The two programs are using different models for the acid calculation. Before I dive into the science, I will mention that
BeerSmith 3.2 it has an option to choose models on the mash tab and in your equipment profile.
You can select between the two major models. The default is the MPH 3 model (mentioned below) but you can also select the BW model if you prefer the BNW estimates. So you can choose either.
The Basic Problem
Without diving too hard into the math, there are two major models in broad use for mash pH estimation and also mash pH (acid) adjustment. These are the
MPH model by D Mark Riffe and the unpublished BNW model. Both use a very similar method for estimating the mash pH from a water profile and a given set of grain additions based on a series of experiments done originally by Kai Troester. So both models will give you a similar estimated mash pH.
Where the two models diverge is in handling of the acid additions. The problem here is simply that the buffering capacity of the grains (which counters the acid addition you are making) is unknown. While D Mark Riffe has done extensive work on a MPH 4 model that does a better job of handling this problem - it uses a variety of grain parameters that are not commonly measured or known, so really is not practical for general use yet.
Unfortunately the
reality is that the best we can do today is estimate the acid needed as there can be a wide variation in the buffering capacity of the malt, and that variation is simply not measured or known.
How to Choose
Riffe's MPH model is well documented on his blog and based on earlier work going all the way back to the 1950's. However in practice, many brewers have reported that it overestimates the amount of acid needed. The BNW model takes a much simpler approach by treating the acid additions much like grain acidity (largely ignoring the problem of grain buffering) which gives what I would call something of a lower bound on the acid needed.
So if we take the BNW model as the lower bound and MPH model as the upper bound, you can probably get into the ballpark here.
A Practical Approach
I recommend doing what I outlined in my article here on mash pH adjustment and measurement. Since the true buffering of a given grain bill is not known with absolute precision, you can add most of your acid adjustment up front to control the initial pH and then once you have mashed in take a pH measurement and make a final adjustment:
As a final note here's a tutorial on how to use the calculator in your recipe to estimate and later adjust mash pH:
Sorry for the long post - but this is a complex topic!
Brad