This question is aimed at folks who use BrunWater, but possibly other people can chime in too...
I've been noticing that when I have a water built on partially-distilled water with a lot of additional salts/minerals in it, the suggested amount of lactic acid from BrunWater (Adjustment Summary tab, cell H25) does not seem to be enough to drop the pH of my sparge water to 5.5. When however I used my own water with no added minerals (for other beers) the suggested amount of lactic acid drops the sparge pH as predicted by BrunWater.
Here's my guess, but tell me if it's correct: is BrunWater somehow including the minerals I'm adding to the mash & sparge in its calculation of what the pH will be, and therefore how much lactic acid to add? I stir in minerals at dough-in, and then when sparging, I also add them to the mash at the same time as the sparge water as it seems more accurate than dissolving them in my sparge water and then adding all that to the mash tun. So my pH reading is off because the minerals BrunWater is expecting are not present in the sparge water. Is this a correct assumption? If so, do I need to change my procedure, or can I just relax, knowing that when the acidified sparge water hits the mash together with the minerals, the pH will all work out fine? Or do I need to start dissolving the sparge minerals directly in the sparge water with the lactic acid?
(FWIW I add lactic acid when the sparge water is less than 70F, as suggested by Martin, although I can't remember exactly where I first saw that)