Modifying water for double mash

[OldGoatBrewing]

All, I also posted this in the BIAB thread. I am doing a double mash this week for a 10% barleywine. I have done this a few times now with good results. However, I recently moved to a new area and have started modifying my water with Epsom Salt, Calcium Chloride and Gypsum. I use EZ Water Calculator to determine the amounts of salts. However, this is the first double mash I have done since using salts to modify PH. The question is, should I calculate for the entire grain bill (18lbs) or should I calculate for 9lbs at a time. My thoughts: If I calculate for the entire grain bill, I will overshoot my PH on the first mash and maybe hit the mark on the second. If I calculate for 1/2 the bill at a time, I will hit the PH on the first mash, but will have to take a PH reading between mashes, recalculate for the 2nd mash. Does either of these sound right. I am definitely not a chemist, but I am hoping someone out there has dealt with this.

 

[ScrewyBrewer]

Why not just split the strike water and grain bill in half and treat them as two separate mashes.

 

[OldGoatBrewing]

This is a full-volume mash so I will be mashing in the same strike water. The reason for the double-mash is the size of my mash tun/boil kettle. Although I suppose I could do it as you suggest and boil much longer to reduce my volume into fermenter...

 

[1fastsedan]

I haven't done this before, but I would adjust your water chemistry for the first mash and then on the second only treat for whatever make-up water you need to add (due to grain absorption losses from the first mash.) I assume you're using the runnings from the first mash on the second mash right? If so, the water minerals will still be present from the first to the second mash.