Quick Look

 Compression Mass Gauge for Very Large Cryogenic Tanks, 18-9181

Principal Investigator
Franklin T. Dodge

Inclusive Dates:  01/24/00 - 05/24/00

Background - As part of NASA's Future X program, the Institute is developing a gauge to measure the quantity of cryogenic liquids in a spacecraft tank in a zero gravity environment. The gauge, named the Compression Mass Gauge (CMG), is the result of more than eight years of development sponsored by both NASA-GRC and SwRI, through its internal research and development program. A demonstration gauge will be tested in space in Boeing's Solar Orbital Transfer Space Experiment (SOTVSE). This quick-look project was prompted by the near-term possibility that the volume of the SOTVSE liquid hydrogen tank would be increased from approximately 70 cubic feet (the size for which the gauge was designed) to 1,000 cubic feet or more.

Approach - As shown schematically in the left illustration, the CMG employs a small bellows to periodically change the tank volume by a small fraction. The resulting change in fluid pressure is measured, and thermodynamic relations are used to compute the volume of gas (and thereby the volume of liquid), assuming that the bellows compresses the entire volume of gas uniformly. The bellows must be operated at several frequencies to compensate for frequency-dependent nonideal effects, as shown in the right illustration. The lowest allowable frequency of approximately 3 hertz is set by the response characteristics of the pressure sensor, while the highest allowable frequency is limited by the need to minimize acoustic resonance in the vapor volume. For the SOTVSE 70 cubic feet tank, bellows frequencies below 12 hertz eliminated all acoustic phenomena of any consequence. However, for a gas volume of 1,000 cubic feet, the highest allowable frequency is much less than 12 hertz and approaches the 3-hertz limit imposed by the pressure sensor. This narrowing of the frequency range for large tanks does not allow the real gas effects to be corrected by the current data analysis method. Therefore, a critical need exists to develop a method of improving the data analysis method or of otherwise accounting for or minimizing acoustic effects in large tanks.

Accomplishments - Computational fluid dynamic simulations demonstrated that acoustic phenomena would significantly degrade the gauging accuracy for gas volumes larger than approximately 100 cubic feet, using the current data analysis model. A pulse mode of gauge operation (i.e., extending the bellows and holding it in the extended position) was analyzed conceptually and found to be a solution to the acoustic problem; but this solution may be difficult to implement because of the need to distinguish a small transient pressure pulse superimposed on a much larger steady-state pressure. An alternative data-analysis method was also conceived in which bellows oscillations for larger tanks can still be used over a sufficiently wide range of frequencies if the data analysis model is modified to include acoustics phenomena explicitly by empirical means. This improved data analysis was developed by introducing an empirical constant by fitting the model to the measured pressure data over the gauging frequency range. The improved model appears to be satisfactory for tank volumes of more than several thousand cubic feet. For even larger tanks, the pulse mode of operation may be required.

Fluid and Machinery Dynamics Program
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