The Specific Gravity scale is relative to the weight of water.
1.000 is the SG of water at
The Potential Alcohol scale is used specifically for alcoholic beverage brewing. The typical scale ranges from 0 to 20 percent. Using this scale before fermentation will tell a brewer how much sugar could potentially be consumed to create a certain percentage of alcohol in the final beverage. In order to determine a final alcohol percentage, a second reading is taken after fermentation. Subtracting the first reading by the second will give the brewer a final alcohol percentage.
The Brix Scale is based on the percentage of sugar in liquid by weight. Typical scales range from 0 to 30 and the number represents the percentage of sugar in the liquid by weight. For example, a reading of
15ºBx in a
100g sample means that there are
15g of sugar in the liquid being measured. The Plato Scale is nearly interchangeable, but instead of being said to measure sugar, it’s said to measure dissolved solids, i.e. not everything factored into ºP is considered “fermentable.”
The Sugar Scale gives a reading of ounces of sugar per gallon. For example, a reading of
15 means that there are
1 gallon of liquid.
There are conversion charts and calculators to convert to different scales for different needs. Typical glass hydrometers are calibrated with liquids at a specific temperature (old ones for brewing at
15C and newer ones at
20C), so it’s important to also take the temperature of the liquid you’re measuring. The weight of a liquid can be calculated based on different temperatures, so it isn’t necessary to try to adjust the temperature before taking a reading.