Reports until 18:09, Thursday 07 August 2025
H1 PEM
samantha.callos@LIGO.ORG - posted 18:09, Thursday 07 August 2025 - last comment - 09:17, Monday 11 August 2025(86257)
20-40 Hz HVAC Noise

Samantha Callos, Robert Schofield

There is a persistent peak at 20 Hz that has been appearing and disappearing intermittently for several months. I have found the times it existed corresponded to work hours during week days. Robert and I looked at the summary pages and DTT for the floor accelerometers and noted that the noise was most present in the area around YCRYO, we ruled out other locations around site and determined the source of the noise was coming from the Vacuum Prep Warehouse. We tested seismic isolation of the various HVAC units around the warehouse and found the Liebert AC unit inside the VPW which can manually be turned on and off and was moved there sometime in the last year.

I cycled through intervals of turning the Liebert unit off and on and checked the CS floor accelerometers for those times (see times below, Fig. 1, and blue arrows in Fig. 4). When the unit is off, the spike at 20 Hz in the accelerometers disappears. I then checked coupling to DARM and noted the noise at 20 Hz was present and that there was a harmonic at 40 Hz as well (see Fig. 2).

Additionally, for the external AC (Daikin), we noted that the springs it is mounted on are entirely compressed, so there is little to no seismic isolation for the entire unit. Noise from the previous unit has been found in DARM before (see alog 77477). The closest accelerometer to this HVAC unit is the YCRYO floor accelerometer and the shut down period can be seen in the spectrogram for it (see Fig. 3). It can also be seen automatically turning off and on the summary page 24-hour spectrogram (see Fig. 4). Comparing this shut down period to when it is on, it does not look like the noise is making its way into DARM for now, however, we recommend seismically isolating the Daikin as we have seen it couple to DARM before. 

Liebert AC on/off times for 08-05-2025:

Liebert AC on/off times for 08-06-2025:

Liebert/Daikin on/off times 08-06-2025:

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shivaraj.kandhasamy@LIGO.ORG - 02:25, Friday 08 August 2025 (86259)
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In the a-log 85984, we noted that in addition to the sharp line at 21.26 Hz in DARM there was also a ~1Hz broad feature around the 21.26 Hz line. At that time, it wasn't clear whether those two features were connected. Looking at the on/off times from this test, it seems they are connected.  The first attached figure is the DARM (GDS STRAIN CLEAN) and the second figure is the LVEAFLOOR YCRYO accelerometer. The feature in accelerometer is sharp, while in DARM we see both the sharp as well as 1Hz broad feature. In the comment to the a-log 85984, we looked at different auxiallary and PEM channels to check if there are any other channels that show both these features. Among the channels we looked at, we saw both these featues in ASC-PRC1_{Y,P} signals. The third figure shows the spectrogram of ASC-PRC1_P_IN1_DQ during this on/off tests during which we see the sharp as well as ~1Hz broad feature in that channel.  It is not clear whether this is just another witness of DARM feature or this is a place where it gets into the DARM. 

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richard.mccarthy@LIGO.ORG - 08:10, Friday 08 August 2025 (86261)
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The liebert unit was installed circa 2018.  The outdoor unit was replace more recently.

elenna.capote@LIGO.ORG - 09:17, Monday 11 August 2025 (86293)
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I just want to add that we do not use PRC1 P/Y in loop in full IFO lock. However, the PRC1 error signal is the POP A DC signal from the POP QPD on HAM3. This means that the POP A QPD may be a good witness of this line, but is not the coupling source of the line.