Virgo Logbook

Infrastructures Monitoring (Infrastructure Monitoring)

fiori, tringali, paoletti - 14:44 Friday 10 June 2022 (56110)

magnetic lines from AHUs

CEB magnetometers sense a few magnetic lines that are associated to the AHUs, both the CleanRoom AHU and the CEB AHU. One magnetic line is associated to the rotation frequency of each fan. The CEB hall AHU has two additional lines associated to the inverters. The amplitude of these lines in the CEB magnetometers is of the order of 0.1 nT/sqrtHz.

Figure 1 shows the magnetic lines associated to the CleanRoom AHU: 5.9 Hz (Return fan speed) and 18.5 Hz (Supply fan speed)

Figure 2 shows the magnetic lines associated to the CEB hall AHU: 3.74 Hz (Return fan speed) and 7.65 Hz (Supply fan speed)

Figure 3 shows the magnetic lines associated to the two inverters driving the motors of the CEB hall AHU: 45 Hz and 46 Hz.

Yesterday, Maria and i took a measurement with a magnetic probe in proximity of the CleanRoom AHU and then about 8 meters away. As shown in Figure 4, the 5.9Hz and 18.5 Hz magnetic lines (and their harmonics) are indeed very strong in proximity of the AHU (the RMS intensity is of the order of 200 nT) and at 8m their intensity decays to less than 1 nT RMS. It seems that these lines are irradiated by the AHU fans.

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Comments to this report:

fiori, tringali - 17:38 Tuesday 21 June 2022 (56215)

A doubt espressed by Federico was about the fact that the measured magnetic noise could be an artifact due to magnetic induction on the probe which was vibrating inside the earth's magnetic field.

Thus, we did further measurements with the same flux-gate magnetic probe. We compared the magnetometer spectra when the probe was attached to the vibrating surface of the Clean Room box and when holding the probe in our hands, so to seismically isolate it from the vibrating surface. This is shown in Figure 1

Figure 2 shows a similar comparison, but for a different location of the probe, that was on the floor at about 1m from the AHU box.

The result is similar: despite the larger low frequency noise due to extra low frequency motion of the probe (impossible to stay perfectly still while holding it), the peaks associated to the supply (18.5Hz) and the return (5.9Hz) fan and harmonics (11.8 Hz) did not change in amplitude. The same seems true for all peaks, actually. We conclude the peaks are a real ambient magnetic field.

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Tringali, Fiori, Paoletti - 11:10 Wednesday 22 June 2022 (56217)

Something similar happens in the magnetometers of the terminal buildings:

a coherence is observed between the magnetometer (W channel) in the experimental room and the accelerometer (*UTA_BOX_ACC) installed on the AHU case at the fan rate: ~10.74 Hz for the NEB AHU and ~10.69 Hz for the WEB AHU, respectively;
in the case of the NEB conditioning system, an additional coherence at 40 Hz (and harmonics) associated with the inverter frequency is observed;
for the WEB air handling unit, the line generated by the inverter occurs in the magnetometer spectrum and not in that of accelerometer and a coherence is observed between the two probes at the third harmonic of the inverter.

The inverter line is not present in the IPS and UPS current and voltage monitors of North and West end buildings.

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