Air Bubble Troubles

Impulse tubing that is not completely full can cause measurement error, as empty tubing runs the risk of trapping air bubbles. In one installation, the transmitter taps in a two bar steam flow measurement system are located at the same elevation as the flowmeter taps. The impulse tubing is routed down 25 cm (10 inches) from the taps and then up to the transmitter to form a condensate seal. What is the approximate pressure measurement error if the air is not bled out of one of the impulse tubes during calibration?
A. 25 cm of water column
B. 12.5 cm of water column
C. 8.3 cm of water column
D. 5 cm of water column

Commentary
The first order of business is to determine whether the pressure unit (bar) is an absolute pressure unit or a gauge pressure unit. Some engineers were educated to always treat bar as an absolute pressure unit, whereas common industry practice is to use bar as a gauge pressure unit. For the purposes of this problem, let’s assume that bar is a gauge pressure unit. In addition, one atmosphere is 1.01325 bar absolute. However, one bar absolute will be used in this problem to simplify the calculations.

The bubble will be 25 cm high before the transmitter is put in service. Putting the transmitter into service will compress the bubble and reduce its size. Therefore, Answer A is not correct.

The size of the bubble is inversely related to its absolute pressure. The absolute pressure is not two bar, so Answer B is not correct.

The absolute pressure of the compressed bubble (three bar absolute) is calculated by adding one atmosphere (approximately one bar) to the gauge pressure (two bar). This will compress the bubble to approximately one-third (1/3) of its original size at atmospheric pressure (zero bar), so its height can be calculated to be approximately 8.3 cm. Answer C is the correct answer. This corresponds to almost 0.3 percent (8.3 / (3 * 1,000)) of the absolute pressure.