Noticed that the two relatively new full-range, hot cathode ion gauges on corner volume exhibit strange behavior at a pressure between 5e-6 and 6e-6 Torr. Last Friday PT-180 suddenly fell in pressure (no one on site making changes to vacuum systems). Today when cycling turbo valves (for leak checking), PT-170 suddenly dropped in pressure. Kyle noted something peculiar with PT-525 at EX on Aug. 31, 2017 after TMDS discharge activity (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=38471). Will contact Inficon for an explanation.