How do I correlate "pHDI" with "Wort pH" for Weyermann malts?

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Just going from 9:1 to 4:1 will drop the PH by approximately 0.35 which in the case of Rahr would give an average wort PH of 5.63.

That would only be the case for an unbuffered system, as AJ has reminded me (or rather forcefully chided me) more than once, after I tried to make the same diluted pH argument as you are making.

As AJ has instructed, and also shown via doing the math, in a buffered system there is only a trivial change in DI_pH as the mash thickness is varied, even when appreciably varied.

Here is a snippet of one of such chidings that I received from AJ:
50 grams of 600L chocolate mashed in 150 mL of DI water will come to pH 4.6995 (its DI mash pH) and the same amount of this malt mashed in 1.5 L of water will come to 4.7065 and mashed in 15L of water, to 4.7660.
 
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As to mash pH vs. wort pH here is a snapshot of what I presume to be one of their Pilsner malts, as taken from the Weyermann presentation:

Mash pH Wort pH.png

The horizontal scale is in half percent of grist weight increments (whereby 12 represents 6% by weight acid malt in the grist).
 
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Perhaps it all comes down to whether or not AJ is correct in his conclusion that mash thickness has merely a trivial effect upon mash pH. Anyone care to mash 50 grams of their Pilsner malt in 150 mL and likewise in 1,500 mL of DI water (or alternately, distilled if DI is not available) and let us know their measured room temperature mash pH results?
 
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These are results for Pils malt with about 40 ppm Ca from gypsum in DI water. These are measured at 60 min.

1.33 qts/lb: 5.57
2 qts/lb: 5.60
4 qts/lb: 5.66

Relatively minor pH difference due to mash thickness, but appreciable. The differences in pH are very much more pronounced at the 15 minute mark. This response is relatively consistent for a wide variety of mash grists and mineral/acid additions.
 
These are results for Pils malt with about 40 ppm Ca from gypsum in DI water. These are measured at 60 min.

1.33 qts/lb: 5.57
2 qts/lb: 5.60
4 qts/lb: 5.66

Relatively minor pH difference due to mash thickness, but appreciable. The differences in pH are very much more pronounced at the 15 minute mark. This response is relatively consistent for a wide variety of mash grists and mineral/acid additions.

Thanks Martin! Your measured pH shift for a mere 3X dilution is about on par with AJ's anticipated 100X dilution induced shift.
 
These are results for Pils malt with about 40 ppm Ca from gypsum in DI water. These are measured at 60 min.

1.33 qts/lb: 5.57
2 qts/lb: 5.60
4 qts/lb: 5.66

Relatively minor pH difference due to mash thickness, but appreciable. The differences in pH are very much more pronounced at the 15 minute mark. This response is relatively consistent for a wide variety of mash grists and mineral/acid additions.

Martin, was 40 ppm Ca++ maintained across all of these dilutions?
 
I've been thinking about the disparity between Martin's measured pH results and AJ's pH theory with respect to dilution impact upon pH drift, as whenever there is a discrepancy, there must be a reason for the discrepancy.

For 1.33 qts of DI water and 1 Lb. of malt, the overall water and malt system has a weight of 1.715 Kg.

For 4.00 qts. of DI water and 1 lb of malt, the overall water and malt system has a weight of 4.237 Kg.

4.237/1.715 = 2.47

40 mg/L Ca ions * 2.47 = 98.8 mg/L Ca ions

Just thinking while I'm typing, and certainly that in itself is rather dangerous and often off base, and therefore the reason for the differences in mash pH that Martin measured clearly may lie elsewhere. But also wondering what the mash pH would be for 98.8 mg/L Calcium ion water used for the case of the 4 quart water and 1 Lb. malt sample alongside of 40 mg/L calcium ion water being used for the case of the 1.33 Qts. of water and 1 Lb. of malt sample. If the 4 Qt mash thereby winds up measuring 5.57 pH, just as for the 1.33 Qt. sample, that might indeed be interesting. And it would likely call for a different model of mash pH prediction software development also.
 
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Where do you get the increased concentration of Ca ions? The water was prepared in a single batch with 40 mg/l and then simply split into two batches of different volume.

The simple answer is that malt brings its own ions (Ca and Mg among them) into the mash and with differing concentrations they will react differently with the water and create PH differences with a proportionality to the mash thickness that is approximately linear (within a reasonable range but nobody mashes 1:100 anyway).

This is what is taught in any professional brewing course and I'd be surprised if that were to be completely wrong. There is certainly some degree of approximation as the mash is a quite complex system from a chemical standpoint but it certainly is not a simple two component buffer and does not behave like one either.
 
By adding what? That looks like a multiplication to me...

The Ca ions don't get concentrated, they stay at 40 mg/l no matter how much water you use for the mash.
 
By adding what? That looks like a multiplication to me...

The Ca ions don't get concentrated, they stay at 40 mg/l no matter how much water you use for the mash.

For the case of 1.33 Qts. of water and 1 Lb. of malt use water with 40 mg/L Ca++
For the case of 4.00 Qts. of water and 1 Lb. of malt use water with 98.8 mg/L Ca++

Mash each as identically as possible, and take their mash pH's. I don't know how to make this any more clear.
 
Maybe it's just me but it's never clear to me when you're proposing someone do an experiment...
 
Maybe it's just me but it's never clear to me when you're proposing someone do an experiment...

Apparently I can be that way sometimes, though it is not by intent.

But cutting to the chase, wouldn't it be interesting if both now mash at the same pH? (not saying that they will, just asking if you [and others] would find it interesting if they did)
 
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