The high 16.8Hz and 17Hz lines on the DARM signal are coherent with the CO2 WI and NI bench displacements, see Figure 1 and 2 (zoom). The structure is visible expecially on the seismometer signals on the benches. The coupling mechanism is not clear but the coherence is visible also in the magnetometer signal and on the WI accelerometer.
It could be worth to do a switch off of the clean room air conditioning.
Moreover on the WI cryoTrap only the 16.8Hz is visible
This morning I searched for the sources of these lines. The two fans serving clean air flux to the SPRB minitower area seems the source. It is advisable to perform a switch off once SPRB is in vacuum. Other tests done with CEB clean Rooms air conditioning, and with Antonio's fan cooling SR turbo pump.
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Using the accelerometer of TCS NI bench (the one which showed the highest coherence with DARM) I tracked down the two lines which today are at 16.7Hz and 16.9Hz (Figure 1, top-left).
With Davide we first switched off the CEB Clean Rooms air conditioning between 9:07 and 9:012 LT: this affected Lines at 17.9Hz and 18.5Hz which disappered from many accelerometers (Figures 1 and 2) but not the hunted lines.
Lines which appear to correspond to the searched ones are emitted by the two fans serving clean air flux to the SPRB minitower area. Figure 3: picture.
I also checked the fan which is being used to cool down the SR turbo (I used channel ENV_SR_ACC_Z temporarely connecting it to a meggit accelerometer attached to the fan' base. See picture in Figure 6). It emits vibration lines at 4.5Hz and around 31.Hz plus several harmonics above 50Hz, which however presently do not show evident coherence with accelerometers or microphones: Figure 6.
This morning after cleaning of the SPRB SAS by Marco, the two clean air flux fans have been switched off around 8pm LT. Two lines around 17Hz disappeared from seismic and magnetic sensors: Figure 1. Yet it was not possible to lock DARM on B1 DC as on April 14Hz so to check if DARM improved.
On May 5th we switched off the SPRB clean air flux fans while ITF was locked with DARM in DC. We did not observe any noise reduction in DARM nor MICH at the fans frequencies (17Hz peak): see Figure 1. For comparison Figure 2 shows the situation on April 14 when we observed the large 17Hz peak coherent with some ENV accelerometers. Indeed presently the 17Hz peak in DARM and MICH (if it is the same) has reduced by a factor 100. This might be due to having put the SPRB in vacuum, or thanks to alpha tuning, or other, to be investigated.
