Getting accurate shot temperature displays on PIDed double boilers without a thermometer

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another_jim
Team HB

Postby another_jim » Oct 26, 2018, 5:05 pm

The attraction of PIDed single and double boiler machines is that you get the shot temperature you see; no fiddling, no flushing. If the shots are too bitter or too sour, you can change the temperature with a few button pushes, and get a predictable result. If you walk up to another PIDed machine, and use the same temperature and brew recipe, you will get the same result. Roasters are taking advantage of this cross-platform consistency to publish brew recipes that are actually useful and meaningful.

There is a catch. The temperature sensor controlling the brew boiler must be set close to the cold water inlet to work properly. At that spot, the brew boiler temperature needs to be several degrees higher than the temperature the puck will experience. This is the offset. The size of that difference varies by machine, and is sensitive enough so that it can vary among the individual machines of the same model. You may get a new machine where the offset is wrong by several degrees, and as the machine ages and scales, the offset may drift. Bottom line, you need to be able to calibrate the offset.

In the past, this calibration required the use of accurate portafilter thermometry like the Scace thermofilter or the earlier Schomer portafilter. Judgment was required using these devices, since their thermal mass delayed the readings for about 7 to 15 seconds. Here is a method that requires no thermometry and no judgement calls, since it relies on the liquid to vapor phase change, which is easily seen as flash boiling.

This technique has been used for at least ten years; but has usually been posted as instructions on how to use for particular machines. I think it should be published as technique that is widely applicable to many PIDeed espresso machines. Here is DaveC's 2008 instruction for the Izzo Alex DB.

Set your brew temp to 100C (or your local boiling point), let the machine stabilize. Flush the group without a portafilter. Do you see flash boiling? Now set it to 99C and let the machine stabilize. Flush the group. Do you see no flash boiling? Congratulations, your offset is perfectly calibrated.

Here is a video of a properly calibrated machine showing the flow at 98, 99, 100, and 101 ceontigrade. There is weak boiling at 99C that takes time to develop; to me this indicates I'm not quite there yet. But if you pick this point for your offset, your calibration is still going to be pretty good. The 98C is clearly to low; and the 101C flow is boiling no harder than the 100C.



The idea is to find the **lowest** temperature at which you see flash boiling, the LFBT (lowest flash boiling temperature). If it is at the local boiling point in your preferred temperature unit, you are set. If the LFBT is X degrees higher than you local boiling point, your offset is X degrees too low and needs to raised by X degrees. If the LFBT is X degrees lower than the local boiling point, your offset is too high, and needs to be lowered by X degrees. (There is a gotcha for Fahrenheit users, the offset is usually in Centigrade even if the panel reading is in Farenheit -- check your manual, or always do this calibration in Centigrade).

Please post your thoughts, criticisms and observations. I tried this technique on the Bianca I'm reviewing. With a few minutes work (along with a few hours of stabilizing time), it arrived at the same offset I got doing several days of thermometry. So I'm quite enthusiastic about how easy and physically elegant this is. However, there may be problems or ambiguities I didn't experience.
Jim Schulman

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yakster

Postby yakster » Oct 27, 2018, 2:16 pm

Sounds like a great technique, Jim.

I assume you'd substitute 100 C for your local boiling point of water where significant due to altitude.
-Chris

LMWDP # 272

pizzaman383

Postby pizzaman383 » Oct 27, 2018, 2:40 pm

I think that sounds like a very good method. Should we call it the Schulman Calibration Technique SCT?

Edited - see earlier description by DaveC: http://coffeetime.wikidot.com/izzo-alex-duetto-offset-settings
Curtis
LMWDP #551

Mesmer

Postby Mesmer » Oct 27, 2018, 4:11 pm

I think many people installing after market PIDs, epecially to single boiler machines (like Gaggia Classic) are familiar with this method.
The big problem is identifying flash boil. Maybe someone with a well calibrated machine can make some videos like flash boil for 99, 100 and 101 c settings.

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another_jim
Team HB

Postby another_jim » Oct 27, 2018, 4:23 pm

yakster wrote:I assume you'd substitute 100 C for your local boiling point of water where significant due to altitude.


Yep, I have "local boiling point" in the body of the text, I've edited it in at the top too.
Jim Schulman

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another_jim
Team HB

Postby another_jim » Oct 27, 2018, 4:32 pm

Mesmer wrote:I think many people installing after market PIDs, epecially to single boiler machines (like Gaggia Classic) are familiar with this method.
The big problem is identifying flash boil. Maybe someone with a well calibrated machine can make some videos like flash boil for 99, 100 and 101 c settings.


I figured I couldn't possibly be the first one to come up with anything this simple. You'll see mild steam and lazy bubbles at lower temperatures than the flash boiling (just heat a pot of water to see what I mean). I used the point where I could hear and see energetic hissing and bubbling. This was the hallmark that got me to the same offset as I got doing thermometry. I'll make a video asap, although I'm not sure how helpful it would be.
Jim Schulman

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ailevin

Postby ailevin » Oct 27, 2018, 4:41 pm

Is the equilibration step done with the portafilter in place? I've noticed that my "cruising" temperature as measured by Eric's thermometer is different if I use a bottomless portafilter vs. larger heavier dual spout portafilter.

Alan

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another_jim
Team HB

Postby another_jim » Oct 27, 2018, 5:01 pm

I'm not sure what you mean.

Changing your PID offset won't change the readout on your group thermometer. Since you have a group thermometer displaying the shot temperature, a temperature readout on the PID display giving the nominal shot temperature is somewhat redundant. You can use the display for something else; for instance, you can put the offset to 0, change the readout from set point to actual, and see what your boiler is doing in real time.

In any case, it's easiest to observe flash boiling from the group when the PF is off.
Jim Schulman

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ailevin

Postby ailevin » Oct 27, 2018, 5:27 pm

Hi JIm,

I understand the redundancy of worrying too much about offset with the GH thermometer present. My observation was that at idle the GH temperature changes depending on the presence and type of portafilter. My question was whether the difference between boiler set point temperature and temperature at the puck is influenced by the thermal mass and radiative properties of the portafilter as well as the head. The implication was that perhaps your offset calibration method might give different offsets depending on the temperature of the GH and that the temperature of the GH might depend on whether there was a portafilter inserted during equilibration after a change in boiler set point on the PID.

Alan

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another_jim
Team HB

Postby another_jim » Oct 27, 2018, 6:06 pm

Ah, a way subtler point than I ever considered: The difference between group and brew boiler temp can vary, if the PF is left on or off, and by how heavy it is. That difference will change the offset.

I did mine with bottomless, which is what I usually use. That would be the drill -- use your regular PF. I guess if you use several PFs, you should see what happens with each. There won't be a correct offset, and you'd have to mentally offset, and change the temperature, if you change PFs. When I get a chance. I'll see what happens with the regular PF.

I suppose the same issue affects calibration with a thermofilter, since that is done with a bottomless as well.
Jim Schulman