Sheila, Timesh, Robert

There have been several alogs about this and there will be others, but I wanted to try make the situation more clear to detchar.

When the new instrument air compressor turns on, for about 5 minutes approximately every 25 minutes, we see a 35Hz line in DARM, which goes away when it is turned off. Around the time the compressor swtiches off, there are terrible scattering shelves in DARM.  The onset of scattering in DARM seems to happen around the time of the compressor switching off (as seen by PEM seismometers), sometimes it is 16 seconds after, sometimes 24 seconds, sometimes before the ground motion drops off by up to a minute.

The first attached screenshot shows the regular switching of the compressor, and the board band large noise in DARM when the compressor goes off.  The second screenshot shows darm spectra in 3 states:  compressor off, compressor on (there is a 35Hz peak added to DARM) and compressor switching off, shelf.  The PRCL/SRCL/MICH spectra also show fringe wrapping when the compressor is swtiched off.  

These are probably interesting times for people who are interesting in looking for scattered light.  Bubba and Tyler have ordered some parts to better isolate the new equipment, so this should be mitigated within a few days.