Tyler, Eric, Richard, Robert

Since the last site-wide HVAC shutdown in August (72308) there have been a number of improvements in interferometer control and reductions in HVAC vibrations and coupling. In addition, last month the ACs in the CER were identified as significant sources which had not been shut down in earlier site-wide shutdowns (73430). So we shut down the HVAC site-wide today, including the CER ACs as well as the usual turbines and chillers at the corner and end stations:

Start  shutting down Nov 2, 18:06 UTC

Start turning on Nov 2, 18:20 UTC

The August shutdown increased range by roughly 10 Mpc to about 160 Mpc and Figure 1 shows that today's shutdown increased range by about 10 Mpc to about 169 Mpc. Figure 2 shows DARM spectra before during and after the shutdown. The reduction in noise seems to be broad band, reaching from 10 to 200 Hz, with the biggest feature being the 52 Hz peak which I think comes from the chilled water pump driving the cryobaffle at EX.

There were a couple of problems, to be expected as we continue to exercise the system, which I had not done since O3.  For example, one of the two CER systems did not turn back on, and will be looked at tomorrow. There appears to be a slight average increase in range associated with having these three ACs off.  I plan to study the coupling mechanisms for the CER ACs more this visit and also use focussed shutdowns to pinpoint other contributions.