The last two days, my M31 has been wanting to overfill the boiler. So after getting home today, I took the boiler probe out and it has a black coating on it. I'm not entirely sure what this is, but I suspect it must be something from the water. I am using ordinary tap water for the machine, BUT I am also using the machine's built in water softener. I have been recharging it using table salt as per the directions from Cimbali. One thing about the water here is that it is good quality except that it has quite a bit of calcium in it.

The other thing is that I've been getting quite good shots the last few days from this machine in combination with the Cimbali Junior grinder!

At this point I think I need to clean up the boiler probe, but also maybe run citric acid through the system. I've not done this before (with the machine assembled) and would someone mind walking me through it?

Oh, here's a picture.

Some hot water and citric acid took it off in less than 10 minutes. So now I just need to do the boiler descale I would say.

The black stuff is Copper 2 Oxide.
Source: http://en.wikipedia.org/wiki/Copper(II)_oxide for complete article.

Wikipedia wrote:It is a black solid with an ionic structure which melts above 1200 °C with some loss of oxygen. It can be formed by heating copper in air.
It is formed along with copper(I) oxide as a side product; thus, it is better prepared by heating copper(II) nitrate, copper(II) hydroxide or copper(II) carbonate.
Copper(II) oxide is a basic oxide, so it dissolves in mineral acids such as hydrochloric acid, sulfuric acid or nitric acid to give the corresponding copper(II) salts.
A laboratory method for preparing copper II oxide would be to electrolyze water containing sodium bicarbonate at a moderate voltage with a copper anode, collect the mixture of copper hydroxide, basic copper carbonate, and copper carbonate produced, and heat it.