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How To Use A Hydrometer in Winemaking

It's important for winemakers to know how to use a hydrometer, because it is used to measure the amount of sugar in must or wine. Winemakers know there is a direct correlation between the amount of sugar present and the ability to make wine. This instrument allows the winemaker to predict and adjust his/her recipe depending upon the readings taken.

This page explains what a hydrometer does, how it works, and how to use it.

What Is A Hydrometer?
A hydrometer is an inexpensive yet essential piece of test equipment used by winemakers. It is generally made of blown glass, with a weighted, bulbous bottom and a long narrow stem. The hydrometer is designed to float in liquid with the bulbous end down. A reading is taken by looking at a scale (contained in the stem) where the surface of the liquid being measured.

What Does A Hydrometer Do?
A hydrometer allows the winemaker to figure the specific gravity (the relative "weight" of a liquid compared to plain water) of wine or must. Specific gravity is also known as SG. Depending upon the readings observed, a winemaker can monitor the progress of fermentation and make immediate adjustments if necessary.

There are many reasons why a winemaker might want to use a hydrometer:

  • To measure the specific gravity (SG) of must or wine
  • To determine progress of fermentation
  • To estimate potential alcohol percentage at time of yeast pitching
  • To calculate percentage of alcohol (using "before" and "after" readings)
  • To measure the amount of sugar present in wine or must
  • To allow the winemaker to determine when fermentation is finished or should be stopped

When you purchase a hydrometer, you'll need to know:

  • The range of readings (highest to lowest), to make sure it will suit your purpose. A standard range for home brewers is 0.990 to 1.120. For example, in order to achieve a 12% wine, you'll want to start your wine at a SG of 1.090.
  • What the hydrometer measures. Some hydrometers only measure specific gravity, but most of them measure three things: SG, potential alcohol (P.A.), and sugar content.
  • The calibration temperature of the hydrometer. The most common calibration temp is 60° F.
  • How to convert the SG readings you get based on the temperature of your sample. See our winemaking calculations page to help you with this.
  • Whether sugar content is expressed in ounces per gallon (US/Imperial), or in grams of sugar per liter.
  • Whether it will fit your test jar / test cylinder.
  • If it comes with a protective case (they're pretty fragile).
Test Jar before fermentation

How A Hydrometer Works
A Practical Example

The easiest way to explain how a hydrometer works is with pictures. Take a look at Diagram 1, on the left, which represents a test jar full of must before the yeast is pitched.

Do you see how the hydrometer is floating rather high? This is because the liquid is "heavy" with all the sugar... the hydrometer is pushed up because of this.

Think back to your childhood visits to the beach... remember how it was easier to float in salt water than in fresh water? Whether you realized it or not, your body was the "hydrometer" on that summer day!

As the yeast turns the sugar into alcohol and carbon dioxide, the wine becomes lighter (alcohol weighs less per unit volume) and the hydrometer doesn't float as high as it once did. Diagram 2 (seen on the right) represents a wine that has fermented to dryness and is lighter than water.

Test jar after fermentation

Diagram 1


Diagram 2


Reading the Hydrometer

How To Use The Hydrometer

It's really pretty easy to use the hydrometer; just follow these simple steps:

  1. Sanitize the hydrometer, wine thief, and test jar.
  2. Place test cylinder on flat surface.
  3. Draw a sample of "clean" must or wine with the wine thief - avoid testing samples that contain solid particles, since this will affect the readings.
  4. Fill the test jar with enough liquid to just float the hydrometer - about 80% full.
  5. Gently lower the hydrometer into the test jar; spin the hydrometer as you release it, so no bubbles stick to the bottom of the hydrometer (this can also affect readings).
  6. Making sure the hydrometer isn't touching the sides of the test jar and is floating freely, take a reading across the bottom of the meniscus (see diagram to the left). Meniscus is a fancy word for the curved surface of the liquid.
  7. Be sure to take good records of your readings!

That's it! Pretty simple, huh?

Diagram 3
Reading The Hydrometer


Here's some other hydrometer pages we recommend: