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When brewing from an extract kit we always seem to be low in the Original Gravity department. It's usually not a lot -- something like 1.055 instead of 1.060. What might be causing this to happen? Is there anything we can do to check and adjust before the fermentation begins?

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are you measuring your gravity at 60 degrees or adjusting the gravity if the temperature is higher? And is the volume at 5 gal? (or higher or lower?) – danafr4 Nov 19 '10 at 19:48
If it's constantly low, you should also check your hydrometer (assuming that's what you're using to measure SG) to make sure the paper scale inside hasn't slipped. – tallie Nov 24 '10 at 0:08

3 Answers

up vote 4 down vote accepted

If you're doing a concentrated boil, it's possible you're just adding too much water to the fermenter when you finally put it together. For a five gallon batch, a difference of about half a gallon of water will cause that exact difference in OG.

If you're doing a full wort boil, it could be the difference between boiling with the lid on and boiling with the lid off.

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Yes this. Or you could not be boiling long enough or vigorous enough. a lower OG indicates you have too much water for the given fermentable sugar content of the recipe. – Morgan Nov 13 '10 at 0:43
Another note, the wort will take up less volume as it cools, so if you measure it as 5 when it's boiling, it'll likely condense to 4.X after it cools. This might be the case if your OG was higher than the recipe, for example. – Morgan Nov 13 '10 at 0:48
up vote 3 down vote

The usual problem is that a brewer will do a partial boil and top up the fermenter with water. Since wort, containing sugar, is heavier than water it will sink to the bottom of the fermenter. When you take the sample, you're getting watered down wort from the top of the fermenter. When doing a partial boil, it's usually easier to compute the OG than to measure it.

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I think this explains the same issue I just had trying my first kit. The kit suggested the OG was going to be 1.042 for a 23 litre batch, but I was getting 1.034 with only 20 litres (so was actually hoping it to be little higher than the 1.042). I didn't want to mess around too much with my wort, so I just pitched the yeast and left it for a few hours. Just gone back and given it a stir and took another reading and got 1.038. I suspect I have some thick liquid stuck to the bottom or maybe I didn't dissolve all the granulated sugar. – Guy C Jan 26 at 1:36
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If you've not adjusted for the temperature of the wort then this might explain a few points difference. Check what temperature you hydrometer is calibrated to (usually 20°C/68°F) and then use this calculator - http://www.brewersfriend.com/hydrometer-temp/ - A 40°C wort would explain a 1.060 gravity being read as 1.055

Another possibility is the kit is just overstating the potential OG. My kit suggested 1.042, but I have just put my ingredients list into http://www.brewersfriend.com/allgrain-ogfg/ and got an OG calculation that was very close to what I measured.

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