Building a lever machine.... from scratch - Page 38

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mathof

#371: Post by mathof »

JohnB. wrote:I'd say that Senor Bosco qualifies as one of those lever masters. My single spring Sorrento peaks at just over 8 bar & drops to around 6 bar as the shot finishes.
Assuming he's Italian, your Neapolitan craftsman should be referred to as Signor Bosco (not Senor Bosco). :wink:

Matt

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JohnB.
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#372: Post by JohnB. » replying to mathof »

Right you are. I corrected that post so he's once again Italian. :lol:
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bidoowee

#373: Post by bidoowee »

@JohnB - I'm not disagreeing with you. I'm sure the Bosco makes fine, fine coffee too. You have a Carravaggio (Signor), mine is a Velázquez (Señor!); both are worthy of study. I note however, that if I lower the pre-infusion pressure with real coffee in the machine, the shot starts at a lower initial pressure (which I cannot measure, but only estimate from the lever position) and I'm not happy with the results in the cup. Perhaps this could be fixed by altering the grind, but I've not managed so far. One of these days, perhaps I'll try to find a lighter spring and see what happens at 8 bar.

So, if a pressure profile is a cat among pigeons around here, here's a fox in the hen house:





What are we looking at? This is a multi-channel temperature log of my early 80s horseshoe HX machine over a period of about half an hour.

Methodology:

Five K-type thermocouples were installed on the boiler, the side of the brew reservoir, the group neck (i.e. extension between the bolt flange and the body of the group), the outside of the portafilter holder on the group and the puck inside the pseudo-Scace. The machine was turned on at roughly 10am and was at idle for at least one hour before the test. The pressurestat is set so that the average boiler pressure is in middle of the recommended band (the green area on the Brugnetti gauge) at 0.9 bar.
At 5:30pm the portafilter was inserted cold into the holder and 6 simulated shots were pulled with the needle valve set to give 20-30 seconds of flow per shot. The spacing of the shots is very quick - about 30 seconds apart - much faster than a 'worst case' scenario of constant use in a commercial setting.
At 5:34pm the machine was left to idle for 3 minutes then a further 5 shots were pulled.
At 5:40pm the machine idled for 5 minutes and 3 shots were pulled.
At 5:59pm the portafilter was removed.

Limitations of the methodology:

A) The pseudo-Scace is by no means a perfect analog to actual coffee for a number of reasons:
First, the Acetal puck, although vaguely similar, does not conduct heat in the same way as coffee grounds.
Second, the puck gains heat from shot to shot unlike coffee, which will always be at (close to) room temperature.
Thirdly, the flow rate through the needle valve is not very repeatable.
Fourthly, fluid flow through real coffee is unlikely to be linear, and this non-linearity is not modeled with the needle valve.
So temperature readings "at the puck" should be taken with a grain of salt.
That being said, for comparative study of different machines, Scace-like instruments are a valuable tool.

B) Also, with the except of the pseudo-Scace, these measurements are surface temperatures not actual water temperature.

Observations:

- The roughly triangular wave of the boiler (blue line) is from the pressurestat controller with roughly 0.2 bar of hysteresis (dead band). Total variation is approximately 4 degrees C.
- The (red) brew reservoir temperature (and by inference also the water inside it) correlates closely with the boiler temperature but the variation is considerable damped. Total brew reservoir variation is less than 1.5 C.
- Average (or baseline) temperature of the brew reservoir is affected only very slightly by continual use rising from 100.5 C at idle to 102.5 C after intense activity.
- Neck temperature (purple) seems to be a better analog to puck temperature than the outside of the portafilter holder.
- The group gains heat with each shot if not left enough time to recover and (based on a small sample of only two points on the group) the heat gain is uniform throughout the group.
- With the exception of the walk-up shot with a cold PF, the temperature gain of the group is between 1.5 C and 2.5 C per shot.
- Group recovery time - that is, the time it takes for the group to cool from any given temperature gain - is about 67 seconds per degree C of temperature gain.
- Puck temperature (bearing in mind the caveats above) is lower than expected, especially for the walk-up shot - but the results in the cup are known to be good with this machine at these settings.

Conclusions:

- Brew reservoir temperature demonstrates a relatively high degree of stability even with a dead band of more than 20% of the operating pressure setpoint.
- Group temperature should be stable if correctly managed. If the machine is left to idle between shots for a reasonable amount of time (between 2 and 3 minutes - i.e. not inconsistent with the time it takes to prepare a shot), it will return to its initial temperature and provide repeatable shot temperatures.
- The methodology should be changed for future tests to reflect a more realistic real-world usage with a sufficiently long period of time between shots.
- Not bad for 1982.


samuellaw178
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#374: Post by samuellaw178 »

Thomas,

Great study!

I did some temperature tests on mine a while ago as well. Here were my take-away notes:

(i) If you aim to do absolutely no flush, 1.5 hour warm up is recommended. 45 min is workable with flush, 1 hour is better, 1.5+ hour is for the lazy bumps who don't want to flush at all. :oops: Though, for good practice sake, I still do a flush to replace the HX with fresh water at the start of the day (despite being warmed up for 1-1.5 hour).

