This hydrometer (second from right in photo) was manufactured by the Taylor Permax Company (formerly Taylor Instrument Companies) in Rochester, New York. It carries U.S. Patent number 2035603, which was awarded to Edward N. Hurlburt, assignor to Taylor Instrument Companies, in 1936. Most hydrometers had graduated scales that were held in place with adhesive inside the instrument’s stem. Hurlburt observed that the adhesive frequently deteriorated, causing the scale to slip and resulting in inaccurate readings. His innovation involved fastening the scale into the ballast, which was anchored into the lower end of the hydrometer.
Hydrometers are the fundamental tool for measuring the alcohol potential of freshly crushed grape must, as well as the progress of a fermentation (the process whereby sugars are converted to alcohol via yeast). Made of glass, the bulb is weighted to allow it to float upright when suspended in liquid. A hydrometer will float to a different height on the measuring stick due to differences in the density of the liquid being measured. Because sugar increases the density of water, it is possible to know how much sugar is present when the density changes, i.e., when sugars turn into alcohol, during fermentation.
Grape juice at harvest contains about 25% sugar, with lesser amounts in cool climate grapes. Winemakers take frequent readings of the must (fermenting liquid) during fermentation. When the density stops changing and it is near zero, the fermentation is finished. There is always a trace of remaining sugar, in the range of 0.1 to 0.3%, but this amount has to be measured by another, more sensitive means. If the fermentation stops before the density is near zero, for instance at 3% sugar, this is considered a stuck fermentation, and a problem for the winemaker. The wine will taste sweet, which is generally not a desired outcome, so the fermentation will have to be restarted. This is generally done by adding yeast from another, active fermentation.
This hydrometer was donated by the Department of Viticulture and Enology at the University of California at Davis.
