Note: a previous version of this page had incorrectly used relative humidity in the DAQ room instead of on the platform. This is now corrected, but the conclusions are unchanged.

 This first plot pair is of field cage differential currents (inner minus outer in [μA]) to remove variations due to the cathode power supply (which has its own humidity dependences that are common to all field cages and subsequently not of high concern), along with humidity inside the blower tube as measured by the new online hyrgrometer that sits inside the blower tube, versus date in May (e.g. 10 = the start of May 10^th, 2018). It is clear that an increased differential current is seen when the humidity inside the tube gets above ~45%. Below that is a similarly-plotted-versus-date record of the humidity on the platform (which experience an outage for several days during this period) and in the DAQ room over the same period. These are not quite identical to the humidity inside the blower tube. Roughly speaking, it appears that the meter inside the tube is showing a number 5% higher than these others. Whether that's a real difference, or just the systematics of different meters, is not definitively known (a new platform meter was installed at the very end of May that appears to read nearly 10% higher relative humidity than the previous meter).




This has the appearance of being a humidity-related problem for the inner TPC field cages (it wouldn't be the first such problem this year), albeit very minor at this point (I wouldn't worry about 0.01 μA differences, but I would worry about 0.1 μA differences, as we already do for the intermittent OFCW short). It is an open question whether yet higher humidities will be seen inside the experiment hall over the upcoming weeks of Run 18.

These high humidities have only rarely been seen before at STAR during TPC operations. While we don't have old records for humidity inside the blower tube, we do have old records for humidity on the platform as a proxy. Here they are from 2012 through now. Humidities close to 70% have been seen, but the maxima are in the summer after TPC operations have ceased for the year.




Using this data, I have tried to correlate periods of high humidity with field cage current behavior. Unfortunately, it's not quite so simple because ionization (from high luminosity operation) in and around the TPC draws current from the TPC field cages (differently for inner and outer field cages). I tried to pick a time when humidity showed a large range, and luminosities weren't high, so I found early UU193 data from days 113-123 of Run 12, which appears to show no correlation between field cage currents and the relative humidity on the platform, though only barely reaching the platform humidities where we see an increase for Run 18. Alternatively, in Run 15 the luminosities were high enough that I was reasonably able to separate out field cage currents during times when there was no beam over days 128-180. This took us later into the summer and even higher humidities, very similar to those now seen in Run 18, but still exhibiting no apparent correlation with humidity on the platform that year.



In summary...

I have no explanation at this time why there appears to be a correlation now, but there wasn't in Run 15.

-Gene


_________



Update on June 5, 2018:

The humidity got even higher over the weekend, causing the current differential to go about 0.018 μA high. The blue and pink small dots are just as in the earlier plot for Run 18 except that now I am using the humidity inside the blower tube and I'm looking at more recent days, and the larger dots are profile histograms to make it easier to read the trend.



Fitting an exponential to the IFCW-OFCE data leads to this function: f = e^{0.177*(humidity - 76.31%)} - 0.0079 μA

If I am concerned when the current is 0.100 μA above the baseline of -0.0079, that would occur when humidity = 63.4% inside the blower tube.

-Gene