3 – Scale Hydrometer

A hydrometer is a simple device that measures the Specific Gravity (SG) of a liquid. SG is a number which compares the densities of a liquid being tested to a reference liquid. The type normally used by home brewers and wine makers is graduated using water as the reference liquid, usually at a temperature of 20*C.
As sugars are added to water the density increases and therefore so does the SG. As these sugars are fermented they form ethyl alcohol which has a density less than water, and so the effect is to reduce the density of the fermenting liquid, and with it the SG. A typical range of a brewing/wine hydrometer is SG 1.120 to 0.990 (note position of the decimal point).
To find out the SG of the liquid simply immerse a clean hydrometer in the liquid with or without the help of a sample jar. Try to ensure that there are no gas bubbles attached to the hydrometer surface since they could cause the reading to be inaccurate; gently spinning the hydrometer sometimes helps. When the hydrometer has settled compare the liquid level to the scale on the hydrometer.
Note - You should check from your hydrometer instructions where your SG scale reading should be taken, relative to either the liquid level or the top meniscus level. Conventionally hydrometers will require a reading to be taken at the liquid level (as shown in the diagram below). However some hydrometers are available where the instructions state the SG reading is to be taken at the meniscus level (i.e. the opposite of what the diagram shows). If in doubt, test (calibrate) your hydrometer using water at the calibration temperature to confirm how the scale compares to the 'reality'.

Basically that's it.

On a Stevenson Reeves hydrometer commonly used by homebrewers, examples of readings are given in the picture below. However note that the example readings '1010', '1014', and '1040', are incorrectly stated and should be 1.010, 1.014, and 1.040 (again note the decimal point).

For complete accuracy a small temperature correction should be applied to a measured SG, since the densities of liquids do vary with temperature. Online calculators are available to do this, typically http://www.brewersfriend.com/hydrometer-temp/.

If you intend to apply a temperature correction you should check the calibration temperature of your hydrometer, and take this into account.

The SG measurement taken at the beginning of the fermentation is usually called the Original Gravity (OG) and that at the end Final Gravity (FG).

Tables and calculators are also available to convert SG values to potential and actual alcohol content using the OG and FG values you have taken.

Finally if you suspect that your hydrometer is not reading correctly simply immerse it in clean water at 20*C (or the calibration temperature of your hydrometer) and it should read 1.000.