Beer Forum

On multiple occasions Tasty has refered to checking his volume throughout the boil. I am trying to set up a spreadsheet to calculate where I need to be at the beginning of the boil and a couple of intervals throughout to maintain a 10% evaporation rate, but I have been unable to get the formula right. I have tried to figure out the formula myself, have tried to search the internet at length and looked in the few texts I have, but have been unsuccessful. If anyone can help me out with this it would be greatly appreciated.

Na Strovia,
Brad

In my opinion your evaporation rate should be specified in a volume per hour (gal/hour), not a percentage. With my system, I boil off 1.6 gallons per hour. With 12 gallons of wort in the kettle, this is 13.3%. If I do a 6 gallon batch, I still boiloff 1.6 gallons per hour, 26.7%. This percentage stuff will never fit into a formula because it won't be accurate for different volumes. If you know your boil off rate, then you just multiply it by your boil time and it makes the calculation very easy.

The engineer in me says that scientifically, evaporation rate is directly tied to the surface area of the liquid being boiled. In most cases, surface area for homebrewers is pretty much the same throughout the boil, whether you've got 10 gallons, 5 gallons, 3 gallons, or whatever. In fact, it's even almost consistent from brewer to brewer because many of our brew kettles are roughly the same diameter -- perfectly sized for putting over a big burner.

The only other variable, really, is how high you crank up your heat. If you have a standard sized brew kettle and you crank 'er up as high as she'll go without gurgling significantly over the side, chances are that your boil rate will be someplace between about 1.2 and 1.5 gallons per hour. Scientifically, it is not appropriate to use a percentage, although if you always make the same batch size, use the same kettle, and boil for the same amount of time, then you CAN utilize a percentage which will probably end up around 15 to 20% for the typical 5 gallon batch. Actually, I brew 3 gallon batches and I know my boil rate is about 22% for my standard batch size and my equipment. Since my brewing software uses percentage, I put 22% in and I always come out real close on all my volumes. When I brew bigger batches, I know from experience that I will need to lower this to the 15-20% range.

But "the right way" would be to figure out how typical your kettle size is, and then assume a certain volume per hour is boiled off, which would NOT change based on batch size. 3 gallons, 5 gallons, doesn't matter. The evaporation rate by volume per hour will stay the same, assuming the same boil vigor (is it just barely bubbling, or is it rolling big-time over the sides?).

Thanks for the help guys.

Na Strovia,
Brad

I think the main reason most use a % evaporation rate per hour is because most of us make the same batch size every time. So the preboil volume is fairly constant. As such, a % is adequate. But I agree that if your volume changes on a batch-by-batch basis, then a rate in gal/hr. or similar is much better (and more accurate).

I think that the reason you hear % boil off so often is that George Fix used it in the Principals of Brewing science in the chapter on Wort Boiling:

"The most widely used indicator is the percent evaporation that takes place in the boil (Narziss, 1992). With standard boiling systems, a general rule is that the volume reduction be at least 7%. However, it has been shown that evaporation rates above 12% may produce level 2 heterocyclics, leaving vegetal malt tones that are accompanied with some astringency. A wide range of level 2 and 3 heterocyclics is possible once evaporation rates exceed 15%. As already stated, the flavor of the finished beer will determine the extent to which this effect is relevant." - Fix 1999

Of course, there Fix is quoting Narziss, who is a German brewing scientist who focuses mainly on commercial brewing. Gallons/Hour is a better predictor of final volume.

In regards to a formula the beer math show's gravity calculation would be applicable here:

ppg = pts Gal/lbs = L °/kg
8.3454
ppg x mass = total gravity points
total gravity points/volume = specific gravity

Also ppg can be found by using the Fine Grind Dry-Basis percentage on the malt report and multiplying that percentage in decimal form by 46 (the 100% extract ppg represented by the ppg of Sucrose). This is helpful, again paraphrasing from the beer math show, because if you have a preboil spcific gravity and volume you should be able to predict your final gravity at your final volume (ex a preboil of 7 gallons of 1.050 wort when boiled down to 6 gallons would be roughly 1.058 [GP/7=50; GP=350; 350/6=SG; SG=58.333333 OR 1.058]).

I think percentages are appropriate when you are speaking generally to someone and you don't know the configurations of their boil kettle.

When you are speaking specifically about your own equipment or need precision in a calculation, you should always use volume / per hr.