Abstract
We have designed a system for the COBE flight dewar to measure its liquid helium fill. We apply a small known amount of heat to the helium tank and monitor the temperature rise in the liquid and the tank. Working with a detailed thermal model of the tank and liquid we can determine the amount of liquid present. COBE uses a 117 mW, 7 mA heater to warm the helium. It is planned to use the mass gauging system only after the projected midpoint of the mission, after one full sky survey. The system is optimized for use with 50–75 liters of helium (≈8–12% of the cryogen tank volume). Ground testing of the system in a one gravity environment is difficult, but from tests conducted so far we estimate an on-orbit temperature rise of ≈2.5mK/min. A similar system is planned for the Superfluid Helium On-Orbit Transfer (SHOOT), a Shuttle-based experiment. The SHOOT’s specific requirements call for a high power pulse heater, applying 40 W for approximately 20 seconds.
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