How to figure out your Evaporation Rate By: Kay Witkiewicz
Perhaps the most important aspect of your boil is knowing the evaporation rate of your brewing set-up in order to accurately match your pre-boil volume to your expected post-boil volume at the requisite specific gravity. While programs such as BeerSmith may give you an idea of your evaporation rate, nothing beats personal experience, and the only way to truly know your evaporation rate is to measure it in the act under your own brewing circumstances. Here’s how I figured out my evaporation rate, and it all starts with making a measuring device:
1. Procure a dowel, a food-grade plastic spoon, or even a very long chop stick to serve as your measuring device. The three most important characteristics of the measuring device of your choosing are that it is the same height as or higher than your brewing kettle; it is an untreated piece of equipment, meaning the dowel isn’t lacquered, the spoon isn’t your usual sweet tea stirrer, and you didn’t eat take-out with the oversized chop stick last night; it can be carved into to mark certain volumes in your kettle.
2. Take your brew kettle and a sizable container with a known volume (I use a one-gallon plastic container, but a very large measuring cup will also do) and head for your sink. Bring along a pocket knife and your measuring device as well.
3. Fill up your container of known volume with water and pour its contents into your brew kettle. Repeat as many times as you need to in order to attain the beginning volume you want to mark on your measuring device.
4. Place your measuring device into the same spot of your brew kettle (preferably the center) at volume increments of your choosing and carve a mark at each water line. For example, I know my brew kettle will never contain less than 3 gallons of wort pre- or post-boil, so the first mark on my measuring device is at 3 gallons with subsequent carvings at each additional gallon up to my brew kettle’s maximum volume of 7 gallons.
5. Once you’re done marking all the volume increments that are convenient for you, you can either boil the water in your brew kettle for a set period of time and get an idea of your evaporation rate that way or you can just dump the water, wait until brew day, and figure it out as you’re boiling wort.
Notes: Carving is a precarious procedure, especially since your hands as well as your blade are likely to get wet during this exercise. However, I recommend carefully carving your measuring device as it is a permanent means of marking with no potential impact on the flavor of your wort, whereas other means of marking, such as masking tape or Sharpie carry the risk of leaking unpleasant compounds into your wort when in contact with the hot liquid. The use of temperate tap water rather than steaming wort is another aspect to consider when making your own evaporation rate measuring device. Water obviously has a lower gravity than wort; moreover, liquid expands as it heats up. Therefore, when you immerse your measuring device in hot, or even boiling, wort, you will receive an upwardly skewed volume reading. While carving your measuring device relative to known volumes of hot wort would alleviate this inaccuracy, the splashing of wort is ultimately more detrimental to your beer than a slightly off volume reading.
Now that you have made your own evaporation rate measuring device, it’s time to put it to work. Useful in more ways than one, this handy stick tells me the volume of the first runnings of my mash and it allows me to gauge the amount of water I need to sparge with in order to hit my target pre-boil volume. Regardless of your brewing method, use it to measure your pre-boil volume of wort, then start your boil and temporarily measure the amount of liquid left in your brew kettle in order to judge the length of your boil necessary to meet your intended post-boil volume, gravity, and timing of your hop additions. If you’ve been brewing beer, but you haven’t actually measured your evaporation rate, chances are you have developed a good feel for it and the measuring device will only improve your brewing practices. If you’ve never paid attention to your evaporation rate, estimating your volumes, timing your boil, and figuring out when to add your hops can be daunting, but buck up and pay careful attention to how your brewing system reacts during your trial batch so you’ll have an intimate understanding of what you need to adjust for subsequent batches. Believe me, after collecting and fermenting more than a gallon more wort than I planned on for an American brown ale that also started 25 gravity points lower than expected, I knew that how much wort you’re boiling and how long you’re boiling it matters a great deal in executing your recipe, even though the beer turned out really delicious. Volumes and evaporation rate are neither as artisanally charming as grist composition nor as amazingly creative as hop additions, but they are fundamental to consistently brewing good beer.
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