A hydrometer measures the specific gravity (SG) or weight of the liquid compared to water. It is a glass instrument with a long thin neck and a bulbous bottom that is used to measure the amount of sugar in the working wine mixture, which is called “need.” The instrument floats in liquid and is read by a scale inside the thin glass tube. Typically, hydrometer readings are taken throughout the fermentation process and recorded to control the amount of sugar converted to alcohol.
- Pour enough wine or must into the jar of a room disinfected, so it is approximately 80 percent of its capacity. Make sure the liquid is clear and does not contain solids. Otherwise, this will give you a false reading. Hydrometers are calibrated to be used in liquids ranging from 59 to 60 degrees Fahrenheit.
- Set the glass hydrometer on the wine and rotate the bulb gently between your fingers. This will eliminate any air bubbles that cling to the bottom. The air bubbles will also give a false reading.
- Let the hydrometer to come to rest on itself. Read the scale at the intersection point of the surface liquids to the inner level. Observe the head on the scale and not an angle. The surface tension of the fluid can give a false reading if the scale is read at any aspect other than a linear reading. Write down this measure.
- Test the wine in small quantities every week until the correct level of fermentation is reached.
- Pure water will have a reading of 1,000. Begin reading the records as the first scale will be in the vicinity of 1.046. This corresponds to 1 pound of sugar dissolved in 1 gallon of water. Over time, the specific gravity will change as the alcohol begins to develop. The final reading will be in the area of 1090. This is the “magical” number for producers as wine is best preserved at this percent alcohol content.