Thu Dec 18, 2008 8:11 am
This is actually a pretty simple computation. Even I can do it.
Most dry malt extract will add 45 pppg. PPPG = Points Per Pound per Gallon. In other words, if you add one pound of DME to enough water to make one gallon of wort, you will end up with one gallon of 1.045 wort.
If you are making 5 gallons of wort, one pound of DME will add 9 points of gravity (45 divided by 5 gallons).
As an example, let's say you are making 5 gallons of beer with an expected OG of 1.045 but you only end up with 1.040. This means you are short 5 points. To bring it back up to 1.045 you will need to add 5/9th of a pound of DME. Being lazy I would just make it an even half pound and settle for 1.046. You won't notice a difference of 0.001 anyway.
The big thing you need to be careful of however is making sure you really are short on gravity. The gravity reading with a hydrometer will vary widely based on the temperature of the wort. Hydrometers are generally calibrated to be accurate at 60° F. At higher temperatures the wort is less dense and the hydrometer will sink lower, giving you a low reading. When you check the gravity, be sure you are correcting for temperature.
Another common problem with extract brewers is stratification of wort, especially if you are boiling less than the full volume and adding water to make up your final volume. The heavier wort will tend to sink towards the bottom, leaving diluted wort towards the top. It takes a LOT more stirring than you might think to get it mixed completely. If you are doing an all extract batch, don't even worry about pre fermentation gravity readings. You will have all the sugars in there that you are supposed to have. The only variation will be if you have less or more volume going into the fermenter than what you were supposed to have.
This exercise in adding DME to fix low gravity is mostly done when doing all grain beers where you have gotten lower than expected mash efficiency.
Hope this helps you understand this topic a little better.
Wayne
