Spitzer


Last month, a differential-pressure transmitter was used to measure the level in a five-meter high atmospheric tank in water service. Changing the application to measure the level of liquid ammonia (SG=0.6) in the same tank, the high-pressure and low-pressure taps are located one meter and five meters from the bottom of the tank. The transmitter is located at the same elevation as the bottom of the tank (0 meters). The impulse tubes constantly slope downward from the tap to the transmitter. What is the calibration of the differential-pressure transmitter?

A. 0 to 2.4 meters of water column
B. 0 to 3 meters of water column
C. 0.6 to 2.4 meters of water column
D. 0.6 to 3 meters of water column
E. None of the above

Commentary
Answers A, B, C and D were calculated by multiplying last month’s answers for water service by the specific gravity of ammonia. As a reminder, Answer D (1 to 5 meters of water column) was the correct answer for water service.

However, the physical properties of liquid ammonia are quite different from the physical properties of water. In particular, liquid ammonia has a high vapor pressure. This means that if the liquid ammonia tank is at (say) 20 C, liquid ammonia in impulse tubing will boil if its temperature is above 20 C. This means that installing a differential-pressure transmitter in a liquid ammonia tank in the same manner as one would install it in a water tank will create havoc with the measurement. In particular, ammonia will boil out of the impulse tubes when their temperatures increase and condense when they cool down.

The best answer would be Answer E, but it might be more prudent to select another level technology for this application.

Additional Complicating Factors
Ammonia presents sealing issues for level transmitters because its vapor pressure is relatively high while the size of its molecule is relatively small.

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.

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