(ii) 1.2 bar is suggested if you want to achieve ca 93 deg C (or 200 F) brew temp measured using the thermofilter. For home use, I find that this is the best setting (for me) with minimal fuss for walk up shot (no flush whatsoever).

(iii) For production mode (continuous shots, say > 5 back-to-back shots), it's better to set to a lower boiler pressure (say 0.7-0.9 bar in the manufacturer 'recommended' range). But do a quick flush (~30-60ml) prior to pulling the first shot. This will replace the brew reservoir water with HX water at a higher temp. Do not flush on subsequent shots. This way you get a solid brew temp from the get go and the rising temp in subsequent shots will be mitigated.

(iv) FWIW, I cranked my PID/pressurestat down to 0.5-0.6 bar and did no flush whatsoever. Expecting the shot to be sour (pretty sure it's far from the 93 deg brew temp), but instead was surprised with a mind-bogglingly good espresso shot with heaps of clarity. Obviously you sacrifice some steam power at this setting, but for espresso purists it's of no consequence.


I also agree that a Scace/thermofilter is not the best method to represent the brew temp in repeated shots. The plastic puck heats up in repeated shots, and could not replicate the heat sink nature of a freshly prepared coffee puck, as well as the temp drop during preinfusion stage. Additionally, the flow rate affects the temp reading too - retard the lever(lower flow rate) and the thermofilter temp drops slightly, push the lever and the temp reading increases. :| This behavior might account for some forgiveness in pulling a shot (higher brew temp helps extract a gusher and vice versa) but can be a nightmare to characterize them as numbers.

I also had some preliminary tests that I documented - link below. Unfortunately the tests that I drew my conclusions from above (with DIY thermofilter) weren't documented completely - they were carried out randomly over a span of months.
'New' lever in the house!! - Brugnetti Aurora Single Group


The brew reservoir on my Aurora holds roughly 60 mL water (65 g cold water), the hourseshoe HX about 100 mL water (110g cold water), and a negligible 7 mL volume in the piping between the brew reservoir and HX. This info is critical if you want to understand how the flush will affect the temperature outcome. For example, a flush within 100 ml/3 oz is a heating flush, more than 160 mL/5 oz and it will become a cooling flush - the HX capacity is exceeded in the latter case.

The horseshoe HX design is pretty foolproof in use, but the thermodynamic is crazy complex if you want to try to understand it. :twisted: Am really impressed with the engineers who designed this back then.

Javier

#375: Post by Javier »

In case you guys have not seat down and realize what is going on, this is a rebirth of the Brugnetti Aurora!!! Don't take that fact for granted, since it is (at least to me) mind boggling! Great job Thomas.

EDIT to add - what is next Thomas? The Arrarex Caravel? :)
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doubleOsoul

#376: Post by doubleOsoul »

LOL! Or a Velox?

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bidoowee

#377: Post by bidoowee »

I did some temperature tests on mine a while ago as well.
@samuellaw. I looked at your excellent post some time ago. It was very helpful. I did this study because I needed a baseline against which to test the new machine. A couple of comments:

i) I concur with your suggested warm-up period with and without flushing. The boiler heats about fairly quickly, but it takes a long time for the heat to permeate the group and for the system to reach equilibrium.

ii) Be aware that the design operating pressure for the copper boiler is 1 bar. This is either because the seams are "soft" (low temperature) brazed for cost reasons (that would be a lot of silver brazing rod) and because of the wall thickness of the boiler. 1 bar is the limit for all "soft" brazed copper plumbing independent of wall thickness after the engineering safety factors are taken into account.

iii) This describes my current practice exactly :).

iv) That is interesting - for reasons that will become apparent shortly.

Re - horseshoe HX
I agree with your observations about heating and cooling flushes being volume dependent - i.e. if the flush exceeds the volume of the HX capacity, it will be cooling the group.

As will be seen in an upcoming post, the horseshoe HX is indeed fine beast, BUT, the diagonal HX is even better.

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bidoowee

#378: Post by bidoowee »

Great job Thomas.
Thank you!

What's next? Are you kidding? I still have to finish this one! :)

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JohnB.
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#379: Post by JohnB. »

bidoowee wrote:
ii) Be aware that the design operating pressure for the copper boiler is 1 bar. This is either because the seams are "soft" (low temperature) brazed for cost reasons (that would be a lot of silver brazing rod) and because of the wall thickness of the boiler. 1 bar is the limit for all "soft" brazed copper plumbing independent of wall thickness after the engineering safety factors are taken into account.

Bosco ships their machines with a 1.5 bar boiler setting. I keep mine at 1.2 bar. Soldered household copper pipe typically carries water at 3 bar + pressure.
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bidoowee

#380: Post by bidoowee »

Soldered household copper pipe typically carries water at 3 bar + pressure.
@JohnB - you are absolutely right. What I should have said it that the 1 bar limit is for copper at operating temperatures over 100 C. And of course, that number takes the safety factor into account which is likely 3x or more, so ultimate failure (at 100 C+) is probably well over 3 bar (I haven't tested that ;) ). It is also quite possible that Bosco uses a different welding technique from the antique Brugnettis; it is possible to TIG weld copper using pure copper rod which would obviously be a game changer.