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Hydrometer Wisdom: Monitoring Fermentation

As with all matters of life, there are two ways of monitoring the fermentation of your mash: the easy way and the complicated way.

If you’re a K.I.S.S. fan – not the band, but the „Keep It Simple, Stupid” philosophy – you’ll prepare the mash and just let it be. A day or two after adding the yeast, you’ll see the airlock bubble – and know the stuff’s doing its fermenting business. After 14 days, it should be about done. If it still bubbles, let it sit for another few days, or until you see no bubbling for at least a minute or two. Once there is no activity in the airlock, your mash is ready to run. This is a non-scientific method but pretty reliable in judging when fermentation is completed.

The scientific method isn’t actually that complicated either, and it will let you know that the mash has completely finished fermentation and determine its potential alcohol. What you’ll need is a beer or wine hydrometer.

The hydrometer indicates the density, or specific gravity – SG – of a liquid, compared to water. As alcohol is thinner than water, the higher the alcohol content, the deeper the float sinks. Pure water has a specific gravity of 1.000 on the hydrometer scale.

Temperature is a key factor when measuring the specific gravity of a liquid – the hydrometer should indicate the temperature it’s calibrated to, and also include an adjustment table. A standard measuring temperature is 20°C or 70 °F.

Original Gravity – OG

Measure the gravity of your mash before fermentation – and before adding the yeast. The reading will be higher than 1.000, because of the sugars present in the mash. During fermentation, these sugars will be consumed by yeast causing the density and therefore specific gravity to lower. The number will be the lowest at the end of fermentation.

Fill your hydrometer tube about 2/3 of an inch from the top with the wash/mash you wish to test. Insert the hydrometer slowly not allowing it to drop. Give the hydrometer a light spin, to remove the air bubbles that may have formed.

Read where the surface of the liquid cuts the scale of the hydrometer.

You can also predict the potential alcohol of your mash from the original gravity.

Original Gravity – Potential Alcohol       

  • 062 → 7.875%
  • 064 → 8.125%
  • 066 → 8.375%
  • 068 → 8.625
  • 070 → 8.875%
  • 072 → 9.125%
  • 074 → 9.375%
  • 076 → 9.75%
  • 078 → 10%
  • 080 → 10.25%
  • 082 → 10.5%
  • 084 → 10.75%
  • 086 → 11%
  • 088 → 11.25%
  • 090 → 11.5%
  • 092 → 11.75%
  • 094 → 12.125%
  • 096 → 12.375%
  • 098 → 12.75%
  • 100 → 13%
  • 102 → 13.25%
  • 104 → 13.5%
  • 106 → 13.875%
  • 108 → 14.125%

Final Gravity – FG

Measure the specific gravity of the mash after the airlock slows down and you’re not getting much activity. If the reading is at 1.000 or less, it is definitely done. If it’s 1.020 or higher, you may want to wait a day or two and then take another reading. Keep taking readings, if needed, until the gravity stops dropping – which means the fermentation is complete.

A good rule of thumb: if the gravity hasn’t changed over the course of three days, then the mash is done fermenting.

Final Gravity – Potential Alcohol

Using the chart above and some math, you can calculate the alcohol content of your mash after fermentation is complete.

ABV = (OG – FG) x 131

For instance, if the OG reading is 1.092 and the FG is 0.99, the math goes like this:

(1.092-.99) x 131 = 13.36% ABV

Remember, this is a rough estimate, as many factors are at play. But the science will at least keep you busy until you’re ready to get your whiskey still running.

Posted by Jason Stone on


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