[SiameseMoose]

I'm having a hard time predicting my boil-off volume. Two consecutive brews with the same boil time, and the first boiled off 2.5 gallons, but today was only 1.5 gallons. As far as I can tell, the only major difference between the two sessions was the humidity. Today I brewed in an on-and-off drizzle, so saturated humidity. The previous session was about 60% humidity. Today I even turned up the burner intensity because I saw that the boil off rate was low. I have to find a way to get a more repeatable figure. Any hints?

[Slowbrew]

Ambient humidity does make a difference in your boil off.  I adjust my boil time to account for the anticipated differences.  On very humid days I add 20 - 30 minutes to the boil prior to my first hop addition.

Other than doing the boil in a controlled environment I haven't found a better way to manage it.

Paul

[tubercle]

I use my stirring paddle with notches calibrated to nearest 1/4 gallon for my boil kettle. I stop when I get it to where I want it. Temp and humidity are no longer factors.

[a10t2]

Humidity will definitely affect the boiloff rate, but I don't think it could explain a 50% variation. I brewed my first fifty-odd batches in the Midwest, so there must have been some major changes in humidity, and I never saw anything close to that kind of discrepancy.

[euge]

Yes it does make a difference but not that bad. Mine is predictable.

[Kit B]

Several factors contribute, including:
Starting gravity
Humidity
Intensity of boil
Surface area of boiling liquid
Wind

[dmtaylor]

I'm sure humidity does matter, although I'm not exactly sure how much it matters.  Probably just a little.  But you also mentioned that it was drizzling rain -- did the boil kettle get hit with drizzle during the boil?  This could also have cooled it down, preventing normal boil vigor.

The wind can also have an effect, e.g., if wind is blowing the flame off to one side.

Also the amount of flame, e.g., amount of propane sent to the burner.  Depending on your setup, it's not necessarily easy to get the flame at exactly the same strength every time you brew.

Perhaps the most significant factor, however, is the diameter of the boil kettle.  You probably didn't change your kettle between batches, but if you ever do, it's obviously something worth considering, as you'll see...  Forgive me for using a little simple mathematics, but if you usually use a kettle with a diameter of 18 inches, let's say, and now you switched to a kettle with 12 inches, this can have a huge effect based on the ratio of boiling surface areas.  Since A=pi*(R squared), you could compare 9*9=81 square inches versus 6*6=36 square inches and based on that the boiloff rate for the 12-inch kettle is 36/81 or less than half as much as the 18-inch kettle.  Quite a difference.

[denny]

Several factors contribute, including:
Starting gravity

Hmmm, something I've never considered.  How does starting gravity affect boil off rate?

[Jimmy K]

Several factors contribute, including:
Starting gravity

Hmmm, something I've never considered.  How does starting gravity affect boil off rate?

Dissolved sugars raise the boiling point of water, so higher gravity means the temperature of the boiling wort is higher. I was trying to figure out how much higher, but can't find a reference.

[denny]

Dissolved sugars raise the boiling point of water, so higher gravity means the temperature of the boiling wort is higher. I was trying to figure out how much higher, but can't find a reference.

Based on my experience, I'd say that while that may be true theoretically, in the real world I've seen it make pretty much no difference.

[hopfenundmalz]

Several factors contribute, including:
Starting gravity

Hmmm, something I've never considered.  How does starting gravity affect boil off rate?

Dissolved sugars raise the boiling point of water, so higher gravity means the temperature of the boiling wort is higher. I was trying to figure out how much higher, but can't find a reference.

If you search enough you will find the answer, and it is not much even for a 1.100 wort.  It doesn't even make up for the decrease in boiling temp. for where I live (at the nose bleed altitude of 900 ft above sea level).

[a10t2]

Dissolved sugars raise the boiling point of water, so higher gravity means the temperature of the boiling wort is higher. I was trying to figure out how much higher, but can't find a reference.

In a typical beer wort it's <0.5°C. The maximum would be ~1.5°C.

[hopfenundmalz]

Sean - I just was at that page. 

Where I live, water will boil at 210F, the calculator said 1.100 wort (24 Brix) will add 1.1 degrees C to the boil.  So if I do a Barleywine that gets me back to 210+(1.8*1.1)=211.98F assuming no big lows or highs are in the area.

[dmtaylor]

It's true that gravity affects the boiling point temperature slightly, but the heat of vaporization of the water is barely affected at all, way down in the hundreds of a percent, i.e., negligible.  So yeah, basically it takes an extra minute or two to heat up your wort to boiling temperature, but once it's there, the amount of heat you need to add to boil it off is about the same regardless of boiling temperature.

Yes, that's right.  I'm an engineer.

[denny]

Yes, that's right.  I'm an engineer.

And I believe ya!