Using a Hydrometer

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One of the most frequent requests we receive is for help with fermentations that may have stuck.

We will always ask for the hydrometer reading, and most of the time the reading indicates that the wine or beer has finished fermenting and is ready for the next stage.

We realise that hydrometers may appear difficult to use, so hope this guide will help.

What is a Hydrometer?

Youngs Hydrometer and Case

A hydrometer is a glass instrument weighted at one end, with a calibrated scale and designed to float in liquids.

What is a Hydrometer used for?

It is used to measure the specific gravity of a liquid. In simple terms how thick or thin a liquid is compared to water. (Water has a reading of 1.000 on a hydrometer.)

What do home brewers use a Hydrometer for?

A hydrometer is used to measure how much sugar is in the liquid we are fermenting. (“MUST” in winemaking or “WORT” in ale brewing.)

At the start of fermentation before the yeast is added (“PITCHING” is the term used for adding yeast), the hydrometer will float high in the must/wort. (A typical value for wine is 1.090 for ale 1.050.)

TIP: When making wine from fruit or vegetables, i.e. anything other than a kit, this reading will help you to decide how much sugar may need to be added.

As the fermentation progresses the hydrometer will float lower and lower in the must/wort until fermentation has stopped. (A typical value for wine is 0.996; for ale 1.008.)

Using a Hydrometer

  • First clean and sterilise all equipment used to obtain a sample of the must/wort.
  • Use a wine thief or pipette to obtain a sample of must/wort and fill a trial jar to within approximately 50mm (2 inches) from the top.
  • Carefully lower a sterilised hydrometer into the liquid, making sure it is not touching the sides.
  • Give it a spin to remove any gas bubbles adhering to hydrometer.
  • Read the hydrometer where the level of the liquid intersects the scale on the hydrometer.

It is not wise to repeatedly take hydrometer readings throughout a fermentation, to avoid the risk of contamination. Take a reading at the beginning of fermentation (just before the yeast is added) and one or two at the end of fermentation.

TIP: If it takes more than one and a half minutes for bubbles to go through the airlock/bubbler then the fermentation has most likely ended.

Armed with the reading at start of fermentation (the original gravity OG) and the reading at the finish of fermentation (the final gravity FG), it is possible to calculate the alcohol content (alcohol by volume % ABV) of the finished wine or ale using the following sum.

Original gravity OG, minus final gravity FG multiplied by 131.25.

For Example, these are some typical figures for the start and finish of a wine.

OG = 1.090, FG = 0.994

1.090 – 0.994 = 0.096 x 131.25 = approximately 12.6% ABV.