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How to Read a Hydrometer at Different Temps

A hydrometer measures a liquid's specific gravity, which is the ratio of its density to the density of pure water. These instruments are used in many scientific tasks that require knowledge of the physical properties of a liquid, both in field and laboratory settings. Different hobbyists also make use of hydrometers, including home brewers and people who own saltwater aquariums.

Things You'll Need

Graduated cylinder or beaker
Thermometer
Temperature correction table

Instructions

- 1
  Prepare the liquid and hydrometer. Pour some of the liquid into a graduated cylinder or beaker tall enough to float the hydrometer. Gently place the hydrometer in the liquid and allow it to float freely. Make certain that it does not touch the walls or bottom of the container, and allow it to stop bobbing.
- 2
  Read the value at which the liquid's surface intersects the scale. You must project the level of the meniscus, where the liquid arcs along the container's edges, to the scale and may need to estimate a precise value by interpolation. Record both the raw measurement of specific gravity and the hydrometer's calibration temperature, which is printed on the scale.
- 3
  
  Take the temperature of the same liquid sample used to measure specific gravity.
  
  Measure the liquid's temperature. Place a thermometer in the same beaker of liquid used for the hydrometer reading and read the temperature from the scale. Record the temperature.
- 4
  Calculate the correction factor from the table that accompanies the hydrometer. Use linear interpolation if the exact temperature measured is not in the table. If no table is packaged with the hydrometer, find a correction table online. Be sure to use a correction table matching the hydrometer's calibration temperature.
- 5
  Add the correction factor to the specific gravity you measured. The correction factor is a negative number if the liquid is cooler than the hydrometer's calibration temperature, meaning the specific gravity is lower than the raw measurement. If the liquid is warmer, the factor will be positive.

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