day wrote:if you dont care for steam why not build it as a gravity fed system?
Good question. I thought long and hard about making the Graziella into a gravity fed, open boiler machine. In the end, there wasn't a good way to do it because of the positioning of the heating elements in the boiler, the location of the water entry tube, etc.
After putting a pid on the Graziella, I realized that I didn't need to shift away from the dipper system. Setting the PID at 240 degrees F and letting it fully preheat for 30-40 minutes let me my typical 3-4 doubles with stable shot temperature.
I've looked at the designs of a lot of spring-lever dipper machines. They all are a little different and it looks like they're trying to have a boiler that's much hotter than the espresso needs with some mechanism to reduce that down by the time it reaches the coffee puck. My theory is that by heating the group head (like the Strega does) I can run a lower boiler temperature and make the brew temperature more consistent.
Most of the newer lever designs seem to use the thermo-syphons to keep the group head nearer to the end temperature so this is just another way to do the same thing. Since I had a dipper boiler and it was a dipper group that became available I decided to take this path. I picked copper as the material for the plate in case I needed the cartridge heaters. It wasn't until I put it the thing together that it became clear that I would need them; I can only get reasonably close shot temperatures when I run the boiler at a very high temperature.
The other variable I can play with a bit is the heat transmission from the boiler to the plate and from the plate to the group head by varying the thickness and material of the gaskets. The current plan is to have fairly thin gaskets for good heat transmission; I'm now using food-grade RTV between the plate and the group head because it's the thinnest-possible seal I can get.
If anybody knows where I can get some soft copper-foil gasket material that would be perfect!