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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.