Using and Calibrating your Hydrometer

[lockwood1956]

Your hydrometer will tell you how much potential alcohol your brew has before you start, when it is actively fermenting (specific gravity still reducing) and when it is dry (no sugar left...1.000 to 0.090)

To calculate the alcohol content of your wine take the starting Specific Gravity and take away the finishing Gravity
i.e. start at 1.090
finish at 0.990 (a specific gravity drop of 100)
divide 100 by 7.36 gives alcohol content of 13.58% alcohol by volume (this will give you a guide figure and it isn't necessarily 100% accurate....but near enough)

to calculate the potential Alcohol by volume take a starting Specific gravity and assume it will finish at 0.090
take that number and divide by 7.36

i.e.
start S.G. of 1.060
assumed finish at 0.990
S.G. drop of 70
divided by 7.36 = 9.51% Alcohol by volume

to make life easy, just work out the specific gravity drop and look it up on this chart

http://www.winesathome.co.uk/downloads/alchoholbyvolumechart.xls

Before using the hydrometer

• Make sure both the hydrometer and hydrometer jar are clean.
• If the liquid to be tested is not at room temperature, allow it to reach room temperature before testing.
• Pour the liquid carefully into the hydrometer jar to avoid the formation of air bubbles. Do this by pouring it slowly down the side of the jar.
• Stir the liquid gently, avoiding the formation of air bubbles.

Taking a Reading

• Carefully insert the hydrometer into the liquid, holding it at the top of the stem, spin it gently to release any CO2 stuck to the side which can affect the reading.
• Note the reading approximately, and then by pressing on the top of the stem push the hydrometer into the liquid a few millimetres and no more beyond its equilibrium position. Do not grip the stem, but allow it to rest lightly between finger and thumb. Excess liquid on the stem above the surface can affect the reading.
• Release the hydrometer; it should rise steadily and after a few oscillations settle down to its position of equilibrium.
• 
• The correct scale reading is that corresponding to the plane of intersection of the horizontal liquid surface and the stem. (see diagram) This is not the point where the surface of the liquid actually touches the hydrometer stem. Take the reading by viewing the scale through the liquid, and adjusting your line of sight until it is in the plane of the horizontal liquid surface. Do not take a reading if the hydrometer is touching the side of the hydrometer jar.
• You should test at 60 degrees F, so will need to adjust your hydrometer for temperature differences
• see here for hydrometer temperature adjustments http://www.winesathome.co.uk/forum/calculator.html

Handling the Hydrometer

• The hydrometer should never be held by the stem, except when it is being held vertically.
• When holding the stem, always hold it by the top, as finger-marks lower down can affect the accuracy of the instrument.
• Always handle with care....they break easily

[lockwood1956]

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.

[lockwood1956]

Calibrating your Hydrometer

Your hydrometer is a valuable tool in your Winemaking, but cheap hydrometers (the ones that we buy for a few pounds/dollars) are not always 100% accurate, and the paper strip inside can move, so it is always worth calibrating your hydrometer from time to time, just to be sure that your readings are accurate.

Water at 60 deg F (15.5 deg C) has a Specific Gravity (S.G.) of 1.000

so fill your test jar with water at this temperature and test with your hydrometer.
If the reading is off it will give you a figure to add or subtract from future readings.

If your hydrometer is a long way out then maybe it would be best to replace it....they are not that expensive.