A couple weeks ago I was at the tower making bad jokes about the liquid cooled EEV ESCIOT tube based Harris PowerCD transmitter being a space station toilet.
Really, it’s a three stall restroom, and today I got all three flushing again… and learned more about how freaking weird and scary *pure* deionized water can be.
First, here’s a questionable explanation of what’s in there. You’re looking at two separate liquid cooling loops. The external one which exits the cabinet at upper left circulates an ethylene glycol coolant solution (similar to automotive antifreeze, but nigh unobtainable outside of ordering it off Shamazon) between the heat sources and a set of fan cooled radiators outdoors. It’s circulated by an external pump station. I marked its flow with the orange arrow emojis. The internal one has a pump in the cabinet as it’s a closed loop within. The vertical accordion looking piece is a heat exchanger. Attached to the door on the left are two filters that keep the deionized (DI) water as pure as possible to keep its electrical resistivity high.
The supply manifold at the top sends the anode and collector water jacket water supplies to the tube cart around the front. The small line coming out the middle feeds the filters; you can set their flow rate with a valve up there. Everything finally returns to the pipe at the left that sends the DI water back to the reservoir on top. Now, have you noticed the middle finger emojis yet? Well.
In the DI water returns from the anode and collector are these Seametrics flow sensors. The pinwheel has magnets in two opposing vanes, and a Hall effect sensor screws into the recess seen at the bottom here. By measuring the interval between pulses, the transmitter controls can determine if there’s enough water flow… until the sensor breaks.
Now let me say this, I see absolutely nothing wrong with the design and build of the Seametrics sensor. It’s actually damn cool for what it is. No metal parts contact the working fluid, and it rides on a ceramic shaft and ruby bearings like a fine watch movement (and that wouldn’t even have ceramic shafts… Or would it?)
The Seametrics is even completely field rebuildable!
So, uh, time to be creeped out and amazed by mere water. In the picture of the cabinet you’ll see there’s one more sensor mounted horizontally in the glycol line. This never fails, as the glycol solution has some lubricity to it – that is to say it’s slick and forms a film that tends to isolate facing surfaces from direct contact, just like an oil would. The DI water, however…. No. When I got some on my hands, it felt really weird, almost more like I’d just rubbed them with a really cheap and nasty hand sanitizer that was stripping the oils and leaving behind sticky yackage. So let’s see what it does to those extremely hard, smooth bearings:
The bore of the bearing above has become egg shaped. This wasn’t even the worst one — that distinction goes to the one that was in the collector flow meter:
I wasn’t able to pull this one apart for further inspection but didn’t need to. You can see the axle right through the plastic — it chewed completely through the ruby bearing and started digging into the plastic. Funny thing was this one would work perfectly UNTIL the water temp rose to about 46 degrees C when I put the cabinet in Beam On (normal RF output state), at which point it’d abruptly start ticking down from 12.6 GPM to 10 and the controls would kick the beam supply off to avoid meltdown. After rebuilding both sensors on the DI water side, the flow readings come up the moment the pump starts and stay stable.
Want to read more about how damn weird pure water is? There’s a somewhat sensationalized (in their usual style and don’t even get me started on that Supermicro fiasco) article from Business Insider about the Super Kamiokande which is a massive subterranean neutrino detector tank lined with the stuff that physicists have had to enter on a rubber boat for maintenance. Just imagining what that’d feel like across a large area of skin makes me want to go rub an Aloe Vera leaf on my entire body.