I have a coffee machine which can make espresso. It has a grinder and a brewing unit, and it takes water from a dedicated container, not from a tap. It looks mostly like this picture:
It detects scale accumulation (using just a counter of cups of coffee, I guess), and now it tells me I should run a descaling program using an original descaling liquid. I did this in the past, using a small liquid container which came with the machine. The descaling program runs about 30 minutes, doing weird things like sucking the liquid into the machine, heating it and spitting it back.
I imagine that such a descaling liquid is just a weak acid (maybe acetic or citric acid), chosen so that it will not dissolve the machine's innards. The size of the original container was about 250 ml, and the machine diluted it to about 1 L.
How can I prepare such a liquid myself? Which acid to use? Which concentration?