The Mighty Hydrometer

by Connor Creighton May 31, 2019 4 min read

The Mighty Hydrometer

Several times a week we will have a customer come into the store to troubleshoot their fermentation problem. It might be that their wine is not fermenting, or their beer doesn’t seem to want to stop fermenting. Other times it is that their cider tastes thin and acidic. The first question I always ask is “What does your hydrometer say?” Nine times out of ten I get a quizzical look in return with the response “errr, I don’t know”.

The hydrometer is a lovely tool. It is not the instrument that will fix a fermentation gone wrong, instead it is the first step to diagnosing the problem.

What IS a Hydrometer

A hydrometer is a long weighted glass tube. It kind of looks like a thermometer. More importantly, it is used to measure density or specific gravity (s.g.). Specific gravity is a very important term in all wine, beer, cider, kombucha, mead, etc brewing. In the case of fermentation, specific gravity tells you the amount of fermentable sugar in the liquid you are fermenting.

Why is it Important?

The beauty of a hydrometer is that it can in a very short period of time tell you a lot about your fermentation, potential alcohol percentage, and can reveal red flags.

Finding the Alcohol Percentage

The biggest application for a hydrometer is to use it at the beginning and end of a fermentation. If you put both of those specific gravity numbers into a s.g. calculator it can tell you the alcohol percentage of what you made. Typically, most wine yeasts will finish below a reading of 1.000, and most beers will finish anywhere from 1.000 – 1.020. Most yeast strains are fairly predictable. Meaning, you can figure out how much alcohol you will probably have by just measuring the initial specific gravity, this is often called the original gravity (o.g.). Right before adding the yeast to whatever you are making, take a hydrometer reading first and write it down! Because trust me, it is easy to forget what the o.g. was 10 days later. Especially if you enjoyed a couple cold ones while brewing.

Low Gravity Reading?

If your reading is low, it is really easy to fix before you add yeast. For wines, sugar can be added to boost gravity. You don’t want to add too much, but it is a viable option to increase the S.G. We typically recommend dextrose for this. Start with a small amount (500g/20L) and mix with boiling water. Add to your wort/must and take another reading. Add more as needed.

For beer, this is a very similar process. It is very easy to finish a brew day and be way under your efficiency target for a variety of reasons. In the past I have mixed in 1lb of DME with boiling water and added it to my wort. It tends to bump up your s.g. by about 5 points.

These sorts of remedies are possible only if you take that first hydrometer reading. You don’t want to be in the dark about your potential alcohol percentage!

Troubleshooting Your Fermentation

A lot of homebrewers make the mistake of using visual cues to figure out if their fermentation is healthy, or complete. While visual cues can be helpful, the hydrometer is a 100% objective way to know the status of your fermentation. Countless times I have had homebrewers come into our shop and say that their wine stopped fermenting and they could not do anything to fix it. I respond with, what is the specific gravity? -> They don’t know -> We measure it and it turns out it was completely finished fermenting!

Other times, a hydrometer will diagnose a stuck fermentation. In this instance, the fermentation did not complete even though it has been 10-30 days. Without using a hydrometer brewers may believe its finished fermenting, when instead there is a bunch of unfermented sugar. This can lead to all sorts of issues going forward. A stuck fermentation can be restarted though! We’ve resuscitated many stuck fermentations here at KJ over the years, but we would never have known they were stuck unless we used our hydrometer.

How does it work?

A large part of the utility of the hydrometer is that it is really easy to use and quick to give results. Simply submerge the hydrometer in the liquid you want to measure and it will float at the specific gravity level. You can submerge it right in the fermentation pail, or liquid can be removed into a hydrometer tube. We don’t recommend tossing the hydrometer inside a carboy as it will be difficult to get out! A wine thief works great at extracting liquid from a carboy for a measurement.

Important Tips

  • Temperature can sway a reading quite a bit. The warmer the liquid the lower the specific gravity reading will be given. Whereas the colder the liquid the higher the density will be given. The ideal temperature for taking a reading is 20c. For brewing, don’t use your hydrometer until the temperature has been brought down. If you need to take a reading when it is at a higher temperature use a refractometer.
  • Pre-calibrate with distilled water at 20c. It should read 1.000. Calibrate yearly.
  • Always inspect your hydrometer so that there are no fractures or crack. They are very fragile! (They especially like to roll off tables and shatter.)
  • Read below the meniscus.
  • Make sure the hydrometer is completely free floating, not touching the sides. (An astute observer would notice in 2/3 of our photos the hydrometer is touching the side)

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