[SiameseMoose]

The only significant change in any variable is the humidity. I brew outside, but under a cover, so nothing extra entered the kettle, The burner is located at a walk-out basement door, which has a six foot wall right behind it which acts as a great wind shelter. I have a gas line plumbed from inside the house, so the pressure is the same. It's obviously the same kettle. In this case, I actually ran the flame higher for the batch with the lower boil-off. Finally, this is a frequent problem for me, and I've given it much consideration over the last few years, I'm convinced humidity is the factor at play here, and I was just looking for others experience with it.

Thanks for all of the replies!

[Malticulous]

I brew a lot and still have not gotten my boil off perfected. I live in about the driest place in the US but it does get humid time to time. I end up topping off or just ferminting 6 gallons instead of 5.5 sometimes.

[hopfenundmalz]

The only significant change in any variable is the humidity. I brew outside, but under a cover, so nothing extra entered the kettle, The burner is located at a walk-out basement door, which has a six foot wall right behind it which acts as a great wind shelter. I have a gas line plumbed from inside the house, so the pressure is the same. It's obviously the same kettle. In this case, I actually ran the flame higher for the batch with the lower boil-off. Finally, this is a frequent problem for me, and I've given it much consideration over the last few years, I'm convinced humidity is the factor at play here, and I was just looking for others experience with it.

Thanks for all of the replies!

On another forum,a guy in Canada was talking about the boil off rate he got in the winter when it was subzero (F) and the relative humidity was in the single digits.   That was a crazy amount.

Anyone ever hike in the desert at 108F and 8% humidit?.  Not that bad, dry heat and all.  Then you take a break in the shade, remove the back pack and start to get the chills from the evaporation off of your back.  Just saying humidity is a factor in this.

[dhacker]

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?

I think everyone has some problem with this sometime in their brewing career. Several years ago I found a way to get real close to the final volume I was looking for. I threw out all the theories, formulas and guess work and installed a site gauge on my brew kettle. I also made 90 minutes my standard boil time. I put 13.5 gallons of pre-boil wort in the kettle, and end at 12 post boil. You get a feel for the personality of your system after a number of batches, so now I watch to see how much volume has gone out in the first 30 minutes. I can tweak the boil rate up or down slightly to end up at 12 when 90 minutes has elapsed. This way, the first 30 minutes is spent getting your final volume dialed in without screwing up the timing on your hop additions, etc.

Yes . . my boil off is repeatable!   

[tubercle]

Let me try again.....

Why try to predict all of this using all the complicated science and formulas, consulting the Myan Calender (adjusted for the Moon phase), gravity calculations, and throwing in the current price of eggs in China while burning the feathers of a black chicken and rattling cat bones?

 Get a piece of wood - mash paddle or something  - and hold it upright in your boil kettle. Pour in a known amount of water, say 1/2 gallon, and take your trusty Barlow and carve a notch where the wet and dry meets. Pour in another known amount, say 1/2 gallon, and repete the notch.

As you boil away, occasionally put the stick in and see where you are at. Cut the flame down and let it settle a few seconds and take a measurement. Not there yet? fire up the flame. At your desired volume? Start cooling; open another beer.

 This takes away every variable, temperature, humidity, altitude, etc....100%  reproducible volume.

I know, expansion due to heat...bla, bla, bla.

Enjoy the brewing experience.

[tschmidlin]

Anyone ever hike in the desert at 108F and 8% humidit?.  Not that bad, dry heat and all.  Then you take a break in the shade, remove the back pack and start to get the chills from the evaporation off of your back.  Just saying humidity is a factor in this.

I'm not convinced it is a huge factor.  I haven't done the math, but boiling will drive off more vapor than any effects from the humidity.  When you consider that at the surface of the boil the humidity is damn near 100% and all of that steam is going up and out because your kettle is uncovered, ambient humidity just doesn't seem likely to play a major role.

I agree with tubercle though, it shouldn't be a huge source of stress.

[Kit B]

Let me try again.....

Why try to predict all of this using all the complicated science and formulas, consulting the Myan Calender (adjusted for the Moon phase), gravity calculations, and throwing in the current price of eggs in China while burning the feathers of a black chicken and rattling cat bones?

 Get a piece of wood - mash paddle or something  - and hold it upright in your boil kettle. Pour in a known amount of water, say 1/2 gallon, and take your trusty Barlow and carve a notch where the wet and dry meets. Pour in another known amount, say 1/2 gallon, and repete the notch.

As you boil away, occasionally put the stick in and see where you are at. Cut the flame down and let it settle a few seconds and take a measurement. Not there yet? fire up the flame. At your desired volume? Start cooling; open another beer.

 This takes away every variable, temperature, humidity, altitude, etc....100%  reproducible volume.

I know, expansion due to heat...bla, bla, bla.

Enjoy the brewing experience.

This is exactly what I do.
While several factors play a role, I find that this is really the only simple method to which I have access.
It is fairly predictable, except on days of extreme humidity or a lack thereof.

[davidgzach]

Agree with the no-nonsense way of determing your boil volume.  The more you brew, the more you will get to know your brewery and the more consistency you will see.  I can now pretty much tell just by checking my boil kettle where I am and where I need to be.  I'd say I end up within a pint of each brew regardless of the above stated factors.

But I do like the stir paddle notch idea.  May have to incorporate that to idiot proof myself for the days I have too many homebrews......

Dave

[euge]

Another way is to take a tape-measure and calculate gallons per inches. One can mark the inside of the kettle if desired or just use the tape-measure.

[Kit B]

...Or, yard stick

[richardt]

Using a sharpie to mark the long plastic stir spoon does NOT work.   

It probably has something to do with the heat and/or acidity of the wort.
The marks basically come off when you stir the wort.

[tygo]

Another way is to take a tape-measure and calculate gallons per inches. One can mark the inside of the kettle if desired or just use the tape-measure.

Yep, this is what I do.

[dbeechum]

I have a spreadsheet that I plug the kettle height into and it spits back a number of gallons both pre and post boil expansion, calculates the final gravity if I hit my boil off numbers and tells me how much sparge water I need based on the target starting volume.

[jeffy]

I fill up the kettle to the high dirt mark, boil for an hour and accept what's left.  It's usually about 10 gallons of beer.

[euge]

I fill up the kettle to the high dirt mark, boil for an hour and accept what's left.  It's usually about 10 gallons of beer.

Fill my 80qt nearly to the brim and heat for batch-sparging. After it is all said and done I get about 12 gallons of beer for a long boil and 13 for a short boil.