Understanding Hydrometers: What they are and How they Work

man holding the hydrometer

A hydrometer is an instrument used to measure specific gravity or density of liquids. It typically consists of a glass tube with a bulb weighted at one end and calibrated markings along the length of the tube. The use of hydrometers is crucial in many industries, including food and beverage production, automotive engineering, and scientific research.

By measuring the density of a liquid, hydrometers provide valuable information that is used to determine the desired taste, texture, and performance of products. This article will cover the working principles of hydrometers and the correct methods for utilizing them to achieve precise measurements of liquid density and specific gravity.

How Does a Hydrometer Work?

Hydrometers work based on the Principle of Buoyancy, which states that an object immersed in a fluid experiences an upward force equal to the weight of the displaced fluid. A hydrometer typically consists of a glass tube with a large weighted glass bulb on the bottom and a scale to read the measurement.

When a hydrometer is placed in a liquid, it displaces a certain volume of the liquid and experiences an upward force equal to the weight of the displaced liquid. The reading on the scale indicates the density or specific gravity of the liquid.

Density is the mass per unit volume of a substance, while specific gravity is defined as the relative density, which is the ratio of the density of a substance to the density of a reference substance, usually water. A liquid with a higher density or specific gravity will cause hydrometers to sink deeper and give a higher specific gravity reading on the scale.

Types of Hydrometers

hydrometer measures potential alcohol

There are several types of hydrometers depending on the specific application, including:

Glass Hydrometers

These are the most common type. It is made of glass with a weighted bulb at the bottom and a thin stem rising from the top with calibrated markings. They are used for measuring the density or specific gravity of liquids in a wide range of applications.

Floating Hydrometers

These hydrometers are commonly used in the petroleum industry for measuring the density of fuel, including very light oils, which are measured in a variable volume container. Floating hydrometers are often preferred over glass hydrometers because it is less likely to break or shatter in harsh environments. They are also used in other industries, such as in the production of chemicals or pharmaceuticals.


It is a medical hydrometer designed specifically to measure a urine's specific gravity. They are similar to glass hydrometers but are smaller in size and are typically made of plastic. Urinometers are commonly used in medical settings, such as hospitals and clinics, to monitor the hydration level of patients and to help diagnose certain medical conditions.

Soil Hydrometers

These are used to measure the specific gravity or density of soil suspensions, typically in the context of agriculture, civil engineering, or environmental science. This soil analysis can provide important information about soil composition, texture, and fertility. Soil Hydrometer Analysis is also used to monitor soil compaction and moisture levels, which can be important factors in agriculture and civil engineering projects.

Different Hydrometer Scales

In addition to density and specific gravity reading, hydrometers can read other scales that are used in different industries. Here are some of the other hydrometer scales:

  1. Brix Scale: The Brix scale is used to measure the sugar content of a liquid, such as in fruit juice and wines. It is measured in degrees Brix, which represents the percentage of dissolved sugars in a solution.
  2. Baumé Scale: The Baumé scale is used to measure the density of liquids heavier than water, such as acids and brines. It is measured in degrees Baumé, which is the ratio of the density of the liquid to the density of water.
  3. Potential Alcohol or Proof Scale: The Proof scale is used to measure the approximate alcohol content of distilled spirits, such as whiskey and vodka. This can be done by subtracting the post fermentation reading from the pre-fermentation reading.
  4. API gravity Scale: The API gravity scale is used in the petroleum industry to measure the density of fuel oils and other petroleum products. It is measured in degrees API, with higher values indicating lower densities and vice versa.

Applications of Hydrometers

application of a hydrometer on brewing

Some of the industries where hydrometers are commonly used include:

Brewing and Distilling: Hydrometers are used in the brewing and distilling industry to measure the sugar content and alcohol level of wort or mash, which helps to determine the quality of the final product.

Food and Beverage: In the food and beverage industry, hydrometers are used to measure the sugar content, acidity, and alcohol level of various products, including wine, beer, and fruit juices.

Automotive: In the automotive industry, hydrometers are used to measure the concentration of antifreeze in the coolant of cars and trucks. This helps to ensure that the engine is protected from freezing and overheating.

Battery: Hydrometers are commonly used to test the state of charge and health of lead-acid batteries. By measuring the specific gravity of the electrolyte, battery technicians can determine if the battery needs to be charged, maintained, or replaced.

Petroleum: Hydrometers are used in the petroleum industry to measure the density of crude oil and petroleum products. This information is crucial for determining the quality and value of the product.

