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hot porridge

Hot Porridge, Hannah Clarke Preston MacGoun, 1910

[Ed: re-post from Nov 17, 2018]

The new results in my ongoing quest for Goldilocks porridge (aka boiler-group thermodynamic interaction and stability, but porridge sounds much better) are in and I have to say that I’m rather pleased.

What are we testing?
This is temperature profile of the new boiler with diagonal HX and injector. The boiler and the HX chamber are both made of stainless steel but, unlike the previous Horseshoe HX prototype, the brew reservoir is now bronze (for reference, stainless is roughly 20 times less thermally conductive than copper and copper-based alloys). The diagonal HX configuration eliminates the separation between the HX chamber and the brew reservoir and they both form one single volume of hot water at a lower average temperate than the boiler water. Cold water is injected directly into this volume and the resulting mix, now at a lower temperature, moves on into the group during a shot.

Methodology is similar to previous tests: the machine was turned on several hours in advance to make sure that everything is at its ultimate idle temperature. The probes are K-type thermocouples placed in the same spots as prior tests – the only difference being that the brew reservoir now has a dedicated threaded thermocouple socket – no more tape coming unstuck or clamps falling off. Shots are simulated by using a flow restricting valve placed on the outlet of the portafilter.

Shot simulation procedure is:

  • Pull
  • Pre-infusion 7 seconds (lever in down position) –
  • Shot 20-25 seconds for lever to return to the cam inflection point (lever just past straight up and down)
  • Post-flow 10-30 seconds (lever returns to rest position)

Various timings between the shots are tried: 5 minutes, 4 minutes, 3 minutes, 2 minutes, 3 minutes.

The pseudoScace™ device (puck temperature readings) has too large a thermal mass to give meaningful results for peak puck temperatures when inserted cold. I therefore left it in place, before, during and after the test to minimize its impact. The one second sampling time period is too long to give reliable readings at the moment of the pull. On a few of the shots there is a significant drop seen at the puck at the moment of the pull. I believe that this is due to the piston creating a vacuum as it is raised and drawing cold water back up through the pseudoScace from the waste line. A change in equipment would be required to eliminate this if this hypothesis is correct.

Conclusions and observations
The original Aurora diagonal HX I profiled back in May demonstrated uncanny thermal stability at the brew reservoir, but the group suffered per-shot heat-gain and was slow to return to its baseline idle temperature. These results show that the brew reservoir temperature is dipping significantly but the group and the puck temperatures are, by comparison to the antique machine, rock steady. The maximum overall delta at the puck is 3.1 C (between the walk-up and the third shots) but the inter-shot maximum delta is 1.8 C (the minimum inter-shot delta is 0.5 C for 2 minutes between shots).

So, to summarize: best performance at 2 minute intervals, significantly lower puck temperature fluctuation than the antique machine and little to no group heat-gain. This, I think, may be a slightly better mouse trap – though not really by design, rather by accident of the thermal interaction of the materials. I’m not going to complain.

If you will permit me, and at the risk of tooting my own horn:

Courtesy of UC Davis, Special Collections
Title: Magazine ad for Bank of America: hammer and nail montage.
Creator/Contributor: Halberstadt, Milton, Photographer

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Goldilocks part 3

The set of the bears. Plate 7, 1664, by Marcus de Bye, after Marcus Gheeraerts I, 1559. Gift of Bishop Monrad, 1869. Te Papa (1869-0001-67)

Though I’ve run out of three bears analogies, I’ve stuck with the story’s structure: first the porridge was too hot, then it was too cold, and finally Goldilocks found one that was just right. This iteration of the HX caused me to take my hat off, yet again, to the Italians. Many months ago, Dr. Pootoogoo brought a boiler from a later-model Brugnetti to my studio. The flange bolts were so badly rusted that it wasn’t ever going to go back into service without replacing them. It happened to be one with a diagonal HX that I hadn’t examined before and it inspired my to try a similar concept with the horseshoe HX prototype. I was quite surprised that the 60ml HX (baby bear) didn’t deliver water that was cooler than the brew reservoir temperature even though the HX volume was close to the 50ml shot volume. I also started thinking about what the ‘correct’ temperature for the brew reservoir should be. It occurred to me it might not be the best thing for it always to be the same. For example: if the group is at 75 C and the water coming in is at 102 C, the resultant water temperature at the puck is 92 C (these are roughly the numbers for both vintage machines) and we know that the group gains heat after a shot, let’s say for the sake of argument it gains 3 degrees and requires about 2 minutes per degree to recover i.e. 6 minutes. It follows then that for the next shot, if it is to be pulled [i]before the end of the recovery time,[/i] it would be preferable to have the brew reservoir water at a lower temperature than 102 degrees so that when it reaches the puck it will be at same magic 92 degrees. Because of the difference in thermal properties of the materials (i.e. the brass group and the water) and their relative volumes (i.e. big thermal mass of brass vs 50ml of water) it isn’t a one to one relationship. But it is linear – i.e. it will be a constant times the temperature rise of the group. So at any given time during group recovery, the required brew reservoir temperature is the reservoir idle temperature minus the group temperature rise times some constant. In math not English:

