GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

helium at 1.8 K. It does not appear, however, that the effect of the vapor is quite so negligible: When the maintank is at 10% of capacity, the heat capacity of the vapor is nearly 6% of that of the liquid. In the analysis of theHPM results, it was therefore assumed that the vapor was in equilibrium with the liquid, and its contribution tothe overall heat capacity was taken into account. This turned out to be a questionable assumption, as will bediscussed below.12.5.2 Results from HPM operationsSix HPM operations have been performed with the first being on 6/28/04, and the last on 7/29/05. In each case,the duration of heater operation was selected to yield a temperature increase of approximately 10 mK with aheater power of 1.44 W. This was chosen to yield a good signal-to-noise ratio for the temperature rise (~ 1%)without perturbing the system too greatly or significantly reducing lifetime (~15 hours for the firstmeasurement and proportionately less for subsequent measurements). An example of the data produced bysuch a measurement is shown in Figure 12-4, and the results of all six measurements are shown in Figure 12-5together with a linear fit and extrapolation to depletion. Also shown in the latter figure is the fact that abackward extrapolation of the linear fit to the date of launch disagrees with the initial quantity of 337 kg byabout 13%. This disagreement is indicative of a possible scale factor error, discussed above, which does not alterthe zero intercept on the time axis (end of life).Figure 12-4. Main tank temperature in response to fourth heat pulse measurement with lineartrendlines before and after heat pulse. Temperature increase is taken to be the difference in thetwo trendline values at the middle of the pulse.342 March 2007 Chapter 12 — Cryogenic Subsystem Analysis