Saturday, August 21, 2010

Steam fail: latte, home

The Silvia has two thermostats: one to get the temperature right for pulling shots, and one to get the temperature right for steaming. As thermostats aren't terribly intelligent ("Turn the heater on if it's not hot enough in here.") my new PID setup has replaced both of these. The PID is a box with a blue LED temperature readout that measures the current temperature and figures out whether to turn the heater on or not, taking into account a few important points:
  • What was the temperature a moment ago? How fast is it changing?
  • How far away from the target temperature are we?
  • How long does it take the heater to start heating after we turn it on?
  • How long does it take the heater to stop heating after we turn it off?
Etc. That's the gist, I think. I'd go into more detail, but honestly I tried reading a serious document about how PID works, and my brain nearly melted. But that's not what this post is about.

It turns out there is also a third thermostat in there. If, somehow, the temperature gets to 155ºC (yes, very bloody hot) then something has obviously gone wrong, and the third thermostat shuts the whole thing down to prevent anything from seriously screwing up. The "I'm heating up" light will still come on, but the heater will be disabled. Somehow, I did this today while steaming milk. Yikes. In Silvia's defence, I was trying to do five things at once this morning. If I'd just stayed at the machine and focussed on the coffee-making thing, it probably wouldn't have happened. Pressing the reset button requires a screwdriver and moving all your stacked-up glasses off the top of the machine, so it's a bit annoying. Not quite as annoying as when you have to walk outside in the rain when you blow a fuse trying to boil the kettle and cook toast and at the same time, but almost that annoying.

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