[dhacker]

Well . . I was able to prove that my "Reverse Reckoning" works no mater the original kettle volume.

Y-day I brewed the Tripel that had a  31 lb.  grain bill . . about all my mash tun wants to choke. For whatever reason my sparge water needed for a 13.5 gallons original BK volume didn't seem to fit. and I had to batch sparge twice . . or so my mistaken calcs told me. Long story short . .14.5 gallons in the BK. 1.5 hours later I was at 13 gallons. One hour to go, so hops went in, all other additions in, and 12 gallons at flameout. Nailed my OG @ 1.074.

I guess I pretty much have it down to a 1/2 gallon per 1/2 hr. boil off rate no matter how I try to screw it up.  

[oscarvan]

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 what I do too.......BUT.....BUT...and it's a BIG BUTT........there is the amount the volume decreases as a result of cooling....so I leave myself and extra 1/2 gallon above where I want to be at boiling temp, and then when it cools down I get what I am shooting for.

[Will's Swill]

Well, if you don't consult the Mayan calendar, then you might ending up dramatically changing your hop utilization as you try to fiddle with the boil volume.  But instead of doing a kabuki dance, why not plan to overshoot your boil-off target and then top up if needed?  That's what I do, but I generally have a good feel for what the boil volume looks like and I fiddle with the burner (though never my boil time) and I usually hit my volumes dead nuts.  5 gal exactly yesterday (to the accuracy of my measuring spoon).

[oscarvan]

Well, if you don't consult the Mayan calendar, then you might ending up dramatically changing your hop utilization as you try to fiddle with the boil volume.  But instead of doing a kabuki dance, why not plan to overshoot your boil-off target and then top up if needed?  That's what I do, but I generally have a good feel for what the boil volume looks like and I fiddle with the burner (though never my boil time) and I usually hit my volumes dead nuts.  5 gal exactly yesterday (to the accuracy of my measuring spoon).

You measured out 5 gallons with a spoon? 

[Will's Swill]

Yeah, a teaspoon - "One million, four hundred eighty-five thousand, six hundred two; One million, four hundred eighty-five thousand, six hundred three; What, honey? Crap, lost count..." No wonder it was so accurate...   

[Will's Swill]

Thinking about this a little more...  The ambient air conditions won't make any difference actually while boiling, but in the run up to the boil and during chilling it might.  Does it take longer to get to a boil on the days where you have greater boiling loss?  Perhaps cover the pot while heating to a boil and chilling.  Watch for boilovers while heating, of course...  I sometimes heat my wort this way - I use a remote probe thermometer with an alarm set to 195F to let me know when I'm close to a boil so I can remove the lid prior to any mishaps (wort boils around 202F here).

[denny]

there is the amount the volume decreases as a result of cooling

4% is the ROT

[mainebrewer]

I've heard and used the 4% ROT.
Wondering, is this a linear thing? For example, 212 degrees to 60 degrees is a 4% loss in volume. What would the percentage loss be at, say, 100 degrees?
I know I'm over thinking this, just curious.

[hopfenundmalz]

More than you want to know.  From 32F to 212 F it is 1.0 down to .958.  4% works for me.

http://www.engineeringtoolbox.com/water-temperature-specific-gravity-d_1179.html

[mainebrewer]

Curiosity satisfied, thanks.

[kgs]

I have a wooden spoon marked in half-gallon increments with a laundry marker. the markings haven't faded in a year of brewing.

I way overboiled my first outdoor batch (a little over a year ago). since then I've become better at modulating it. but I do wish I had something on the propane line that would be the equivalent of the wooden spoon. indoors, the boil is easier to control on the range.