As a thermo-syphon design, stability is not related to the boiler material so much due to the flow. The material used in the heat exchanger is important, though.

Temperature transfer in such a small boiler isn't a big deal. In larger machines, it is.

I would need to see the temperature data and profile to have any opinion at all, but I doubt that the stability has anything to do with boiler composition. Given how surprised Richard was at the results, by guess is that he may have gotten lucky. It is now up to the gear heads to figure out what's going on by reverse engineering it.

Michael

Some relevant material properties on water vs copper vs stainless...


------------------------------Water------------copper----------stainless---------units
Specific heat----------------4.2----------------0.38-------------0.5--------------J/g-C
Thermal conductivity------0.6-----------------385-------------16.2-------------W/m-K


Now, before you freak out and try to get all the water out of your espresso machine system...
Remember that water can move... That is, there is heat transfer thru water due to convection, rather than just conduction.

A metal can only move heat thru conduction (in its solid form). Yes, I'm ignoring radiation at these temps.
A liquid, such as water can convect heat -- this is exactly how a thermosyphon works, the hotter water moves around because it's a different density, as it moves it carries a HUGE amount of heat with it (look at the specific heat compared to the others).

Well, I'm starting to ramble... Sorry.

ciao

lino

lino wrote: ... different density, as it moves it carries a HUGE amount of heat with it (look at the specific heat compared to the others).

So the flush time to cool off an E61 has more to do with the amount of water in the thermosyphon than the hunk of brass. This would mean the lighter Thermosyphons Faema built later, E68 and no-stop, would have had roughly the same properties.

On the other hand, there's about 9 lb of brass, and (on my machine) 3/4 of a pound of water max in the loop, so the heat stored in the brass still exceeds that stored in the water

Exactly!

Because the movement of water directly affects the brew head temperature and radiation (loss) of heat, manufacturers restrict the flow of water with adjustable regulators or teflon disks with sized holes in the feed tubes. The size of the tube (surface area) and the length of the cold water injection tube affect balance.

If the LM could flow water through its saturated group, it would be much better balanced to the temp of the boiler. Likewise, if the whole system was copper or brass.

It is what it is. There are tradeoffs to everything. Solutions too.

Thanks Lino.

Michael