What is the hydrometer for?
The hydrometer is one of the most important instruments for home wine and beer making.
The hydrometer provides valuable information of the following things:
- What the dissolved sugar content of the wine must / beer wort is before adding additional sugar.
- How much sugar should be added to achieve a particular alcohol strength.
- Helps ensure that too much sugar is not added.
- How the fermentation is going.
- When the wine or beer has been completely fermented.
- Information for calculating the alcohol content of finished wine or beer.
- How much sugar should be added for sweetening wine.
The floating hydrometer is used to measure specific gravity (SG) of liquids. In wine and beer this practically means the amount of dissolved sugar in the wine must or beer wort. The most commonly used hydrometer scale in home brewing is Oechsle (°Oe).
The starting point is the specific gravity of water (1000 g / liter at 20°C). This is marked on a scale differently, depending on the manufacturer. Either by reading 1.000 or 0. The scale usually ranges between 1.130 (130) and 0.985 (-15), of which the latter is 15 oechsle degrees below the specific gravity of water. Dissolved sugar, being heavier than water, raises the specific gravity.
Alcohol, being lighter than water, lowers it. 2,5 grams of sugar in 1 litre of water increases the specific gravity value by one degree oechsle (1°Oe = 2,5 g/l sugar).
One degree on the oechsle scale equals one gram of weight per litre. A liquid of 1.020 on the oechsle scale weighs 1020 grams per litre.
The hydrometer is used by letting it float freely in the liquid to be measured. Measuring is easiest in a transparent container high enough to float the hydrometer freely. A measuring cylinder, for instance. The hydrometer must float freely and may not touch the sides of the cylinder. When the hydrometer settles in the liquid, a reading is taken at the surface level of the liquid on the scale. The higher the reading, the more sugar there is in the liquid, and vice versa.
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Estimating the amount of sugar needed Example: We want to prepare 20 liters of wine with an alcohol content of 14% ABV (alcohol by volume). Our main ingredient in this wine would be 5 litres of concentrated berry juice. 17 grams of sugar per litre of water results in one percent of alcohol by volume. Thus, 20 litres of wine at 14% ABV would need 4760 grams of sugar. The formula is as follows: 20 (liters) x 17 (g sugar / liter) x 14 (alcohol%) = 4760 grams. Berry juice contains sugar of its own, as well as possible added sugar. But how much does it contain? We can calculate this by measuring the specific gravity of the juice. If the reading is, for instance, 1.060 (+60), then that means 150 grams of sugar per liter of juice (2,5 g/l x 60). And because the total volume of juice is 5 litres, there will be a total of 750 grams of sugar (5 litres x 150 grams = 750 g). When this amount is deducted from the total amount of sugar, the amount of additional sugar required is about 4 kg (4760 g - 750 g = 4010 g). With this total amount of sugar (if fully fermented) we will have a wine with an alcoholic content of about 14% ABV. Note! The original gravity (OG) of wine must should not exceed 1.120 (120) when pitching the yeast. This is to avoid overstressing the yeast. Therefore, strong wines are usually started with only 4 kg of sugar, with the remaining amount of sugar added after a few days, when the fermentation already is well underway. |
Calculating the alcoholic content of finished wine
How much sugar was there originally? And how much is there left? - When the amount of fermented sugar is known, we can calculate the alcoholic content.
Example: Our wine raw materials were estimated to contain about 750 grams of fermentable sugar, while another 4 kg was added to make a total of 4,75 kg of sugar. The amount of wine must (excluding the volume of solids) in our example was 20 liters.
Given these values, there was 237,5 grams of sugar per litre. The original gravity (OG) is thus about 1.095. The fermentation of our wine had for some reason stopped at a final gravity (FG) of 1.010 (+10). If the wine had fermented completely, the final gravity would have dropped by yet 15 degrees to 0.995 (-5).
The difference between the desired final gravity and the actual final gravity is therefore 15, which is equivalent to approximately 37,5 g per liter of unfermented sugar. The fermented amount of sugar is therefore 200 g per liter of wine (237,5 g - 37,5 g = 200 g). Since every 17 grams of sugar per litre results in 1% ABV, our wine would contain about 11,75% ABV (200 : 17 = 11,75).
The other way around:
Approximately every 7,5°Oe decrease on the specific gravity scale during fermentation increases the alcohol content by 1% ABV.
When the original gravity (OG) and the final gravity (FG) are known, the difference between these two values can be converted into alcohol content by simply dividing it by 7,5. If the difference in our wine is for example 85 (1.095 - 1.010), then the alcohol content of our wine is 11,3% ABV (85 : 7,5 = 11,3).
Please note that these two calculation methods result in slightly different values (11,75% vs. 11,3%). In home making, we rarely get 100% accurate results by calculating with these rounded numbers, but the results we get are still usually close enough..
