On Monday (Dec 10) both fan trays of the Sa_PR crate had been moved from FAST back to SLOW in order to help with the alignment investigation of SPRB. They remained with this setting till yesterday (Dec 12) when we performed the following tests, and finally set them to FAST, that is the condition that removed the 70Hz bump from Hrec.
In order to understand if the "loud" vibration and acoustic noise originates from just one of the two trays, and which one, we changed the velocity of the trays once at a time:
starting condition: BOTH cooling fan trays at SLOW (since Monday morning)
8:51 UTC cooling unit n.1 set to fast (level 15)
9:02 UTC cooling unit n.1 set to SLOW (level 5)
9:05 UTC cooling unit n.2 set to fast (level 15)
9:10 UTC cooling unit n.2 set to MEDIUM (level 10)
9:20 UTC cooling unit n.2 set to SLOW (level 5)
9:23 UTC both units set to FAST and left there
The evolution of the noise (monitored by the accelerometer placed on the top tray) shows that the nasty (approx) 70Hz vibration onsets when switching to SLOW the cooling unit n.1
In more details: the second plot: shows that unit n2. behaves has expected: at 9:05 we see 3 lines corresponding to the 3 fans of the tray moving from about 36Hz (2160rpm, the slow rotation speed) to about 48Hz (2880rpm, the fast rotation speed). At 8;51 when unit n.1 is set to fast strange things happen: two lines (two fans?) appear at about 48Hz but not very stable, a third line (third fan?) instead moves down and starts fluctuating around 35Hz., in addition the nasty noise at 71Hz appears.
A doubt arises if some nasty noise is also produced by cooling unit n.2 BUT it is not detected by our accelerometer on the top tray. Therefore, the bottom plot shows what is measured by a distant accelerometer, placed on PR tower, closer to the coupling location: only the noise associated to cooling unit n.1 is seen, nothing from n.2
The indication is that cooling unit n.1 is faulty and should be replaced (there are several spare fans available). The operation needs however to power off the crate.