The 59 Hz chiller pumps at EY produce turbulence in the water lines that results in a peak at about 9 Hz in seismic spectra at Y-end. This peak creates problems e.g. for ISI. We have been planning to install a variable frequency drive (VFD) at EY so that we could reduce the water flow, in hopes of reducing the turbulence. The VFD was installed at EY this week and I ran some tests at various frequencies (pump speeds). Figure 1 shows that the peak is nearly at background with the VFD set to 30 Hz, instead of the nominal 60 Hz. 40 Hz is also pretty good so we decided to run at 35 Hz from now on. We believe that the cold water flow rate is down by less than a factor of 2 and that this should be sufficient for our needs. We will find out for sure when it gets hot next summer. We also now have VFD at EX and the corner station so we can adjust these as needed.

End Y is, as of now, in a seismic condition pretty similar to what we plan for aLIGO science mode. There are still a few pumps making sharp lines, but the broad band should be pretty close, especially when people are not out at End Y. To test the pointing of the seismic array, I placed a shaker on the floor at two locations Wed. night, running at 12 Hz. The times were:

Nov. 15 2012 0:54:54 UTC to 2:20:40 UTC (last ten minutes were the most quiet) and

Nov. 15 2012 2:29:40 UTC to 3:00:00 UTC (first fifteen minutes were the most quiet).

Robert S., Richard M., John W.