This sensor has applications in cryogenic liquid storage tanks.

Although there are several methods for determining liquid level in a tank, there are no proven methods to quickly gauge the amount of propellant in a tank while it is in low gravity or under low-settling thrust conditions where propellant sloshing is an issue. Having the ability to quickly and accurately gauge propellant tanks in low-gravity is an enabling technology that would allow a spacecraft crew or mission control to always know the amount of propellant onboard, thus increasing the chances for a successful mission.

The Radio Frequency Mass Gauge (RFMG) technique measures the electromagnetic eigenmodes, or natural resonant frequencies, of a tank containing a dielectric fluid. The essential hardware components consist of an RF network analyzer that measures the reflected power from an antenna probe mounted internal to the tank. At a resonant frequency, there is a drop in the reflected power, and these inverted peaks in the reflected power spectrum are identified as the tank eigenmode frequencies using a peak-detection software algorithm. This information is passed to a pattern-matching algorithm, which compares the measured eigenmode frequencies with a database of simulated eigenmode frequencies at various fill levels. A best match between the simulated and measured frequency values occurs at some fill level, which is then reported as the gauged fill level.

The database of simulated eigenmode frequencies is created by using RF simulation software to calculate the tank eigenmodes at various fill levels. The input to the simulations consists of a fairly high-fidelity tank model with proper dimensions and including internal tank hardware, the dielectric properties of the fluid, and a defined liquid/vapor interface. Because of small discrepancies between the model and actual hardware, the measured empty tank spectra and simulations are used to create a set of correction factors for each mode (typically in the range of 0.999–1.001), which effectively accounts for the small discrepancies. These correction factors are multiplied to the modes at all fill levels. By comparing several measured modes with the simulations, it is possible to accurately gauge the amount of propellant in the tank.

An advantage of the RFMG approach of applying computer simulations and a pattern-matching algorithm is that the predictions can be verified through testing on Earth, and the results can be extrapolated to low-gravity liquid configurations using simulations of liquid configurations that would be likely to occur in low gravity. Such liquid configurations can also be solved using other computer software tools such as the Surface Evolver code. RF computer simulations are routinely used in the RF and communications industry to design or predict performance of RF devices. The same software tools can be used to calculate the electromagnetic eigenmodes of large tanks with a two-phase fluid distribution. By having a pre-built library of tank eigenmode simulations, the measured tank eigenmode spectra can be compared with the library of spectra to determine the unknown amount of propellant in the tank.

This work was led by Gregory A. Zimmerli, David A. Buchanan, Jeffrey C. Follo, Karl R. Vaden, and James D. Wagner of Glenn Research Center; Marius Asipauskas of National Center for Space Exploration Research; and Michael D. Herlacher of Analex Corp. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18373-1.

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