TJ, Sheila
The fast shutter guardian runs a check in DRMI on AS_A and AS_B, it checks that the power on those PDs are above and below thresholds, which are set by the 'SYS-MOTION_C_SHUTTER_G_TRIGGER_LOW' and 'SYS-MOTION_C_SHUTTER_G_TRIGGER_HIGH' channels. This test was passed when we had the pressure spikes, because the beam to those diodes wasn't clipped.
TJ and I have been discussing adding an additional check to try to catch the type of problem we had in our recent pressure spikes, where there was clipping on AS_C. The table below shows values with DRMI locked, to see if we can detect the problem before fully locking the IFO. It seems that a check on the power on AS_C would cause false alarms, because of the variablity of DRMI build ups. A check on the ratio of AS_C to AS_A or AS_B seems more viable, but still would probably risk false alarms. The difference between the lowest "good" ratio for AS_C/AS_A and the highest bad ratio is only 3.5%, for AS_C/AS_B that difference is only 7%.
| AS_C NSUM (W in HAM6) | AS_A NSUM | AS_ B NSUM | AS_C/ AS_A | AS_C/ AS_B | |
| 6/6 20:46 UTC (bad) | 0.046 | 3908 | 4240 | 1.17e-5 | 1.08e-5 |
| 6/7 2:30 UTC (bad) | 0.049 | 4162 | 4515 | 1.17e-4 | 1.08e-5 |
| 6/7 3:20 UTC (bad) | 0.048 | 4145 | 4488 | 1.15e-5 | 1.07e-5 |
| 6/6 12:08 (good) | 0.052 | 4240 | 4433 | 1.22e-5 | 1.16e-5 |
| 6/6 7:10 UTC (good) | 0.0506 | 4098 | 4267 | 1.23e-5 | 1.18e-5 |
| 6/6 5:21 UTC (good) | 0.053 | 4361 | 4575 | 1.21e-5 | 1.16e-5 |
| 6/6 00:56 UTC (good) | 0.048 | 3943 | 4105 | 1.21e-5 | 1.17e-5 |
| 6/5 23:44 UTC (good) | 0.053 | 4319 | 4513 | 1.23e-5 | 1.17e-5 |