A Saccharometer is a hydrometer that is calibrated to measure by weight the amount of sugar in the solution (or your wort).
Technically, you should draw off a bit of the wort into a graduated cylinder or testing jar (sanitized, of course), however, we simply spray down the hydrometer with grain alcohol to sanitize it and test in our bucket. [Note: They may be a higher risk of contamination with this method, but we have not had a problem yet.]
The saccarometer is a long, hollow glass tube that is weighted at the bottom with a rolled up piece of paper inserted along the stem to give the measurement. The saccharometer is spun gently (to dislodge any air bubbles that may interfere with the reading) and allowed to float in the liquid sample. As it comes to a floating rest, a measurement is taken at the bottom of the meniscus (the liquid clings to the edges of the sides of the saccharometer and test jar).
If using a test jar to sample with, the sample should not be returned to the batch, as contamination may occur. Contamination can cause off odors and flavors in your batch, potentially ruining the entire 5 gallon batch.
How it works:
Sugar is heavier than water. Alcohol is lighter than water.
The saccharometer will float higher in a sugar solution than in water or an alcohol solution. The more sugar there is in the unfermented wort, the more syrupy it is, and the higher the saccharometer will float.
How it functions:
1) Measuring the amount of sugars present in the wort (Starting Specific Gravity) allows you to calculate the potential alcohol of your brew.
-Fermentation is yeast acting upon sugar to turn it into alcohol (and carbon dioxide, which escapes as gas).
2) Measuring the final specific gravity to let you know when you beer is finished.
-As your reading approaches your expected final gravity, you are able to judge more reliably when your yeast is finished working. In the final days, there is not enough gas being produced to create bubbles in the fermentation lock.
How it is calibrated:
1. Specific Gravity scale
a. More exact
b. 1.000 is the weight of water, higher numbers indicate more weight (ie. sugars in solution).
c. Sometimes the number will be simplified and communicated differently : 1.000 is "zero", 1.045 is "45"
2. Balling (Brix) scale
a. Less accurate
b. More useful in measuring the brix of fermenting wine
3. Potential Alcohol scale
a. Simplifies conversion of S.G. reading to potential alcohol.
b. Estimate of the alcohol by volume of the final brew.
The temperature at which you take the reading could affect the actual reading, as saccharometers are calibrated to give the correct reading at 60 degrees F. However, the difference is slight and should not affect the outcome for homebrewers.