There are reported cases where a pressure transducer was installed at the bottom end of a liquid-filled instrument tube for measuring pressure in a recirculation pipe. For example, in one case, a small diameter instrument tube extended a long distance downward from a power plant recirculation pipe to the transducer. A continuous pressure oscillation was observed, which was not consistent with any other measurements in the recirculation system. However, it was found from paper-and-pencil analyses that such an oscillation could be traced to a resonant frequency in the system if the vertical liquid column rested on a postulated trapped gas bubble, which could have remained after venting. The relative compressibility of the liquid column is also compared with the gas bubble compressibility to show when acoustics of the liquid column are important.

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