QUIZ CORNER: Pressure Measurement Errors

March 17, 2008

Impulse tubing that is not completely full can cause measurement error. When empty, impulse tubing can trap air and form air bubbles. In one installation, the transmitter taps in

Impulse tubing that is not completely full can cause measurement error. When empty, impulse tubing can trap air and form 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

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 (0 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 * 1000)) of the absolute pressure.

Additional Complicating Factors
The transmitter and the flowmeter taps are typically not located at the same elevation and the installed impulse tubing routing is typical more complicated. Both of these complicating factors should be considered in the analysis (when applicable).

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.


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