Server Farm Power Monitoring
This was really not a critical thing to do, but...
I bought off a Hong-Kong seller a pair of digital meters, with blue LED backlighting. One is an AC voltmeter capable of measuring 80-500V, the other is a frequency meter capable of measuring 40-70 hz, also on 80-500V. The neat thing about both meters is they're powered off the AC line they monitor, thus no batteries or wall warts are necessary.
So, this is what I started with. The 50A transfer switch on the left (black) and the subpanel for the server farm on the right. Ignore the dangling and disconnected electrical wire just above the panel, that's been there for a year plus. I've been meaning to unsnake it and reuse it elsewhere.
The two meters fit a rectangular hole, "almost" the same size as the "decora" light switch, so I bought a junction box and a mud ring that is for decora switches. All I had to do to allow the meters to fit is notch where the mounting tabs go. I used my milling machine for this. I probably could have used a hand file but the milling machine is more fun :)
Got it installed and wired across the 240V feed, and clearly I don't have the expected 240V or 60Hz. I tested with my multimeter (which can be set to give both measurements) and it turns out the voltmeter is correct and the frequency meter is wrong. I can adjust that with a jewelers screwdriver which I'll do later.
Here is a closeup of the digital meters operating. Very nice.