La Pavoni Brew Group Temp Control

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(Finished project on
http://kostverlorenvaart.blogspot.nl/2015/01/finished-pavoni-fuji-pxg4-pid-probe.html)

In the previous post I had a PID made by Fuji, their PXG4, control the temperature in the bottom of the boiler of the La Pavoni.

Since I had figured out by measurements and tasting that the extraction temperature inside the puck is at the optimal value if the probe on the top of the brew group measures 88ºC I has subsequently determined that a boiler temperature of 116ºC ensures that at my current room temperature the brew group maintains the desirable temperature.

Having the probe that the PXG4 works with on the bottom of the boiler is convenient because the SSR output of the PXG4 also connects to the heating element on a spot below the boiler, so both cables can be bundled which makes for less hassle during transport, for instance when emptying the boiler in the kitchen sink.

Most other espresso machines that flaunt a PID also use the boiler connection for their probe. This way the consumer has one less cable to worry about, as it is hidden away in the casing of the machine. The manufacturer usually assumes a certain "offset" for the difference between the Boiler Temperature and the desired Brew group Temperature. For instance, if the Brew group Temp needs to be 88ºC as in the case of my La Pavoni, and the Boiler Temperature matching that is 116ºC, the offset is 28ºC.

Flint, a reader on Home-Barista, suggested I should measure this, as I have done in my previous post and I enjoyed taking this one step further: having the PXG4 work with the Brew group Temperature probe directly and letting it figure out the optimal Boiler Temperature for me.

PXG4 connected to Brew group Temp probe, top left of lever
Warming up the machine from a cold start was not ideal yet, just like having your car on highway cruise control speed while still leaving your parking space is not ideal. I worked around this by doing a few flushes, but this is not the way to go. I had not run the auto-tune for this situation yet, so having done that might help on the next start-up. In order to warm up the group, the PXG tries to heat up the boiler more than usual.

Probably the best approach would be to warm up the machine using the p-stat and once the group is near its operational temperature, switch on the "autopilot" / "cruise control". I would need to add a switch to the La Pavoni to go from one control to the other. Not a problem.
Warm up flushes and auto-tune
Above, you see the result of the three flushes, and the meandering of Boiler Temperature and Brew group Temperature during the time the PXG4 is in auto-tuning mode.
After auto-tune, PXG4 ensures a perfectly stable Brew group Temp
 Highlighting the auto-tuning and the resulting stability of the Brew group Temperature.

Locking in the cool portafilter is easily compensated by PXG4
Normally, having the room temperature mass of the portafilter locked in changes the temperature of the brew group a little. This is in practice never a real problem because at extraction time, hot water flows in and these small differences hardly affect the extraction temperature in the coffee puck. Nevertheless, I like to see that the PXG4 easily compensates by raising the Boiler Temperature 2ºC.
Temperature stability over idle time
The result is a very stable brew group temperature during idling. Seeing how it behaves with a series of extractions will be a possible next test.

Related posts:

 http://kostverlorenvaart.blogspot.nl/2015/01/plotting-pavoni-pulse.html

 http://kostverlorenvaart.blogspot.nl/2015/01/time-to-pid-pavoni.html

http://kostverlorenvaart.blogspot.nl/2015/01/wiring-la-pavoni-for-fuji-pxg-4.html





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