Download FG5 Absolute Gravimeter User`s Manual - Micro

Micro-g LaCoste FG5 User’s Manual
Figure 15. Characteristic Residuals signal when the vacuum is poor. Note
the vertical scale is about 10X larger than normal.
4.2.2 Feedthrough Issues
The ferro-fluidic feedthrough is what allows the external motor to drive the dropper
cart through the vacuum wall. When the FG5 has been sitting idle for a while (10s of
minutes), it is normal to see a momentary increase in ion pump current for the first
few drops. This is due to the release of molecules in the feedthrough mechanism.
The ion pump then “catches up”, and the vacuum returns to normal.
The feedthroughs typically have a lifetime of about 2 – 3 years. When they begin to
fail, you will notice that the ion pump increases are larger, and take many, many
drops to return to normal. When the feedthrough finally fails completely, the increase
is “permanent” through all the drops, and the vacuum does not recover until the
dropper is idle. At this point, it is necessary to replace the feedthrough. Though the
replacement procedure is beyond the scope of this manual, it is something an
advanced user can perform. Please contact Micro-g LaCoste for instructions on the
replacement.
4.2.3 Dropper Tuning Adjustments
Again, a full discussion of the dropper tuning circuit is too large for this manual, but
there are a few common “tricks” that can be useful.
With time and/or temperature changes, it is possible that the servo circuit needs to be
adjusted from the original factory settings. The common symptom is that OSC mode
will “time out” and the dropper controller will go into RESET mode (DROP mode
normally still works well). To correct this, open the lid of the SIM unit and locate the
dropper controller circuit board on the left. In the middle of this board is a
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