A hydrometer is a tool that measures the density of a liquid relative to the density of water at a specific temperature. This measurement is called specific gravity. Brewers and vintners commonly measure specific gravity, because it allows them to calculate alcohol content.
Although it's possible to calculate the specific gravity at any temperature, each hydrometer is calibrated for a specific temperature. Higher liquid temperatures will cause hydrometers to produce lower estimates of specific gravity.
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Things you need
- Graduated cylinder or similarly shaped vessel
Examine your hydrometer to determine its temperature calibration. It should indicate the calibration somewhere along the side of the scale. Hydrometers are commonly calibrated to 15.6 degrees Celsius, because this is an easy temperature to achieve.
Fill roughly three-quarters of the graduated cylinder with the liquid you intend to test. Use a thermometer to determine the temperature of the liquid, and then gently lower the hydrometer into the liquid.
Swirl the hydrometer gently to make sure that no air bubbles are clinging to the bottom. Make sure the hydrometer isn't touching the sides of the container.
Read the scale on the hydrometer from the bottom of the curved surface of the liquid, which is known as the meniscus.
If the liquid you tested was the same temperature as your hydrometer's calibration, you are finished. The reading from the hydrometer represents the liquid's specific gravity.
If the liquid's temperature was different, you will have to consult a conversion table. The Fermsoft website offers an automatic converter tool to correct for temperature differences (see Resources).
Tips and warnings
- Make sure the liquid you're testing is clean. The presence of solid particles may cause the hydrometer to give an incorrect reading.
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