Rob Connolly's Blog

Wow, I just managed to combine three of my favourite things in a single title! Recently, I’ve been getting further into home brewing, with a book I received as a Christmas present (Home Brewed Beers and Stouts, by C.J.J. Berry). Since I’d never actually measured the Alcohol By Volume (ABV) of my beer I decided to write some Python code to automate the process. The simple modules I came up with work from the command line and also on Android phones via SL4A, which makes them very useful when doing quick measurements.

Measuring ABV involves taking Specific Gravity (SG) measurements at the start and end of fermentation, adjusting them for temperature and pushing them into a simple formula. The SG measurements are taken with a Hydrometer, which is supposed to read at 20C (hence the need to adjust for other temperatures).

The temperature adjustment is done via a simple table (which I took from the book). In the script I used Linear Interpolation, to adapt it for values which weren’t available in the table. Initially I used the numpy.interp() function. However, I found that numpy isn’t present in SL4A and that installing it would be a pain since it is a C module which would need cross compiling for Android.

I therefore wrote my own interp() function, which keeps the same interface:

def interp(x, xp, fp):
    i = 0
    for p in xp:
        if p > x:
            i = xp.index(p) - 1
            break
    return ((x - xp[i])*fp[i+1] + (xp[i+1] - x)*fp[i]) / (xp[i+1] - xp[i])

I split up the code into two separate modules, sg.py (which just does specific gravity adjustment) and abv.py (which does ABV calualation). Splitting the code up enables me to take SG measurements and adjust based on temperature, without doing a full ABV measurement. In sg.py the reverse adjusted SG is what you would need to read from the hydrometer at the specified temperature, in order to achieve the adjusted reading you specified.

Since the calculations are fairly trivial the main bulk of the code is in the user interfaces. I implemented a simple command line interface which either takes arguments from the command line, or prompts the user via the python input() function. I also use the SL4A API to implement a simple UI for Android, basically the user is prompted for each quantity by a dialog box.

Anyway, there’s not much else to say, except that I’ve put the code up on Gitorious for anyone who wants it (it’s licenced AGPL). The code is in a git repository for useful Python scripts I’ve written, right now it’s the only thing there, but I’m going to track down some of the scripts I’ve written over the years and add them too – I might even get a few blog posts out of some of them!