J. Kissel

Since I put the the power into PRM on to the ops overview (see LHO aLOG 51612), in addition to the power out of the PSL -- and noticed it has been dropping by 0.1W every day or three, I plotted a trend of these two input power metrics over the past ~4 months of O3.

Recall that we "permanently" started observing at "37 Watts" of "input power" on June 10, which is marked by the cursor; see LHO aLOG 49783.
Also, note that when we say "input power" in terms of "35 Watts" and "37 Watts" during O3, we mean the power out of the PSL. 

The revealed three things to me:
(1) The power loss between in power out of the PSL the power in to PRM is significant; consistently around 4.5 W, or a 12% difference.
(2) Since we've initial requested "37 Watts," and the powers were 37.5 into the IMC and 33.0 W in to PRM, we've lost about a Watt: the power is now 36.7 W into the IMC, and 32.1 W, and thus a loss of 2-3%
(3) There was a substantial increase after maintenance day on Jun 26 05:20 UTC (Tuesday Jun 25 2019 22:20:00 PDT), and a similar level drop Aug 30 02:44 UTC (Thursday, Aug 29 2019 19:44:00 PDT; after the new ALS pick-off was installed in the PSL; LHO aLOG 51610). The most recent drop was (32.4 - 32.1) / 32.4 = 1% drop in input power to the PRM.

For now this is just at the level of "huh," but given that the sensitivity is roughly proportional to the square root of input power, perhaps we should pay a bit closer attention to these kinds of changes and drifts, if possible.