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On site we have a 324 GE generator hydrogen cooled. 8 months before GE made a MAGUC inspection when it removed the end shield and seal ring from collector end. During the last month we have experienced sudden drop of H2 purity while the unit starts up and mainly when it synchronizes. From 98% we go to 93% during the period from 2700 RPM to synchronization. It is also interesting that during the last two outages we have the lube oil pumps stopped along with turning gear and the aux seal oil worked to keep the dif pressure. Is there any suggestion for the sudden drop of H2 purity? From the time that the H2 pressure is higher than the oil we can't have entrapped air. We also don't have oil drainage to the seal oil enlargements drain pots, but bear in mind that due to a mistake last outage 8 months before a lot of oil was inserted in the generator. If we don't have air or oil contamination then it should be something from the generator itself (e.g. dust from fretting?) that creates the contamination.