I was listening to a show from The Brewing Network today and heard an ad for the iphone/ipad brewing app Brew Pal, which I bought months ago and had intended to write about here. Obviously I'd forgotten to do so, but hearing the ad reminded me of my oversight.
Short version: Brew Pal would be a great app, if only it worked, which it doesn't.
Long version, an Example:
I created an example recipe with a grist of 10lb of Pilsen malt (1.036 ppg). I've set my brewing system variables as follows: efficiency is 70%, the evaporation rate is 15% per hour, the boil length is 1 hour, the trub loss is .75 gallons and my batch size is 5 gallons. A very basic recipe. Let's see what Brew Pal does with it.
- Boil Volume: 7.06 gallons
- Pre-boil gravity: 1.036
- Post-boil gravity: 1.050
This is flat out wrong. Let's go through the math.
The first thing to understand here is that only two things change the gravity of a wort: the addition of sugars and the evaporation of water. That's it. In this example I'm not adding sugar after the boil begins, so we only have to concern ourselves with the evaporation of water. To determine the gravity change due to evaporation, the math is simple:
Pre-Boil Gravity / (1 - (Evaporation Rate per Hour * Boil time (in hours)))
Or, in our example
36 / (1 - (.15 * 1)) = 42.35
That is, if I have a pre-boil gravity of 1.036 and boil away 15% of my volume during one hour, the gravity of the wort will have increased to 42.35. Yet Brew Pal says my gravity should be 1.050. Why?
Here's why, it calculates post-boil gravity using the your 'batch size', which Brew Pal defines (as far as I can tell) as the volume of finished beer you get out of the fermenter.
- Pre-boil volume: 7.06 gallons
- Post-boil volume: 6.01 gallons (7.06 * .85)
- Fermenter volume: 5.25 gallons (6.01 - .75 gallons [trub])
- Batch size: 5 gallons (5.25 gallons - .25 gallons lost in the yeast cake)
Now, I told Brew Pal that my efficiency was 70% and that I was using 10lb of grain with 1.036 ppg. 10 * 36 = 360 gravity points. 360 * .70 = 252 points extracted. If I dissolved 252 points into 7.06 gallons of water, I would end up with a specific gravity of 1.0357, which rounds up to 1.036. That's my pre-boil gravity, which Brew Pal figures correctly. Those same 252 points would give me a gravity of 1.042 in my post-boil gravity of 6 gallons, that's what Brew Pal should report as my post-boil gravity. But Brew Pal gives me a post-boil gravity of 1.050, which is 252 points dissolved into 5 gallons -- or my Batch Size volume.
Remember what I said before -- gravity only changes if sugar is added or water is evaporated away. Losing water to trub or to yeast cake does not change the gravity of your beer. This is basic stuff, yet it's something that Brew Pal gets wrong.
I thought that perhaps I was setting my batch size incorrectly. Perhaps if I set it to the volume of wort left at the end of the boil then all the math would work out -- after all, my own log sheets use post-boil volume as the batch size. But if I change the recipe to have a 6.01 gallon batch size, then Brew Pal just scales up the boil volume to 8.25 gallons -- now everything is wrong.
It's entirely possible that there's some hidden setting I'm missing that would make this all work. But after looking at all of the screens and fiddling with all the variables, I can't find any way to make Brew Pal calculate gravity correctly.
As much as I would like to have an all-in-one recipe app on iOS, Brew Pal just isn't it. When I first recognized this problem I emailed the app's author. I have not received a reply. That the author apparently doesn't think that this is a problem should be enough to warn anyone away from Brew Pal.