Chemical: Hydrometers are used in the chemical industry to measure the density and concentration of various chemicals, including acids, alkalis, and solvents.

Agriculture: Hydrometers are used in agriculture to measure the specific gravity of soil and water, which helps farmers to determine the optimal conditions for crop growth.

Lockout/Tagout: Hydrometers are important tools in lockout tagout (LOTO) procedures as they measure the density or specific gravity of hazardous liquids. They help workers accurately determine the concentration of hazardous substances, ensuring that lockout tagout procedures are carried out safely and effectively.

How To Use a Hydrometer

how to use a hydrometer

Hydrometers are sensitive instruments that require careful handling and proper calibration to ensure accurate measurements. Follow these steps to effectively use a hydrometer:

  1. Ensure that the hydrometer is clean and free of debris or contaminants that could affect its accuracy.
  2. Fill a sample jar or container with the liquid you want to measure, making sure it's deep enough to accommodate the entire hydrometer.
  3. Gently lower the hydrometer into the liquid and release it. Avoid any sudden movements or splashing that could affect the readings.
  4. Wait for the hydrometer to stabilize, usually a few seconds. Read the specific gravity value at the surface of the liquid, where the meniscus (the curved surface of the liquid) intersects with the hydrometer scale.
  5. If the hydrometer has a temperature scale, check the temperature of the liquid and adjust the reading accordingly using the temperature correction chart that usually comes with the hydrometer.
  6. Rinse the hydrometer with distilled water after use and store it in a safe place.

Advantages and Limitations of Hydrometers

Hydrometers are a reliable and cost-effective tool for measuring liquid density. One of their key advantages is their ease of use, making them accessible to a wide range of industries. Additionally, hydrometers provide accurate readings, making them useful for quality control and testing purposes.

However, there are limitations to using hydrometers. They are sensitive to temperature and atmospheric pressure changes, which can result in inaccurate readings if not properly accounted for. Additionally, hydrometers require manual handling and interpretation, which can lead to human error. Despite these limitations, hydrometers remain a valuable tool for many applications, and their benefits often outweigh their drawbacks.

Maintaining and Storing Hydrometers

storage area for lab devices like a hydrometer

Proper maintenance and storage of hydrometers are crucial for ensuring their accuracy and longevity. Here are some ways to do that:

Proper cleaning and maintenance procedures: To maintain the accuracy of hydrometers, it is important to clean them thoroughly after each use. Rinse the hydrometer with distilled water and wipe it dry with a soft cloth. Avoid using abrasive materials or solvents that could damage the glass or markings on the hydrometer. Store hydrometers in a clean, dry place.

Importance of regular calibration and verification: Hydrometers should be calibrated and verified regularly to ensure their accuracy. Calibration involves comparing the hydrometer readings with a known standard, while verification involves checking the hydrometer against a reference standard. Both processes are essential to ensure that hydrometers provide accurate readings. Consult the manufacturer's instructions for recommended calibration and verification intervals.

Safe storage and transportation of hydrometers: Hydrometers are fragile instruments that can be easily damaged if mishandled. Store hydrometers in a protective case or box to prevent breakage. When transporting hydrometers, make sure they are securely packed and protected from any shocks or impacts.

Choosing the Right Hydrometer for Your Needs

Selecting the right hydrometer requires considering several factors, including the liquid being measured, the required accuracy of the measurement, the temperature range of the liquid, and the frequency of use. Additionally, the hydrometer's measuring range, construction material, and type of scale should be evaluated to ensure they are appropriate for the application. By carefully considering these factors, you can choose the right hydrometer for your specific needs.

The material provided in this article is for general information purposes only. It is not intended to replace professional/legal advice or substitute government regulations, industry standards, or other requirements specific to any business/activity. While we made sure to provide accurate and reliable information, we make no representation that the details or sources are up-to-date, complete or remain available. Readers should consult with an industrial safety expert, qualified professional, or attorney for any specific concerns and questions.


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Author: Herbert Post

Born in the Philadelphia area and raised in Houston by a family who was predominately employed in heavy manufacturing. Herb took a liking to factory processes and later safety compliance where he has spent the last 13 years facilitating best practices and teaching updated regulations. He is married with two children and a St Bernard named Jose. Herb is a self-described compliance geek. When he isn’t studying safety reports and regulatory interpretations he enjoys racquetball and watching his favorite football team, the Dallas Cowboys.