Tbr = Tbr_idle – K(Tgroup – Tgroup_idle)

In other words if the heat gain curve of the group could be inversely mirrored by the brew reservoir, then water will be at the right temperature when it reaches the puck [i]no matter when it is pulled[/i]. This is really just destructive wave interference:

If the brew reservoir temperature curve is positive (i.e. there is heat gain), then it will compound the problem of heat gain at the group. But if the brew reservoir temperature drops after a shot, then it will compensate.

The diagonal HX design consists of a large diameter pipe which connects directly to the back of the brew reservoir – essentially increasing the volume of the brew reservoir four-fold. In fact, the concept of the brew reservoir is pretty much gone altogether in this design – the group flange actually becomes one end of the heat-exchanger. A small diameter injector tube runs through the middle of the large diagonal pipe. Line water comes in through the injector too fast for the surrounding water to heat it to boiler temperature and mixes with the water behind the group to get the really stable results that we saw in the earlier testing.

I replicated the basic principal minus the diagonal tube and in so doing figured out why the diagonal design ended up that way i.e. diagonal.

The last kink in the 6mm tubing was only way to thread all the larger diameter fittings onto the injector. And this is a one-way operation: once it is brazed together you can’t take it apart again.

Brazed and (sort-of) cleaned.

And here are the results:

Blue – Boiler
Red – Brew reservoir
Purple – Group neck

Now, although the results aren’t perfect, it shows that the concept works. The group heat gain for successive (unnaturally) rapid shots has been significantly diminished and the recovery time for the group is less than half of what it was (less than 3 minutes). The length of the injector plays an important role in how much boiler temperature HX water mixes with the line water and consequently in the temperature of the water that reaches the brew reservoir. But, as I said, this is a one-shot fabrication and is too much trouble to alter. It would be much easier to change and/or maintain if the injector tube screwed into a straight length of larger diameter tube that maybe ran directly to the group right through the boiler, maybe on a diagonal…

Wait – someone already thought of that.

Enough porridge already.

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Very fine

August is very fine. People take holidays. In France and other places in Europe where the sun shines more than occasionally, things start slowing down in July and people “faire le pont” until sometime in late September when they finally remember they had a job. Trying to do anything other than sitting by the pool and taking four hours for lunch (i.e. an hour longer than usual) in August in Italy just isn’t worth attempting. Here in Montreal we are more organized; we like to do things together: along with Moving Day, July 1st, when everyone moves at the same time (Which is insane. Try to find moving truck on that day. I’m not making this up.) we also, as anyone who lives here can attest, tear up all of our roads and rebuild all our overpasses and bridges at the same time. It is more efficient to wait fifty years and then get it all done in one go, ripping-off-the-band-aid-style. Economies of scale you know. We also have this thing called the The Construction Holiday. Towards the end of July, once we are done ripping up the asphalt and putting out the traffic cones, we all go on our government mandated holiday for exactly two weeks. On the same day. We also all come home at the same time. Over the interchanges that we are rebuilding and along the roads we tore up before we all went on holiday. Sensible I call it.


Welding the boiler together is a complicated set of procedures. Each type of weld requires a different setup and either a judicious application of inert welding gas, a custom heat-sink or both. Some operations render others either difficult, or in some cases impossible, so it is critical to get the assembly order right. To further complicate matters, each weld introduces some distortion in the parts: more or less depending on their geometry and the amount of heat that goes into the weld.

The HX tubes are easy. The size and fit of the parts makes for a simple weld that is almost invisible.

Ditto (once the heat-sink is made) for the bolt ring.

Just load your nine-shooter and fire away!

All the bolt rings were welded up in about an hour.

Fitting the HX tube in place requires a more complicated setup as both the inside of the boiler and the HX tube have to be purged with inert gas during the weld.

The group mounting flange and brew reservoir meet for the first time.

The end flanges are also done using the turntable (a.k.a. the Ouroboros machine).

A few welds later and after some clean-up: the first full-stainless diagonal heat-exchange boiler off the production line.

This one is now ready for a few tests before the rest are assembled. Mistakes at this point would be, ermm, disappointing.





1 – MONTREAL, QUE.: AUGUST 21, 2014 — Construction cones line Rene Levesque Blvd east of Atwater Street in Montreal, on Thursday, August 21, 2014. (Dave Sidaway / THE GAZETTE) Web 4×3 ORG XMIT: POS1410031753473482