question about gravity?

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thebamaking

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ive had a hard time finding any good information on the importance of and different measurements of gravity. i know it is used to tell when your brew is done fermenting, but what is og and sg...is there a forum that goes over all of this i cant find it
 

Cheaton

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SG = Specific Gravity = the ratio of the density of a sample to the density of water.

Hydrometers are devices used to measure specific gravity. Distilled water at 60 degrees F has a specific gravity of 1.000. This is your calibration point.

OG = Original Gravity = The specific gravity of the wort after boiling and cooling before pitching the yeast.

FG = Final Gravity = The specific gravity of the beer when fermentation is complete.

The importance is, if you don't take gravity measurements you have no way of calculating your ABV (Alcohol by Volume). You also have no way of knowing when fermentation is complete without SG readings (the general rule is, if you have 3 successive gravity readings spread across three different days that read the same, if the gravity continues to drop fermentation has not completed).
 

Cheaton

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NP. I should also mention that you have to adjust your gravity readings for temperature. You have to add 2 points (+.002) for every 10 degrees over 60. So if your wort is at 80 degrees then you need to add +.004 to your reading. So if you read 1.042 on your hydrometer at 80 degrees your OG is 1.044.

Also, make sure you cool your wort before measuring... hot wort is a sure way to end up with a broken hydrometer ale.
 
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