Recently, I made a rookie mistake while venturing into new territories of homebrewing. I’ve been doing all-grain for a while now and I’ve gotten used to working with a recipe scale of 5-gallons. It’s pretty much the standard batch size. It never occurred to me that the evaporation rate would remain a constant when I downsized the batch to a single gallon. I assumed that the evaporation amount would be a fifth of the amount of the normal 5-gallon batch. Dead wrong. It stays the same (give or take) regardless of the volume of water you’re boiling. The two things you need to account for is the surface area of the water (or the diameter of the kettle), and the temperature of the boil. The larger the diameter is, the more area the vapor has to escape – and the more vigorous the boil, the faster the water molecules are moving, essentially causing more vapor. I haven’t brewed enough to put this to the test, but I imagine that the climate would also have an affect on the evaporation rate.
Here’s what I did
I wanted to know what my evaporation rate would be for my single-gallon experimental batches so I can compensate for the loss. I do them indoors, so I know the climate conditions won’t be much of an issue. I grabbed my stock pot and measured out 6 quarts, or 1.5 gallons of water and brought it to a boil. Once it came to a boil, I set a timer for 60 minutes and checked the boil rate every so often. Tip: Throw in a cinnamon stick if you have one – it makes the house smell great. Once the boil was over, I cooled the water down a little bit and measured what was left and it came out to just under 3 quarts. Over half of it evaporated! I’m going to use this constant for the next small batch I brew indoors and we’ll see how close I get to the final volume.