The Specific Gravity (SG) of a liquid sample is measured by an instrument known as a "Hydrometer". Specific Gravity is defined as the Ratio of the density of your sample at 60F (15.6C) to the density of water at that same temperature. If temperature differences are unavoidable, then correction tables can help to adjust your Hydrometer readings.

Types of Hydrometers:

• Plain Form hydrometers do not have a built-in thermometer; a seperate thermometer should be used to measure the temperature.
• Combined Form hydrometers have a thermometer built-in to the body or stem to measure the temperature of the sample.

Scales of Hydrometers:

• Specific Gravity: this scale is displayed as a ratio. These are commonly used for liquids that have a density heavier than that of water, such as Marine Aquarium water.
• Degrees Baume: this scale is displayed in degrees. These are commonly used for liquids that have a density much heavier than that of water, such as syrups.
• Salt: this scale is displayed in percent(%). These are used to measure the % saturation or % by weight of salt in a solution.
• Brix: this scale is displayed in percent(%). These are used in the sugar industry. The dgrees of the scale are equal to the percentage of sucrose by weight in a solution.
• Degrees API: this scale is selected by the American Petroleum Institute(API), the United States Bureau of Mines, and the National Institute of Standards and Technology(NIST) as the standard scale for petroleum products.
• Alcohol: this scale measures directly in percentage ethyl alcohol, by weight, in water from 0 to 100%. The proof scale indicates the proof of alcohol from 0 to 200 proof. Proof is twice the percentage of alcohol by volume in a solution.

How to read a Hydrometer:

1. Pour your sample into a smooth, clear cylinder or "Hydrometer Jar" that is dry or well rinsed with a portion of the sample.
2. Make sure your sample is thoroughly mixed before testing.
3. Immerse the Hydrometer in the liquid to a point slightly below the location where it naturally floats. Make sure the Hydrometer and liquid are at rest and free of air bubbles.
4. Measure the temperature of the sample. Ideally, the sample temperature should be equal to the temperature standard of the Hydrometer, generally 60F. If temperature differences are unavoidable, correction tables can help to adjust your Hydrometer readings.
5. Take your reading at the point where the surface of the liquid crosses the Hydrometer.