Roast acidity varies quite a bit, but it doesn’t seem to be well correlated to color rating. Review the results in Kai’s paper on malt acidity and you will note that base and crystal malts tend to correlate well with color rating, but the roast is all over the place. About the only thing I could get out of the data was that roast has a somewhat consistent level of acidity. I just eye-balled an average value. The good thing is that we don’t typically use that much roast in our brews.
Right. Thanks for the info !
Thank you Martin! I had been wanting to blame my sometimes low mash pH in a couple of dark beers on the *L of the roast. Sounds like I have to suss out another culprit. 
I went with an Omega PHH-7011 about a year ago as I finally tackling the topic of water chemistry + needed a more accurate way to measure mash pH. This meter is easy to use and works well, but ran into a silly issue last week where the batteries died + I couldn’t remove a screw that holds the cover on. None of my usual tricks worked and so I sent the folks at Omega an email asking if they had any tips. Within a day I had a great email from customer service saying they’d be happy to replace the meter free of charge. Say what?
Not only is this a great meter but Omega really stands behind their products. That’s worth more than the meter in my book. Highly recommend.
I also have the Omega PHH-7011, and I challenge anyone to find a comparably priced meter that is better.
I did a full review on the meter - and like the OP, I am not a paid spokesperson.