# Calculating boil evaporation loss as a function of pre-boil volume (PBV)?

I had done some cursory digging online and found that at my elevation the boil evaporation loss was about 14% per hour. So to test this out I boiled 3785mL (1 gallon) of water for an hour in my brew kettle. If 14% is true, then I expected to be left with about 3221mL left (about 86%) at the end of the hour.

An hour later, I was left with 1300ml!!! So more like an evaporation rate of 66% when starting with 1 gallon.

So, being of little mind, I'm guessing evap rate is a function of starting volume. If so, does anyone know the formula to estimate evap rate (per hour, ideally) that takes initial volume into consideration?

• I'm half drunk but here's a comment. Evaporation rate should never be measured in percent, and is a function of surface area of the boiling wort, and has basically nothing to do with volume at all. If you have a 14-inch diameter kettle, and use about the same heat to boil 5 gallons or 2 gallons, your boiloff rate should stay the same at roughly 1.2 gallons per hour or something like that. Find out what it is for you and your kettle. Don't measure in percents. Measure in gallons (or ml) per hour. Nov 5, 2021 at 3:02
• Hey maybe you're half-drunk, but that was twice as well as I could have articulated any day of the week! Thanks! Nov 5, 2021 at 10:02
• @dmtaylor: Evaporation is a function of the energy put in the liquid (electric heating, gas burning), and of the energetic losses from the kettle to the environment. Add 2257 kJ of energy to your boiling wort, and you will lose 1 litre (kg) of water, under ideal circumstances. Nov 5, 2021 at 16:25

I found this off of Physics.SE, and I'd be genuinely interested in what others here think.

But the consistent advice I've seen is to do just what @dmtaylor suggests: get to know your own boil kettle and tweak your recipes/plans accordingly.

The two biggest factors contributing two boiloff rate that I could find are the diameter of the kettle (hence surface area exposed to the air) and the amount of heat transfer going into the kettle (boil vigor). In the case of my super-simple stovetop setup, this computes to how "high" or "low" I have the dial set.

So I conducted trials. The constants in every trial I ran:

• Same 16-quart brew kettle used each time
• Same burner (I have different sized burners on my stove) used each time
• Lid placed in the same 7/8 position each time
• Dial turned to the MAX setting which produces a violent boil

The only thing that changed was the volume of water I started with each time. In every case I hit more or less the same rate: 0.68 gallons per hour (GPH). There was some variation (which could have been chalked up to a million inconsistencies in my non-laboratory, kitchen-based "experiments"). But 0.68 GPH was the average. For that kettle, under those conditions.

I will also do this for whatever dial setting I need to maintain a smooth, rolling boil as well as a vigorous boil (there are differences in all of these, apparently). All different amounts of heat transfer.

That way, if I know I need a rolling boil, a vigorous boil, or a violent boil in that 16-quart kettle, on that burner, and with the lid placed at the 7/8 position, I will know what setting to place the dial at and I will know the boiloff rate in GPH to expect.

• The lid doesn't do anything. At a certain point in the boil, the lid also gets to 100° C (or your boiling temperature), and so, no condensation takes place on the lid any more. The steam will just move faster through the gap between the lid and the kettle. Nov 5, 2021 at 16:28
• Good to know @chthon, thanks (and +1), also, does my process described above sound reasonable to you? Could it be improved at all (given my stovetop/brew kettle setup)? Nov 5, 2021 at 17:27

Yes, "dialing in our equipment" is important. Also, factor in the time of the year. By this I mean, for me in the Fall/Winter and part of the Spring it's very dry (humidity wise). In later Spring and Summer it's humid and I get different boil off rates.