PID offset calibration - Help finding flash-boiling point

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#1: Post by MrSourAndOrBitter »

I am trying to set-up the PID offset on my Lelit Bianca and have trouble determining the "flash-boiling" point.
I have followed the method described in ... t-settings and Getting accurate temperature displays on PIDed double boilers. My local boiling point is very close to 100 °C.

Starting with 98 °C I increased the temperature to 104 °C one degree at a time with stabilization periods in between (with PF mounted):
As you can see the water already starts boiling at 98 °C after a few seconds but I believe this is not the flash-boiling point yet. At 103 °C the water starts boiling stronger and at 104 °C even more steam is visible.

For me I guessed the "flash-boiling" point must be at 103 °C due to the strong boiling. In consequence I increased the PID offset by 3 °C.

I then set the PID to 94 °C (for a roast on the lighter side), let the machine stabilize (1 hour) and pulled some water again:
The water starts boiling after a few seconds. This seems wrong, for my understanding at 94 °C nothing should be boiling out of the group head.

Could someone please help me identify the "correct" "flash-boiling-point". Of course this wouldn't be dead accurate, but to make sure to be in the right ball park.

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#2: Post by another_jim »

Immediate steam and boiling, aka "flash boiling". The water in the boiler is always above boiling -- run it long enough and it will eventually heat the group and boil as it exits, the offset is about getting the immediate water temperature right
Jim Schulman

MrSourAndOrBitter (original poster)

#3: Post by MrSourAndOrBitter (original poster) »

Hi Jim, thanks for the clarification, appreciate it. I would determine 102 °C as the point where the water leaves the group head boiling immediately and increase the offset by 2 °C.

Would you agree on the 102 °C?

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#4: Post by another_jim »

Sorry, I'm not much good at interpreting videos; trust how it jogs your own memory.
Jim Schulman

MrSourAndOrBitter (original poster)

#5: Post by MrSourAndOrBitter (original poster) »

Hi Jim, thanks again. It is hard to trust your guts when you have no idea what you are looking for, hence the questioning.

I will see if I can find a more definitive answer somewhere and report back for documentation purposes and anyone else seeking advice.

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#6: Post by Nunas »

You might try another approach, albeit a very old one if you're having trouble finding the flash boil point. Get a styrofoam cup and insert a thermometer with a thermocouple (the thinner, the better). With no coffee in the portafilter, pull some "shots" into the cup and note the temperature. You'll soon get a feel for the actual brew temperature of your machine. Once you know that, then you can fiddle with the offset or just with the temp setting on the PID to make it come out to where you want it. Or, find someone to help you who has an older HX machine. They'll be used to spotting flash boil, as it's part of the process for them.

MrSourAndOrBitter (original poster)

#7: Post by MrSourAndOrBitter (original poster) »

Hi Maurice,
thank you very much for your suggestion. You inspired me to create a crude temperature probe (see pictures attached) consisting of an old NTC and my chepo multimeter. I stuck the NTC through the bottom of the portafilter and plugged in some rubber bands to create some resistance.

With this measurement I measure between 95.3 °C and 96.3 °C with the machines PID set to 93 °C. With a single "calibration" measurment in boiling water beneath the surface 99.6 °C are measured (so little offset should be subtracted).

For now I trust the measurment (more or less) and reduce the PID offset by 2 °C.