Could someone put me right on this?

[butterman]

I understand that distilled water is free of all ions while RO water is nearly free of ions and that both sources are very soft water and I have read that they are supposed to reduce the hardness and alkalinity of brewing water in proportion to the amount of RO or distilled water added to tap water.

I was surprised when I did a couple of tests to see what the actual effects were of adding proportions of RO water to tap water. My tap water is in the range of 7.4 to 7.6. In one case with an RO addition the pH increased to 9.2, in another it didn't alter the pH of the tap water at all.

I think my confusion may be to wrongly believe that the supposed reduction in alkalinity should be reflected in a commensurate reduction of pH, which obviously didn't occur in my tests.

Could someone please help with this mental block?

Thanks

[spiderwrangler]

I'm not the best water man in here, but basically (no pun intended), the pH of your water is only the end result of a series of sometimes complex interactions between ions, etc in the water, all buffering each other and interacting. So, if you take two samples of water from different places and times, they may be the same pH, but arrive at that pH through different means. Disturbing that pH by adding an equal volume of RO water may skew them in different ways by disrupting that balance. I think... the more I ponder this, the more the water messes with me...

[butterman]

OK, if we put aside the complexities of water chemistry for a moment, when & for what reason would a brewer add RO water to tap water?

[BDawg]

If their tap water is high in mineral content, then adding RO water will dilute it to more desirable levels.

[hoodie]

It's not about your water, it's about your waters affect on MASH pH. . Listen to the water shows on BREWSTRONG. It's all about buffering.

[butterman]

If their tap water is high in mineral content, then adding RO water will dilute it to more desirable levels.

Is there an objective method (or if not, a rule of thumb) to tell you how much to add to get to a certain mineral level (desired level). From my OP it seems using a pH meter wouldn't help!

[spiderwrangler]

If you have a water report for your area, you can use that as a start to get anything that is overly high diluted down...

[Ozwald]

If their tap water is high in mineral content, then adding RO water will dilute it to more desirable levels.

Is there an objective method (or if not, a rule of thumb) to tell you how much to add to get to a certain mineral level (desired level). From my OP it seems using a pH meter wouldn't help!

Pretty simple actually. Let's say your water report says you have 100ppm of calcium. Mix 1 gallon of your water with 1 gallon of RO, you now have 50ppm. Just remember to do the same math to each mineral.