From winemaker magazine
THE HANDY HYDROMETER
The hydrometer is the first, and most indispensable, measuring instrument in your home lab. This is a simple device that measures the relative density - or "specific gravity," usually contracted to "SG" - of a liquid. This figure is important for determining, first, how much sugar is contained in the juice (and therefore what the alcohol yield should be) and, finally, when fermentation has reached completion. The hydrometer consists of a thin sealed glass tube with graduated values marked on it, usually in the form of a strip of paper inside the tube, held in place with a spot of glue. The tube has a bulbous, weighted bottom. This makes it float upright, but partially submerged, in a liquid (in our case, wine must). The method is to place the hydrometer in a "testing jar," which is actually a glass or plastic cylinder sealed at the bottom, containing the wine or must to be tested. Ideally, the liquid should be at 60° F when the reading is made. In rough terms, 0.001 should be subtracted from the reading for each 10° F (5° C) below 60° F (15° C), and 0.001 added for each 10° F above 60° F. Since the density of a liquid changes with temperature (density decreases as the temperature rises), the readings will be incorrect if the wine must is at a temperature different from that for which the hydrometer is calibrated. In this case, if the wine must is at 70° F (21° C), a reading of 1.085 actually means a value of 1.086. It should be noted that some hydrometers may be calibrated at a different temperature (for example, 68° F); before applying a correction factor, ensure that you know what the calibration temperature is (it should be marked on the hydrometer itself).
To obtain the reading, the hydrometer is floated (not dropped into) the liquid. It is important to handle the hydrometer with reasonable care, since it is somewhat fragile. The hydrometer is then given a couple of quick twists between the fingers to dislodge any bubbles (which would otherwise affect its buoyancy, again leading to inaccurate readings), and then viewed at the point where the stem breaks the surface of the liquid. The scale marked on the hydrometer will give the specific gravity. It should be noted that the liquid actually forms what is called a "meniscus" at the edges, where surface tension causes it to climb slightly above the level. The reading should be made at the point where the surface is penetrated, not at the higher level of the sides.
Many hydrometers on the market, especially professional ones, use a different scale, known as Brix (or Balling in Europe), which directly measures sugar content. While the concept is identical, the scale is different. Roughly speaking, 1 degree Brix is equivalent to .004 specific gravity points, so that a Brix of 0 equals 1.000 specific gravity, while a Brix of 24 degrees is equal to 1.100 specific gravity.
When a fermentation is started, the liquid will consist primarily of water, fruit solids (grape solids in the case of grape wine), and sugars. The specific gravity reading, which should be in the range of 1.075 to 1.090, will - with readily-available tables - tell you not only how much alcohol may be produced, but by measurement on a frequent basis, how the must is progressing to completion. By definition, water is the standard, and has a specific gravity of 1.000. That of pure ethyl alcohol is 0.792, so that a "dry" wine - one containing no sugar, just water, alcohol and some dissolved solids -will have a specific gravity of less than 1.000 (typically in the range of 0.990).
Over the course of the fermentation, readings drop, showing that sugar is being converted to alcohol. When the readings stabilize, and the total drop indicates that alcohol conversion is complete, the new wine is ready to be removed from the fruit and yeast debris it has thrown. A wine thief can be a helpful device for the home winemaker during fermentation, since it is wide enough to accommodate a hydrometer. It can be used to take samples as well as to take hydrometer readings while you ferment in a glass carboy. You can keep a thief-hydrometer in a carboy with sanitizer so you only need to rinse it off before using it.
A number of tables have been produced that attempt to relate initial sugar content to final alcohol level, but there is considerable disagreement between them. The reason for this is that some are based on a "laboratory situation" of a pure sucrose solution in water, whereas grape juice also includes "non-fermentable" substances that add to the specific gravity, but not to the alcohol content. Further, calculation of alcohol production depends on total drop in specific gravity during fermentation, not just on the starting specific gravity (a juice that ferments out to a final specific gravity of 1.010 will have a lower alcohol content than one which finishes at 0.992, if both started at the same specific gravity). Some tables take account of this by showing that a fermenting must at SG 1.000 actually still has some sugar present, while others do not take this into account and show no sugar content at this level. However, consistent use of the same table will allow you to achieve consistent results, which is more important than accuracy to three decimal places.
N.G.W.B.J.
Member of 5 Towns Wine and Beer Makers Society (Yorkshire's newest)
Wine, mead and beer maker
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