chandra.romel@LIGO.ORG - posted 10:31, Thursday 05 May 2016 - last comment - 19:59, Thursday 05 May 2016(27027)
unexplained vacuum gauge signal stepping
After Tuesday activities, noticed a step change in pirani gauge signals. Attached is plot of PT100 (on HAM 1) and PT140 (on BSC4). Note: PT100 CC gauge keeps turning on and off due to pirani set point and stepping. PT120 also shows a step in signal. Yesterday Richard & Gerardo pulled the leads on PT120 while measuring the voltage. Voltage read higher valve when lifted from rack. PT140 pirani trend over 15 days shows a major step from 4/26 (when Beckhoff was installed) to 5/3 when power was switched and system rebooted.
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Sounds like the input impedance of the Beckhof ADC is lower than the original VME card ADC. These form a voltage divider with the gage head's output impedance. I/O impedances should be listed in the respective instrument datasheets. There may be a jumper/selection available.
Beyond recalibrating epics records, good to confirm that the gage heads are happy driving whatever impedance they see full-scale.
Good idea Chandra. I hadn't even considered looking at the piranis!