Dear Home-Baristas
After a discussion with Maxwell Colonna-Dashwood today it turned out that they did indeed use test kits that measure in ppm CaCO
3, meaning the same ones that are standard in the US and many other countries worldwide - and not as they state in the book and the corrigendum in mg/L of the respective ions. Also they did derive their "ideal brew zone" as well as the "acceptable brew zone" based on a one reagent titration (one for general hardness, one for the buffer/alkalinity) which measured in ppm CaCO
3 (identical to mg/L CaCO
3). So in practical terms this means that the lower third of their "ideal brew zone" overlaps perfectly with the higher half of the SCAA standard - or put into numbers the range of 40-85 ppm CaCO
3 of total hardness at 40 CaCO
3 alkalinity).
What remains still a largely unchartered territory which could be very interesting to explore is their upper part of the "ideal brew zone" which extends as far as 175 ppm CaCO
3 in total hardness at alkalinity values of 40-75 ppm CaCO
3 - the higher values in alkalinity being recommended/"allowed" for the values higher in total hardness.
So if you just switch out any mention of ppm (of the ions) in their book with ppm CaCO
3 you are fine. Except for chapter 6 where they give instruction on how to mix water from scratch - there you have to transform the mg/L into mg/L CaCO
3 - which is as they also mention in the corrigendum go as follows:
To convert [HCO3-] as [CaCO3] to [HCO3-]: multiply by 1.22
To convert [Ca2+] as [CaCO3] to [Ca2+]: divide by 2.5
To convert [Mg2+] as [CaCO3] to [Mg2+]: divide by 4.1