homeburrero wrote:Note that the solubility number you quoted from Wikipedia is expressed in terms of [Ca⁺⁺] ion. In terms of CaCO3 it would be 117 mg/L. My recipe has only around 16 mg/L CaCO3 in the final mix. Points 2 and 3 are true. This recipe is based on purified water with negligible CaCO3 already in it, and the amounts of calcium and carbonate in the recipe are low enough that it should not drop scale even at steam boiler pressures and temperatures. (A Langelier Saturation Index calculation at 130 ℃ gives me a CaCO3 saturation pH of 7.2)
That LSI thing is interesting, but looks like a rather bad model for water under temperature to me, and the 117mg/L incorrect, have you tried putting in 117mg/L along with the rest of your mix, heating it to 130C, venting steam for a minute and letting rest for the night and then sending a sample off to the lab for a water test?
AFAIK you can get proper quality test done by the EPA or local waterworks for a fraction of the price here.
I'd guess that the reason the water is PH 7.2 is that the CaCO3 is starved of CO2 and wanting to drop out of solution or have access to open air to regain it?
As to what's the point of the calcium carbonate, that is a good question. I do it only because it's easy and it makes my water resemble a soft natural water or an RO that comes out of a remin cartridge. That means it's likely to be similar to the water that a roaster might be using to taste and tune their coffee. It's also along the lines of the water they now use at WBC competitions. It does have zero sulfate, chloride, or silica, but I see that as a good thing for espresso machine health.
With respect to machine health it's no better than the rpavlis recipes that have no hardness minerals at all, and although some would argue that hardness minerals are needed for tasty extractions I can't say that I'm convinced of that.
The water lingo is hard to understand sometimes, probably even worse across languages. But putting potassium bicarbonate in the soft water category sounds like it would be unable to neutralize harsh acids in coffee or otherwise which is not at all the case, even though it does not "harden water" it is an excellent buffer against negative PH and predictable because it does not drop out of solution or have a narrow base solubility that's negatively impacted by temperature.
homeburrero wrote:That looks like a reasonable guess to me. Using LSI calculations as a very crude estimate and assuming the hardness starts out purely from CaCO3, and that the pH stays up around 7.5, then at 100 ℃ the LSI is still positive until the calcium and the alkalinity drop to below 34 mg/L CaCO3 equivalent.
That's a good thing to keep in mind if fooling around with cupping taste experiments with hard water -- if the hardness is due to calcium hardness, most of it, and the associated alkalinity may be left behind in the kettle,
Yeah I think so too, if the machine is busy and directly fed with saturated CaCO3 water (from the increased solubility of CO2 in water under pressure at low temperature) then a lot of it is going to end up in the cup (which is tasty) but if it's directly fed and less busy a fair bit will drop out depending on construction and CO2 offgassing so the cup will have more pronounced sour notes.
At home though it seems so much easier to just do that in a kettle and feed the machine offgassed water with potassium bicarbonate tempered to roast.