The ETMy has been slowly sagging over the last few months. Recently, the sag rate has increased. While the temperature control at EY appears to be noisier during this December's cold spell, the overall temp has not changed by a large (greater than a deg C) amount, so I doubt that is the full cause of the drift. However, there have been a few pressure changes that the ETMy chamber has witnessed, according to the VE pressure channels I picked off and plotted (attached). I can't correlate all of the vertical motion humps exactly to these pressure steps, but the direction is correct: lower pressure means a a lower QUAD - that said, the sag usually happens much faster than weeks when the pressure or temp is changed, so wt-heck?. The most recent step in pressure shown on the plot was Dec 11th when the VE crew was doing some work on an IP bakeout at Y2-8. I'll poke them to see what the nature of the pressure step is. I worry a bit that there is something wrong with the channel after the step feature since it goes very quiet past that point.
The corresponding ETMy pitch motion change is much more subtle, which is good from an alignment standpoint. The BOSEMs still have plenty of signal, so we shouldn't have any issues with drive due to this drift.
Another day, another mystery. Keep H1 weird.
This pressure change is too small to account for the vetical drift. In log 15887, ETMY is measured to sag by 120 microns during pumpdown, a pressure change of 760 torr. For a pressure change on the order of 1e-8 torr, as seen here, you would therefore expect a vertical change of
(1e-8 / 760) * 120 microns = 1.6e-9 mincrons or 1.6e-15 meters. This is well into the noise of the OSEMs.
I think it is more likely the in-vacuum temperature is different from the VEA temperature sensor. Are the any in-chamber temperature sensors on the ISI table?
A consistency check you can do is see if it is just the suspension, or really a change in the whole chamber is to see if the ISI is sagging as well; or rather trying to sag since it has active position control. So, if the vertical ISI actuators are pushing up harder proportional to how much the quad is sagging, then there is at least some consistency throughout the chamber.
Since the suspension pitch isn't changing (which is great) that suggests that either the suspension is super well balanced, or it is just the top springs at the ISI table that are sagging. The temperature of these springs likely follows the temperature of the ISI much more closely than the ones lower down.
