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

This implied that the dewar flow meter results were about 10% too high. It should be noted that there is a reasonto be concerned about the dewar flow meter calibration: The calibration of the flow meter was performedduring payload test and prior to a replacement of a dc-dc converter in the ECU after the vehicle had beenshipped to Vandenberg. Thus there is a possibility that the power being applied to the flow meter heater isdifferent than that used during calibration. There is a provision for four-terminal voltage measurement on thisheater, but unfortunately this monitor circuit suffers the same problem as the HPM heater monitor.In addition, there is one other piece of evidence in support of the conjecture that the dewar flow meter resultsare about 10% too high. If the ATC flow data are plotted together with the dewar flow meter data reduced by10%, and the dewar thermal model results (based on measured dewar shell temperatures) are also plotted, allthree results are in reasonable agreement. This is illustrated in Figure 12-8. Also, by using the 13% scale factorcorrection on the HPM results and inferring an average flow rate, the HPM results can be brought into thecomparison: The average flow rate thus inferred from the HPM is 7.80 mg/s, which compares to the long-termATC average of 7.72 mg/s. Hence, it would appear that although there were some unresolved issues, the variousgauging techniques could be reconciled with each other in a consistent way by making some reasonableassumptions.Figure 12-8. Flow meter data reduced by 10% and plotted with ATC flow rate and flow rate predicted by thedewar thermal model. Thermal model is based on measured shell temperatures rather than predicted shelltemperatures.12.8 Comparison of lifetime prediction with final resultSubsequent to the last HPM operation, updated EOL predictions were made by integrating the ATC flowdownward from the last HPM result (17.8 kg) to the then-current date, and then integrating to zero using aprojected flow rate based on the flow rate measured a year ago. The resulting EOL predictions varied a little butGravity Probe B — Post Flight Analysis • Final Report March 2007 345