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There is phosphoric acid 10% & 80% listed, but not 25% - a concentration I think many of us use.
Then I try to add it in the list, the calculations for mash and sparge amounts are way off.
I can't figure this one out.
To to 'miscellaneous items' profiles and click on the phosphoric acid 10% (water agent). Copy and paste this into the profiles below the original. Edit and change the name to Phosphoric acid 25% and then change teh acid content to 25%. Save this new item and you will now be able to enter it into a recipe. The response of the pH to this mirrors the % acid content.
Calculated in Bru'n water the phosphoric acid 25% is 17.7ml to drop my sparge water from pH 7.5 to pH 5.6 for 6.08 gallons.
The BS3 calculation drops the sparge water to pH 3.4 since it incorrectly based the volume on phosphoric acid 10%
Hang on, are you talking about the default value when you add the acid? This is set in the ingredient profile and will adjust automatically only for batch volume. If you change between the two acids, you will see a difference in the estimated mash pH on the mash tab. This is what you should be using to adjust the acid volume.
making up a recipe to try this out, and I have attached snippets of the same recipe with 10% phosphoric acid and 25% phosphoric acid. Make sure that you make the adjustment in the mash tab to the correct concentration you are using for the final pH adjustments. Note also, that the amount of acid will vary based upon the volume of the water source and buffering content of that water.