What is a Hydrometer? How Does it Work and Where is it Used?

What is a hydrometer

Table of Contents

  1. Introduction
  2. What is a hydrometer & what does it consist of?
  3. Types of hydrometers
  4. Hydrometer scales
  5. How does a hydrometer work?
  6. Hydrometer vs hygrometer: difference
  7. Applications
  8. Conclusion



In the worlds of brewing and winemaking (and in other applications as well), various tools are used and each one has its function. In this article, you will learn what a hydrometer is, how it works, what types of hydrometers there are. We are also going to explain the difference between a hydrometer and a hygrometer so you don’t confuse these two terms. Let’s start!

What is a hydrometer & what does it consist of?

Essentially, a hydrometer is a glass bulb that is weighted and floats in a hydrometer cylinder.

What does a hydrometer measure? It is designed to measure the relative density of a liquid. Relative density (or specific gravity) is the ratio of the density of the substance to the density of a reference liquid (for example, water).

What are the elements of a hydrometer? Take a look at the diagram below.

elements of a hydrometer

You can see that a hydrometer is immersed upright in a hydrometer test jar (or cylinder) with theliquidthat is being tested.

A hydrometer consists of a sealed hollow glass bulb with a wide bottom that isweighted at the end for buoyancy. A ballast is typically lead or mercury. There is a scale on the narrow stem of the hydrometer. There are hydrometers with one scale, however, you can get dual-scale hydrometers as well. The scale of the hydrometer increases as you go down the tube. Why so? The heavier the substance you are testing, the higher your hydrometer will float.

Types of hydrometers

There are various hydrometers that are used for specific applications. For example, lactometers are used to check the purity of milk, saccharometers are used to determine the amount of sugar in a solution, etc.

A hydrometer for battery testing measures the specific gravity of the battery’s electrolyte so you know the state of your battery’s charge. There are hydrometers for fleet maintenance, automotive services, general industrial applications, and more.

A soil hydrometer is irreplaceable for soil analysis. It is used for the measurement of the particle size distribution of fine-grained soils and other purposes.

An antifreeze tester measures the ratio of antifreeze to water. Using an antifreeze tester is an easy and affordable way to check the condition of the antifreeze/coolant in the vehicle. This tester will help you prevent freeze up in winter and boil-over in summer.

There are also hydrometers with thermometers. These are 2-in-1 devices that allow you to take both the specific gravity as well as temperature readings at the same time, which is very convenient.

Hydrometer scales

There are different scales that can be used on a hydrometer. Let’s consider them.

Specific Gravity Scale

Hydrometers with the Specific Gravity scale are widely used by home wine makers. You will know about the progress of the fermentation of the wine you make. Plus, many wine recipes mention the specific gravity scale.

Proof & Tralles Scale

Hydrometers with the Proof and Tralles scale help determine the alcohol content of distilled alcohol or spirits. If there is no sugar or other dissolved substances, the specific gravity of a solution of ethanol in water can be directly correlated to the concentration of alcohol.

Brix Scale

Hydrometers with the Brix scale are irreplaceable in the food and beverage industry, in wine making, etc. These hydrometers are used for the measurement of the amount of sugar in the liquid.

Baume Scale

Hydrometers with the Baume scale are used for liquids heavier than water.

API Scale

Hydrometers with the API scale are used to measure how light or heavy a petroleum-based liquid is compared to water.

How does a hydrometer work?

Let’s consider the working principle of a hydrometer. There is a sequence of several easy steps.

1. The liquid for testing is poured into a special hydrometer cylinder.

2. The hydrometer is immersed upright into this liquid in the cylinder. Note that the hydrometer should float freely.

3. The point where the surface of the liquid touches the stem of your hydrometer correlates to the relative density.

How does a hydrometer work

4. Reading

If you work with a transparent liquid, your eyes should be positioned below the plane of the liquid’s surface and raised slowly until the ellipse-like surface appears as a straight line.

If the liquid is not clear, read from above the surface and try to estimate as accurately as you can the point where the liquid touches the scale of the hydrometer.

Operation of a hydrometer is based on the Archimedes’ principle. A solid suspended in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. Consequently, the lower the density of the substance, the farther the hydrometer will sink.

Note that the accuracy of your readings depend on several factors, let’s consider the main ones.

  • The cleanliness of the hydrometer itself, the hydrometer jar, and the liquid. For example, if your hydrometer is dirty, the dirt can interfere with the uniform rising of the liquid.
  • The temperature of the hydrometer and the liquid should be the same as the temperature of the surrounding atmosphere. It will prevent changes in density. Liquids can expand and contract with temperature changes.
  • The proper immersion of the hydrometer is also important. The hydrometer jar should have an inside diameter of approximately 1 inch larger than the outside diameter of your hydrometer.

Hydrometer vs hygrometer: difference

Hydrometer vs hygrometer

What is the difference between a hydrometer and a hygrometer? Have you ever wondered? Well, despite these two words sound and look very similar, they denote two different measuring devices.

As we have already mentioned, a hydrometer is used for the measurement of the relative density of a liquid.

A hygrometer measures the humidity (water vapor) in air, soil, or confined spaces. If the humidity level is too high, there is a risk of mold occurrence, bacteria growth, and so on. In addition, your furniture and musical instruments can be damaged. On the other hand, if the level of humidity is too low, people can suffer from dry and itchy skin, coughing, etc. With the help of a hygrometer, you can easily control the level of humidity at home or in the office, etc.

So now we have figured out the difference. Don’t mix up these two words and get the right instrument to meet your needs.


A hydrometer for alcohol testing is used by home brewers, wine makers, etc. With a hydrometer, you will know how strong your drink is. In addition, it will help you make a better drink as well.

alcohol testing

Hydrometers can help you determine the levels of salt in the water for the fish in aquariums. In addition, with this device, it is possible to know the levels of sand in the water.

fish in aquariums

Another sphere where a hydrometer is used is cooking. The device helps make sure that there is no excess salt in the food.


We described only a few possible applications. Hydrometers are also used in many other spheres.


In this article, we explained what a hydrometer is and how it works. It is an invaluable measuring instrument that is used in a wide range of applications. There are different hydrometers intended for different purposes. Therefore, it is important to know what type is the most suitable for you. In addition, we hope that now you understand the difference between a hydrometer and a hygrometer and won’t confuse these terms.

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