pH questions

Okay, there's a lot to this so bare with me:

My water profile here is a bit high (6.2 pH according what the city posted a year or 2 ago) so I have to adjust for that. Usually I look at what percentage crystal/roast malts I have and then subtract (-0.3 for 10%, -0.5 for 20%) If I still need to drop I use acidulated malt (1% of grain bill will drop the pH by 0.1). In my mash yesterday I decided to use a 1/4 tablet of campden (potassium metabisulphite) for every 5 gallons water. When I checked my pH I was down to 5.0 which is lower than I expected. Another factor to consider is I used Munich 15 as 23% of my grain bill.

Was this due to the campden tablets? Does Munich malt effect the pH? or do you think it might some other factor that I haven't even considered?

Your water's pH has nothing to do with what your mash pH is going to be unless you know the rest of the ionic content.

Bobby- That seems odd, since in the past, using the crystal/roast deductions and the acidulated calculations, it has worked perfectly. (i.e. I have been able to control my mash pH starting with the water calculations and doing the deductions as noted) As far as I know the water pH is calculated from the ionic content, otherwise you wouldn't know it's pH (isn't that what pH is after, a calculation of a substance's acidity based on it's ionic content? Please correct me if I'm wrong.)

Another factor I just realized I forgot to mention is that I added 1/2 # oats to the recipe, I guess that might acount for the additional drop in pH, eh?

Really I wasn't that far under my target, and conversion went well (I hit my OG target of 1.054) I am just looking to understand pH in the mash a bit more. i.e. what effects it one way or the other. I've read a bit about it on various sites, but not beinga chemist wasn't sure if the Campden tablets had any effect, or perhaps the acid rest at 133 degrees (I did a double decoction 133/154), or what it was exactly that lower the pH an addition 0.2

How sure are you that you've got a really accurate pH meter? I'm not a chemist either, but I took a lot of chemistry, and I remember hearing from a teacher or lab tech that pH meters aren't super super accurate unless you've got a really nice one.

I'm working from the numbers provided by the city for the H2O pH and then just using the same method as always for the mash pH. Being that there was a variance in what I have previous got, I was wondering why and what the effects of the lower pH might be in the end.

Like I said, the OG hit it's target and the beer is now fermenting nicely, so I'm not worried about that so much as just wondering if anyone can explain what a lower pH from the mash would normally mean, and possibly explain the variables that effect mash pH.