Leak rate tests Because both the suspension devices and their  accumulator tanks were to be used in a vacuum  chamber, their leak rate was obviously of concern. The usual method of leak testing,  developed for vacuum chambers, involves  helium bagging the devic e, pumping the interior  down to vacuum, and analyzing the gases  pumped out to see how much helium is present.  Because the suspension devices contain air at several different pressures with vacuum outside  the device, this standard  method would not have  been representative of operating conditions.    Fortunately, the allowable leak rate was fairly  generous, at least by the standards of the  vacuum industry. The method used for the  suspension devices was to pressurize the interior of the device to 1.01 bar (14.7 psi) above  ambient, valve off the device with a low-leakage  valve (Figure 14), and then record the interior pressure and temperature over several days. Mass loss from inside to outside was then calculated from the ideal gas law for air and compared to the allowable.  The same method was used for the tanks except that they were pressurized to 3.45 bar (50 psig). Not  surprisingly, it was found that sensing pressure alone was not adequate. Even with the device under test inside an insulated “doghouse”, pre ssure variation due to diur nal temperature variation was much larger  than that due to leakage. Temperature compensation was essential to the method.    Figure 14  Low-leakage vacuum valve used in leak  rate tests Pressure was measured using the same ultra-high-resolution quartz crystal pressure transducers used in  the ride-height control of the actual system. They use the change in natural frequency of a quartz crystal  due to pressure as the sensing mechanism. A built-in digital frequency counter allows a pressure  resolution of a few parts per million. A built-in serial interface allows transmission of data to the recorder  or controller without loss of accuracy. While quite slow by normal analog standards, their update rate was  adequate for both the leak rate tests and the ride height controller. Temperature wa s measured by RTDs  with signal conditioning that included A/D converters and serial interface to the recorder.             Figure 15 shows typical pressure and temperature time history data from leak testing of one of the  accumulator tanks. Figure 16 shows the time history of air mass in the tank calculated from the measured  temperature and pressure after subtracting the average value. Also shown in Figure 16 is a straight line whose slope represents the allowable leak rate. While the trace showing the measured mass loss is  somewhat erratic, it is clear that the time-average leak rate is less than the specified maximum allowable.  While some test articles failed the test initially and required some lea k chasing and debugging, all  eventually passed in time for delivery per the schedule.  While adequate for the purpose, the leak rate measurement  method was slow and on ly practical because  equipment was available to test more than one article at a time. It would not be practical for testing at the  much smaller leak rates typically specified for vacuum chambers or for the bellows seals of the  suspension devices. The latter were tested by standard  helium mass spectrometer methods prior to  assembly of the suspen sion devices.  326