La Pavoni Europiccola 2021 - Bypassing the pressurestat - Page 3

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#21: Post by vit »

jpender wrote:Thanks for doing the test!

1kW for 40 seconds is 40kJ which is enough to vaporize about 18g of water. I'll bet if you weighed a pitcher of milk before and after you steamed it for 40 seconds that's roughly the weight difference you'd find.
Part of that energy will be used to heat the kitchen as well ... on the other side, part of the energy will come from cooling of the remaining water in the boiler while pressure is decreasing (which will usually happen). For instance, cooling 0.5 kg of water by 10°C will give around 10 kJ, so even with heater off boiler will still be able to produce some steam until water cools near 100°C, with pressure decreasing of course ...


#22: Post by jpender » replying to vit »

Yes, of course. The water gives up some energy as the temperature and pressure drops. And energy goes into heating the air/milk/jug, adding kinetic energy to the air and the milk, and some phase change of the milk. Some steam escapes the jug too. And since saturated steam at 120°C isn't quite ideal there would be some loss of energy due to the self-attraction of the water vapor molecules.

But to first approximation I think the heating element determines what you get. I was hoping to prod @bhr into weighing his milk jug but I guess he has better things to do. :-)

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#23: Post by baldheadracing »

Given my previous indiscretion - BTW, thanks John for keeping me honest (really, not being sarcastic or anything like that, just in case it sounds that way), my next milk test will be comparing the two tips that I have (gen 1 3-hole vs. Francesco Ceccarelli's 1-hole). I'll weigh then.

However, right now I have mostly lighter roasts, and the ones that I have pulled so far would not taste good in oat milk.

Regardless, in my experience, the bigger the boiler, the better.
- My espresso: Swirled, not stirred. My pourover: Stirred, not swirled.

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#24: Post by homeburrero »

Related to optimal steam tips for a given element power, I think folks may find this old thread from the late Dr. Pavlis informative: Optimal steam tips
nínádiishʼnahgo gohwééh náshdlį́į́h
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#25: Post by jpender »

Very interesting, thanks. I miss that guy's posts.

I found this post of his which I think is relevant:
rpavlis wrote:For small reservoir machines the rate of steam release is best set so that the all the energy supplied to the boiler from the heating element is used to generate the steam. With large boilers you can take off steam and the heat capacity of the water and steam in the reservoir will allow you to continue to take off steam at an higher rate than it is being produced but not indefinitely.

With small machines the important thing to know is what it says on the little power label. For the La Pavonis of recent vintage it is 1000 watts. The enthalpy of vaporisation of water is 40650 J/mol. The heat capacity of water is about 75.32 J/mole/degree. Milk is mostly water, so it has a similar, though slightly lower heat capacity. (molecular weight water is 18g/mole)

If we steam 5 moles of milk, we need about 375 J to raise the temperature 1 degree. Temperature of steaming 5 moles (90 grams) milk should thus rise with a 1000 watt element at over 2.5 degrees per second with an optimal steam tip. We can utilise the heat capacity of the boiler water and take off more than this for a while, how long that is depends on the quantity of water in the boiler.

From the enthalpy of vaporisation of water one can compute water loss from the boiler from steaming. A 1000 watt element will be able to vaporise 1000/40650 moles of water per second. i.e. about 18 grams every 40 seconds.

The variable, of course, that controls how fast steam is released is the diameter of the hole or holes in the steam tip.

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#26: Post by baldheadracing »

Dr. Pavlis(RIP) said that 1.4 mm is ideal for 1.0-1.1 bar, and 1.5mm is ideal for 0.65-0.7bar. Ref: Single hole steam tip diameters for lever machines

Hmm. I might have to lower my pressurestat back down for steaming - I think my one-hole tip is 1.5mm.

Pressure drop with 3-hole v1 tip from 1bar. Two minutes of (not)riveting video:
Pressure drop with 1-hole tip from - note more violent steam, and pressure holding up much longer over the two minutes of even less riveting video:
- My espresso: Swirled, not stirred. My pourover: Stirred, not swirled.

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#27: Post by guijan12 »

jpender wrote:Very interesting, thanks. I miss that guy's posts.

Does anybody know if his next of kin are aware he is still very much alive in this forum? :?:
A little off-topic, I know.... :wink: