La Pavoni: Fuji PXG4 PID and Pressostat Working Together

(Finished project on

In my previous post I showed how the boiler of the machine almost gets too hot and pressurized when warming up with the PID temperature probe connected to the Brew Head.

I decided it would be best if I could warm up the machine using the pressostat because it is very good  at starting up as fast as possible while keeping the pressure inside the boiler at a modest level.

Once the machine is almost on operational pressure & temperature, the PXG4 could take over and make sure the brew head reaches its ideal temperature and stays there too.

Thinking this through, it struck me how very convenient it is that at start up, the pressostat is ON, whereas the Solid State Relay of the PXG4 box is OFF as long as the internal program has not been started.

So the PXG4 can be ON and helping me monitor the temperature, but it will not start controlling the system until I press the button that fires up the program in its memory.

If I set the pressostat to stop heating the machine at a pressure that matches a boiler temperature close to 110ºC, I can start the PXG4 program and the PXG4 will take over control, switching the heater ON and OFF as much as is needed to reach and maintain the correct brew head temperature.

This will get the machine the best of both worlds. Not a PID replacing a P-stat, but P-stat and PID working together, each in their own best area.

I also moved the Boiler Temperature probe from below the boiler to the outside, at the back of the boiler. Below the boiler seemed a great spot earlier on but it is so close to the heating element that it does not "see" the general temperature changes in the boiler water very well.

After today I will leave off the Boiler Temperature probe altogether. I want to avoid turning the La Pavoni into a Xmas tree of gadgets and wires. More wires facilitate more tripping accidents later on, and more parts that can fail.

And also, by now I feel I know enough about the boiler temperature. It goes up and down but that's fine as long as the brew head temperature remains straight as an arrow. That means my extraction temperature will be stable as well.

In the graphs below you can still see the Boiler Temperature results.

The pressostat kindly warms up the machine here, the boiler warming up in a fairly straight line. After reaching the boiling point of 100ºC I close the steam wand as soon as serious drops of water spit out and steam follows. I also do a short flush of the group to help heating it up. After that the machine idles with the group around 75ºC and the p-stat cycling, switching the element on and off  .

After 12 minutes I push the start button of the PXG4. It has been on, but not in control until then, just passing on information about the group head temperature and the inactive SSR. As soon as the PXG4 drives the machine, you can see the red line showing the target group temperature and the orange line showing the percentage of heating element power applied to get there and to maintain the best temperature.

Next I focus on an extraction. During the extraction, hot water from the boiler flows in, the group reaches its optimal temperature of 93ºC towards the end of the extraction and the heating element stays off for a few minutes to let the group cool down to the situation before the extraction.

In close up:

So next time the blue line will be gone.

I will visit Tije to see if he can make me a secure connection for the Brew group Temperature probe. This is needed to make the system robust. Until now I had the probe fastened to the group with a piece of copper tape that's mostly used by builders of electric guitars to shield the internal wires from any interference. If this probe would accidentally come loose, the PXG4 would switch on max power to get the probe hot, but it would fail and overheat the boiler.


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