I don't question that you saw what you saw but I don't see how it is possible. I measure MO at 5.6 or higher in DI/RO water, as noted above. In RO water with 5 grams of CaCl2 or gypsum a base malt only mash would be expected to come in at 0.08, call it 0.10 lower than the DI water mash or, in the example I measured, minimum 5.5. And that is about where I measure MO mashes even with a fair amount of colored malts (Barley wines and stouts are the only non German ales I do). It always takes an acid addition to get below this. And it is consistent with what's in the brewing texts and, AFAIK, other peoples' experiences. So I am totally mystified as to how you could be getting as low as 5.0 for what is essentially a base malt mash.
I have been mystified as well. My mashes are not quite base malt mashes, but it still seems weird why I am usually going up instead of down.
Now that I am home, I went back through my beersmith files to look at some sample mashes.
12.00 lb Pale Malt (2 Row) US (3.0 SRM) Grain 90.57 %
0.50 lb Cara-Pils/Dextrine (2.0 SRM) Grain 3.77 %
0.50 lb Wheat, Flaked (1.6 SRM) Grain 3.77 %
0.25 lb Amber Malt (22.0 SRM) Grain 1.89 %
This one came in at 5.10 with R/O + 1 tsp CaSO4 per 5 gallons. This one I brewed at a friends house and I had him double check with his pH meter for the very same reason that I posted in this thread. I have been suspect of my meter or my methods. Both readings with both meters were +/-.03 pH of each other.
10.00 lb Pale Malt (2 Row) US (2.0 SRM) Grain 86.96 %
0.50 lb Cara-Pils/Dextrine (2.0 SRM) Grain 4.35 %
0.50 lb Corn, Flaked (1.3 SRM) Grain 4.35 %
0.50 lb Munich Malt - 10L (10.0 SRM) Grain 4.35 %
This one came in at 5.35 with 1/2 tsp CaCl and 1/2 tsp CaSO4 per 5 gallons.
7.00 lb Pale Malt (2 Row) UK (3.0 SRM) (MO) Grain 73.68 %
1.00 lb Caramel/Crystal Malt - 40L (40.0 SRM) Grain 10.53 %
0.50 lb Honey Malt (25.0 SRM) Grain 5.26 %
0.50 lb Munich Malt (9.0 SRM) Grain 5.26 %
0.25 lb Caramel/Crystal Malt -120L (120.0 SRM) Grain 2.63 %
0.25 lb Chocolate Malt (200.0 SRM) Grain 2.63 %
This one came in at 4.9 with 1 tsp CaCl and 1/2 tsp Pickling Lime added to the water the night before. I corrected with another 1/2 tsp Pickling Lime and checked post boil going into the fermenter and it was 5.23.
My beers have been coming out great regardless, but it has been bugging me. I have sealed containers that I store the buffers in between calibrations. I do spray the electrode off with DI water and shake off the excess before going between buffers. I go through buffers every couple of months.
I use an MW-102 meter with auto calibration and temp correction.
I have not checked the R/O water before mashing. I will make a note to do that with the next batch.
On a side note, that may or may not be relevant, I never treat my sparge water with any minerals or acids. I will usually(if I remember) add an additional tsp of either CaCl or CaSO4 to the boil to get my Ca up for fermentation.