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What temp offset are you using with your PID?


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I have MrShades PID kit on my 2020 Classic Pro, and in his instructions he recommends and offset (Psb) of 14.4f. If I measure the water temperature when it comes out the brew head, I need an offset of 26f to get the PID reading to match the brew head temperature. I am using a Thermapen to measure the temperature so it may be not 100% accurate, but I seem to come up with an awfully large Psb compared to what's recommended. I know the thermocouple is measuring the boiler wall temperature and there is heat loss from the boiler wall to when the water exits the brew head, but just trying to get an idea of what everyone else is setting the offset to? 

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See the Auber report I posted here: 

 

Conclusions from their report: 

Quote

Conclusion When making a double shot (60 ml), the brew water temperature of Gaggia Coffee deluxe can drop 10 C (18 F) at the end of the shot. However the temperature change profile is reproducible and predictable. When correctly operated, the Gaggia can produce a very reproducible shot. The brew temperature will take at least 20 minutes to stabilize after start from room temperature. The time needed between the shots should be at least 4 minutes or longer. The optimal PID setting temperature should be between 210 to 225 F (99 to 107 C) 

Note this is for a Gaggia Coffee Deluxe, not a Classic. 

But without knowing of any Scace experiments on the Gaggia Classic, the Auber recommendation seems to the one to follow. If you want to choose, I guess you can take the midpoint of the two recommendations.

I personally prefer to keep the temperature without the offset. The temperature is measured from the boiler and that's what the display should indicate. Otherwise you're placing it with an offset that you don't know for sure. That's just my personal taste but I definitely understand why people prefer to use the offset feature.

Edited by phario
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Soon after I spoke and said that I didn't know of Scace measurements on the Gaggia Classic, I dung one up: http://www.gaggiausersgroup.com/index.php/topic,187.msg3985.html#msg3985

Their image there, reproduced for archive:

 696281706_ScreenShot2020-07-15at21_09_37.png.06313e038d8f623402c306acc00c87fc.png

I've not plotted it, but eyeballing the numbers indicate it's very close to the Auber report. 

Notice as well that according to Scott Rao (from his "Professional Barista's Handbook") 

Quote

It is fair to say all professionals prefer temperatures in the range of 185F-204F (85C to 96C). 

It's a pretty big range.

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Thanks very much for the great information phario :)

Looking at the Water Brewing Temp vs Set Temp graph, I get a temperature offset from the brewing temp and set temp to be 24f which is what I'm measuring as an offset using my thermopen as compared to the PID reading, so maybe I'm not that far off from being correct after all. 

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3 minutes ago, flyboy320 said:

Thanks very much for the great information phario :)

Looking at the Water Brewing Temp vs Set Temp graph, I get a temperature offset from the brewing temp and set temp to be 24f which is what I'm measuring as an offset using my thermopen as compared to the PID reading, so maybe I'm not that far off from being correct after all. 

I guess you're looking at this one:

1934314598_ScreenShot2020-07-15at21_30_04.png.1dae9cde9f432e7008751946c6dde031.png

For example, there is a point (average, blue) at around (SV = 215F,  water temp = 196F), so that's an offset of 19F. 

If I understand what you're doing, you're measuring the temperature further down as the water drips into the shot glass, rather than in the portafilter. It would have further cooled by then, and that might account for the missing few degrees.

 

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6 minutes ago, phario said:

 

If I understand what you're doing, you're measuring the temperature further down as the water drips into the shot glass, rather than in the portafilter. It would have further cooled by then, and that might account for the missing few degrees.

 

No I'm trying to measure the temperature just as the water exits the group head so I know the water temperature as if it would just first hit the ground coffee during a shot. It's this difference I want to set in the PID (Psb setting) so the displayed/set PID Sv is actually what the brew temperature is.  

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26 minutes ago, flyboy320 said:

No I'm trying to measure the temperature just as the water exits the group head so I know the water temperature as if it would just first hit the ground coffee during a shot. It's this difference I want to set in the PID (Psb setting) so the displayed/set PID Sv is actually what the brew temperature is.  

Maybe I'm being a bit dim, but what are you doing exactly? Is the portafilter in and you have the probe within? Where is the probe placed? At what point in time are you obtaining a reading? 

Edited by phario
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I'm measuring the temperature as water exits the group head without the portafilter using a Thermapen. I hold the Thermapen so the "business end" is catching the water just as it exits the group head which would be the temperature of the water as it first hit the puck when pulling a shot. This way I have a good idea what value Psb should be so the displayed temp on the PID is the same as the group head temperature.

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26 minutes ago, flyboy320 said:

I'm measuring the temperature as water exits the group head without the portafilter using a Thermapen. I hold the Thermapen so the "business end" is catching the water just as it exits the group head which would be the temperature of the water as it first hit the puck when pulling a shot. This way I have a good idea what value Psb should be so the displayed temp on the PID is the same as the group head temperature.

Yeah...that's pretty inaccurate. 😬

I guess I could give a more technical answer, and we can estimate how much water will cool as it meets ambient air by the time you 'catch it'. But that's probably your 6-10C difference there.

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