Log entry time 10:50:53 on April 15, 2012

Entry number 374887

It looks like the helium level probe in the nose temporarily froze up again. James was sitting by me narrating during the show, so here's the rundown of events as they happened:

Looking only at the black line on the Levels.stp graph:

• At about -12 minutes the signal goes flat. This is because the individual sensors on the probe are discretely spaced (vertically), and when one of them locks up, the probe reads the helium level to be exactly where that sensor is. Now the helium is boiling off without the PID loop resupplying it, until eventually boiling down to the next discrete sensor.
• At -7 minutes, the helium level is now low enough for the PID loop to start feeding helium into the nose, which happens at -5 minutes. I don't know why there is a lag between -7 and -5 minutes, but the level probe doesn't seem to be reading.
• After -5 minutes the probe starts reading and helium begins returning to the nose via PID loop control.

The PDP screen shows what happened to polarization during this. When the PID loop stops feeding 4 K helium into the nose, the temperature of the existing helium being pumped on drops, so we see a jump in the polarization. When the sensor tells the PID loop to reintroduce helium, the new helium warms the fridge back up and the polarization bubbles pops like an early 2000's dot-com crash.

Levels.stp

PDP