turboyeast wrote:I would like to have some input on the optimal boiler pressure

I found a great article with regard to boiler pressure and dipper/HX design. See:
http://londiniumespresso.com/blog/how-t ... hine-group

In addition, I was able to pull (literally) my first shot. Unbelievable first result..
Cheers, TY

LS,
Because the element of the prestina is rather undimensioned for the volume of water it needs to heat (1200 W/4 L), I isolated the boiler with 19 mm thick armaflex HT. I used Velcro straps to tie the armaflex to the boiler. In my opinion a superior way of strapping, because the Velcro straps are very resistant against heating. The result of isolation is satisfying but not as significant as I hoped. Isolation reduced the warm-up time with only 5 minutes (now 25 minutes). When the machine is idle though, the cycling time is much longer as compared to the non-isolated sitation (4 minutes vs 1.5 minute).
Next is the polishing stage of stainless steel parts...not my favorite, but I have high hopes of getting everything shiny.
Cheers, TY

LS,
Today I measured the brew pressure with a manometer. I was a bit surprised that the needle stopped at (only) 6 bar. However, the machine has produced some outstanding shots with the same grind as used for my rotary pump machine (which is set at exactly 9 bar).
But perhaps 9 bar is not required for this particular machine? I hope that somebody can shed some light on this.
Thanks in advance, TY

Hi TY,
6 bar of spring pressure is rather common in old commercial levers, my Lambro with a new 160mm spring came out with about the same number.
A Gaggia I once had was measured with 6.5bar and the Urania on the bench even feels "softer", but I haven't made a first shot on her, yet.
For my Empress and a Zenith Express I bought some special made springs that ought to be stronger - time will tell

Cheers,
Holger

Beautiful work!

You're in lever land now and 9 bar does not exist in the leverhead's almanac. As mentioned what you are seeing is what you'd expect from a vintage commercial lever.

Let taste be your guide. The shots are pretty terrific, aren't they? I believe some commercial levers are like the Prestina. They'll make beautifully soft shots. You can also get large, rich shots with a triple basket in a bottomless portafilter. Other commercial levers will start with higher pressure comparable to a pump machine and they'll also do that declining pressure profile.

LS,
Thanks for your kind responses. True, the shots of the prestina are really outstanding and that should be the guide (see below). It just puzzles me that with the same grind -however- at completely different pressure profiles (6 bar lever vs 9 bar rotarypump) I get similar results. That is, perhaps even a better result for the lever. I will ask a coffee-friend of mine (with a sensitive palatum) to put the results to a blind test.
I found a interesting article in the scientific community (Caprioli et al 2012). It compares a pump machine and a lever machine. See figure 1a...Although the lever reaches 8 bar, its pressure profile is completely different as compared to the pump machine.
Cheers, TY

LS,
This restoration is slowly coming to an end. I have outsourced polishing of the stainless steel covers and the result is extremely rewarding. The machine shines like the day she was born.
The only thing that is giving me trouble now is the aluminium cuptray. It is badly scratched and since it is originally anodized it cannot be polished. I have considered powdercoating in a metallic RAL color, but I have decided to go for the hard way. I have bought a brass strip in the same dimensions (30x3mm) which I will try to bend in a radius of 1.5 mm which will fit the machine. This is difficult since I do not have professional tools, and I am going for the MacGuyver option. If the bending is successful I will deliver it for chroming...what a crown that would be...

Cheers, TY

Beautiful. I know some people like to leave the machine naked but it's nice to see it in it's original livery. Nice work!

Looks beautiful! Job well done.