I’ve been playing around with Matt Perger’s water recipe and it’s made a huge difference to my brews. I live next to a Tesco, which has Ashbeck water that is cheaper than I can get distilled water and I was wondering whether I could add the minerals to that instead. Ashbeck has 10ppm calcium, 2.5ppm magnesium and 25ppm bicarbonate. My preference for most coffee has been using 2g of concentrate of Matt’s water recipe to 500ml distilled water. I really don’t understand water math so I decided to work from the recipe Matt has created.
To make things simple if I stick to 2.5g concentrate (100GH, 50KH) but want to reduce the amounts down to 80GH and 40KH and then minus the 12.5GH and 25KH already in Ashbeck, I would need to add 68GH (rounded up) and 15KH.
That means for GH I should use 17g Magnesium Sulphate - 25g x 0.68 = 17g
For KH, 2.6g bicarbonate soda - 8.6g x 2 = 17.2 (100pm) then 17.2g x 0.15 = 2.6g
Add those to 500ml distilled water for a concentrate and then 2.5g per 500ml Ashbeck water.
Could someone please confirm/correct my thinking? Or do water calculations become more complicated than this?