Measuring specific gravity (SG) using a hydrometer

- Why use a hydrometer
- How to take a hydrometer reading
- Pre-fermentation sugar required to reach certain alcohol levels
- Calculating alcohol
- Hydrometer tips
- Hydrometers reading variances under different temperatures

The hydrometer is an instrument for measuring liquid: density - weight per unit volume; or specific gravity (SG) - weight per unit volume compared with water. It was invented by William Nicholson (a hydraulic engineer, inventor, translator and scientific publicist) in 1790.

Why use a hydrometer

Winemaking and homebrewing has developed over many hundreds of years, long before the invention of the hydrometer, so why use one? The hydrometer guarantees the optimum conditions for wine fermenation and preservation (e.g. at 1.090 specific gravity (SG), an alcohol level of approx. 12.3%, wine preservation is ensured). As a result you can happily make wine without a hydrometer, however there are optimum conditions under which to make wine and store wine which are easily measureable and best guarantee your enjoyment of your cost, time and effort. Why unnecessarily take chances?

How to take a hydrometer reading

1. Ensure both the hydrometer (particularly the stem) and hydrometer jar are clean.
2. Holding it at the top of the stem, lower the hydrometer into the liquid only gently releasing it when in equilibrium (i.e. when it you can feel no gravity pull or resistance push).
3. Take an approximate reading. Now gently push the hydrometer a few millimetres further into the liquid (i.e. beyond equilibrium) and hold in position.
4. Now gently let go of the hydrometer; allow it to oscillate (very small movements around the equilibrium) afterwhich it will settle. Take note of tip 4 below and watch the meniscus carefully as this happens.
5. Now take the reading from the bottom of the meniscus (see tip 5).
6. Remove and clean with warm soapy wate, rinse and fully dry before putting away

When taking readings always bend down so your eye level is level with the level of the liquid and read from the bottom of the meniscus (i.e. you have to take the reading through the liquid, not from where liquid actually touches the hydrometer stem

Pre-fermentation sugar required to reach certain alcohol levels

Once you've taken your reading you will likely want to know how much sugar to add to reach an intended alcohol level in the wine, we've provided the table below to do just that. When calculating the sugar to be added before fermentation, it is a good thing to keep the starting SG below 1100 to avoid a stuck fermentation. Here is how both ways of computation can be performed:

* Recommeded minimum to ensure preservation

Calculating alcohol

Hydrometers cannot measure the alcohol in a finished wine. However if you measure the starting SG before adding your yeast (remembering to make a note of it!) and re-measure it once fermentation is finsihed you can calculate a largely accurate alcohol percentage in the finished wine.

Alcohol calculation formula: Alcohol percentage = (Starting SG - Final SG) / 7.36

Hydrometer tips

1. Allow liquid to be tested it to reach room temperature before testing.
2. Do not overly disturb liquids before testing, air bubbles will lead to inaccurate readings.
3. Hydrometers are fragile, handle with care. If holding by the stem always hold at the top, finger-marks can affect the accuracy of the instrument.
4. When taking a hydrometer reading, if during the oscillations in step 4 the meniscus (the liquids curved edges where it meets the hydrometer) is wrinkled or out of shape, it is likley the hydrometer or liquid surface is unclean. Remove and clean the hydrometer stem.
5. When taking readings always bend down so your eye level is level with the level of the liquid and read from the bottom of the meniscus (i.e. you have to take the reading through the liquid, not from where liquid actually touches the hydrometer stem and ensure the hydrometer is not touching the side of the hydrometer jar).
6. At 1.090 specific gravity (SG) there is an alcohol level of about 12.3% which ensures the wine's preservation.(NB: If you allow to ferment a little higher than 1.090 you can compensate for topping up your demi-john with water should you need to).

Hydrometers reading variances under different temperatures

Finally please be aware that hydrometers normally measure accurately at 60 degrees fahrenheit. Therefore if your liquid is at a significantly different temperature you need to adjust your reading to reflect this. As a rule of thumb add or subtract 0.001 for every 8 degrees fahrenheit difference to this (i.e. + 0.002 at 76 degrees). However be aware that at very low temperatures fermentation significantly slows (as yeast loses it's effectiveness) and that temperatures should not exceed 95 degrees as you will kill your yeast and therefore the wine.