Mon Feb 21, 2011 11:55 am
Generally getting a keg for deposit price requires theft. Which is to say, kegs are fairly expensive, and reneging on the deposit for a regular keg costs breweries money.
As disasterbrew said, each setup has a different boiloff rate. I have found that mine also changes pretty considerably with local humidity. When it's dry, I lose a fair bit more in the boiloff. That might be magical / anecdotal thinking on my part, though. It's perfectly all right to add some water back into the pot at the end of the boil if you've overshot your target volume. Just make sure the water you're adding is sterilized- either add it 10 minutes before the end of the boil, or boil and cool it separately and add it when you put everything into the fermenter.
You can pretty much do "points math" when you're figuring out a water addition. If you're targeting 1.040, and you have 4.5 gallons of 1.050, then you've got 4.5 * 50 = 225 points. You want 225 / 40 = 5.6 gallons of wort to get to 1.040, so add 1.1 gallons of water.
Finally, remember that your hydrometer will be calibrated to work at either 60F or 68F, depending on the brand. If you measure hotter wort, it will read low, so you'll need to find out the temperature adjustment for your hydrometer. The adjustment table is usually on the hydrometer itself or in the packaging. There's online calculators too.
