Last year (April 2024), we spent some time focusing on the FEE temperatures (see here and here), worried that they were higher than in years past.  I am unsure whether we really paid sufficient attention in years past, and maybe got excited about nothing.

Nevertheless, the temps today (March 25, 2025) are even a little higher, though we did notice day-to-day variations last year, too.

Unsure whether our temps are too high now, I did some tests, which I record here.  Some points

• When the power supplies are turned on after a long time off, one has to do a one-wire scan to even read out the FEE temperatures, so here, I did the one-wire scan right away.
• As last year, we see the cards warm up a little bit (from ~20C to ~22C) and equilibrate within say 10 minutes
• Changing states from one-wire to OFF does nothing to the temperature. 
• Changing from OFF to PHYSICS makes an "immediate" (less than 1 minute) jump of several degrees C.  Many cards reach 30-32C.  This state change reads in the bias and VPED values.
• Subsequently changing from PHYSICS to OFF has no effect on temperature.
• The temp is not related to the higher dark current that we have today (due to radiation damage to the SiPMs from the high-intensity pp running).  I conclude this by using a settings file that has Vset=0, and I still see the same jump.
• Is it related to VPED (which changes the DC output of the FEEs)?  Maybe, but changing from "normal" VPED values (which vary channel to channel, but range roughly from +/- 20) to all channels having VPED=0 makes tiny change.  Probably a time dependence, rather than a VPED dependence.

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Some images from the tests.  The plots are FEE card temperature versus time.


Figure 1.  Test morning of March 25



Figure 2.  Test evening of March 25


Figure 3: Blow-up of figure 2