Logging E61 group & puck temperatures on the Rocket

In this rapidly changing world, in a time when many of the old values are suddenly losing their meaning and very few people are able to make sense of the moral dilemmas that surround us, the one question that's increasingly on our minds is: "What is the relationship between the temperature on a given spot on the outside of an E61 brew group and the temperature inside the coffee puck during the extraction of an espresso?"

That is why, instead of going out on a Saturday night to drink and dance in a state of ecstatic frenzy, more and more people are staying home to take out their temperature probes and stare at their espresso machines, searching for a pattern which might reveal us what it is all about.

I for one am no exception in this new rage.

Laptop with Artisan software, Amprobe logger, probe on E61 group en SCACE device
Portafilter basket with K-type thermocouple on a German book about home roasting 

Thermocouple embedded in coffee puck
 
Extraction from basket with thermocouple

Espresso from ThermoCoupled coffee puck (notice slight channeling along tip of TC)
SCACE device in action, water outflow same volume/min as average coffee puck
A first result from a SCACE measurement is shown below.

I usually do a short flush from the group to pre-heat the cup while I grind the beans, stir grinds, tamp and lock in the portafilter. 

During flush, the group warms from 93º C to 94º C. During extraction, the "puck" starts at 94º C and ends at 93ºC as the group warms up from a little more than 92ºC to almost 93ºC. 

If we allow a 1º C margin for inaccuracies, we seem to have selected a measuring point on the group skin where the temperature is about the same as it will be in the coffee puck during an extraction. We need to see if we can confirm this over more measurements of course. 

During this measured extraction, both group and coffee puck are receiving heat energy from the same source, namely hot water from the Heat Exchanger, and as the dry puck heats up immediately and then cools off a little as espresso flows from its bottom, the heavy metal group mass is a little slower to respond and heats up, then cools off after the portafilter is removed.

Scace 'puck' from 94 to 93ºC, group from 91 to 93ºC
The above was measured after some idle time. After that the group cools off a few degrees and although the patterns are similar, the level is lower. It's a chilly day and the kitchen is far from the heater two rooms away (no doors), so I have turned up the p-stat from 08.-1 bar to 1.15-1.35 bar. The first extraction with real coffee in the puck around the thermocouple was delicious with a great flow and color, but after that the group warmed up more to 96ºC so I need to check how that affects patterns and taste before moving on. The graph below speaks for itself. The little 'dent' in the curve during extraction seems to me like a reading error that's smoothed out by Artisan's smoothing algorithm.
Extraction in real coffee after turning up p-stat, before full effect of the p-stat raise.

Below might be the last graph of measurements for this brief project. With the p-stat set higher, the temp in the puck as well as in the SCACE remains very close to the temp measured on the outside of the group on the spot of this example. On other machines using the E61 in slightly different modifications and different thermosiphon designs, the best spot to measure the group temp in order to indicate the puck temp will probably be slightly different.

Some people like to stick a probe deep into the E61 group which is also a very good looking, simple and practical solution, even though the insertion of the probe could possibly influence the flow and thus the very environment one wants to measure but it has been proven a great and affordable solution for many.

The Rocket E61 design is stable in my measurements. The group temperature tends to meander around a certain value and not dive below brewing parameters nor climb unchecked above good tasting values, and the p-stat setting is not very critical so users who want more (or less) steaming power can tweak the p-stat a little without much chance of harm to the brew temperatures. A p-stat that's set on a warm sunny day may profit from renewed measurements during colder winter days in an modestly heated kitchen environment.

The short 30ml flush to pre-heat the cup cools the group about 2 degrees if the group is around 95ºC and warms the group about 2 degrees when the group is around 91ºC. So, in the configuration that I tested, the "cooling flush" could as well be a "warming flush" and the flush to pre-heat the espresso cup is just beneficial to the system.

My measurements on this machine demonstrate how a thermocouple on the right spot of the outside of the E61 brew group can be used to get a very good idea of the temperature inside the puck during extraction.

Hopefully this will pose a practical and affordable alternative for others as well.







Reacties

Frans zei…
A friend writes me:

"I guess you want to spend the summer inside a Black Ops metal cargo box on the tarmac
at the Istanbul airport. This is what happens to persons whose stick their temperature
probes (and noses) in the wrong places."

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