David W. Spitzer, P.E.

What is the approximate flow measurement error associated with a differential-pressure flowmeter for liquid service operating at a pressure of four bar where the differential-pressure transmitter is calibrated for 100 inches of water column, and the impulse tubing on the high side of the transmitter has a 10-inch vertical bubble of non-condensable gas?

A. 1 percent
B. 2 percent
C. 5 percent
D. 10 percent
E. None of the above

Commentary
The 10-inch error at full scale flow is 10 percent of the full-scale differential pressure. This error represents an approximate 5 percent flow error at the full-scale flowrate. However, the process pressure will compress the bubble that is initially present at startup, so Answer C and Answer D are not correct.

At operating pressure, the non-condensable gas will be compressed by a factor of approximately [(4 bar + 1 bar)/1 bar] or 5. Therefore, the height of the bubble will be approximately 10/5 inches, or two inches. A two-inch error is 2 percent of the full-scale differential pressure and represents approximately 1 percent flow error at the full-scale flowrate. Therefore, Answer B is not correct.

The approximate two-inch water column error associated with the bubble is independent of the operating flowrate. By way of example, the error associated with the bubble is two inches of water column when the flow is the 10 percent or 100 percent of full-scale flow. At 100 percent of full-scale flow, the flow error is approximately 1 percent. However at 10 percent of flow, the flow error attributable to the bubble (2 inches of water column) is larger than the differential pressure measurement (1 inch of water column). In other words, the flow measurement error varies with the operating flowrate, so Answer A is not correct. Answer E is correct (except at 100 percent of full scale flow).