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What is Density?

All You Need to Know about Density Measurement

Home حسب التطبيق Laboratory Analytical Applications تطبيقات الكثافة What is Density?

Density measurement is procedure that provides a quality parameter of both raw materials and finished products, which is used for several applications in different industries to check the concentration of components, purity, its composition and for regular quality control with density value.

For example, the density of water at 20.00 °C is known to be 0.998203 g/cm3: any deviation from this value +- tolerances would imply that the water sample contains impurities.

On this page you will gain essential knowledge about density measurement.

Quality Parameter
Ready to master density measurement for precise quality control?

What is Density?

Density is a physical parameter that provides information on the mass of a sample or body divided by its volume: in other words, how tightly a substance's molecules are packed together in space. The density it is usually represented by the Greek letter rho "r". The Latin letter "d" is also commonly used.

Have you ever wondered why some oils stays on the sea surface when there is an oil spill? The difference of densities makes some substances lay over others, see in the image some typical densities values.

density formula

1. Temperature

Did you know that a temperature change of 0.1 °C can have an impact of 0.0001 g/cm3 on the measured density?

The temperature exerts influence in the space necessary to fit atoms in a molecule in a certain space, because they vibrate faster the space between them increases.

Therefore, the higher the temperature, the lower the density, or if temperature decreases, the density becomes greater. But, the mass of the substance does not change.

Note: The relation among temperature, volume and density is not a linear function and it depends on the specific heat capacity, heat of vaporization and other factors of each substance.

The only exception to the rule above is liquid water, which reaches its density peak at 3.98 ºC and then decreases if the temperature goes up or down. This is one of the reasons why ice has a lower density than water, and floats in the glass.

formula

2. Air bubbles or impurities

A simple air bubble present in the liquid to be measured can cause a massive difference on its density, the same applies for impurities.

For instance, a glass pycnometer relies on the mass for the calculation of the density value. If there is an air bubble present or a contamination e.g. due to improper cleaning, the mass measured and displayed by the balance will be incorrect. This leads to an incorrect density value.


Learn how to choose, use and calibrate pycnometers

The same applies for a digital density meter, which relies on the U-tube measuring principle for the determination of density.

However, modern digital density meters feature an automatic BubbleCheck™ to support the user during the density measurement procedure.

Moreover, to avoid cross contamination due to improper cleaning, users can measure the density of air, which if within the limits, ensures the measuring cell is properly cleaned.

3. Air pressure of a fluid

If you are in Mexico City at 3.930 meters above the sea level, the local air pressure will be lower than in Rio de Janeiro, which is at the sea level (0 m). This means the air pressure is directly related to the altitude.

As gases and liquids are fluids, the following information must be considered:

  • Gas (e.g air), is a compressible fluid which changes its volume at different pressures.
  • Liquids (e.g. water), is considered an incompressible fluid, so its volume is constant at different pressures.

Note: Unless if extremely high pressure is applied, this is not the case for analytical density measurement purposes.

In density measurement, the air value is not used in manual techniques but only in digital density meters, this is for 2 reasons:

  • During instrument adjustment water and air value must be measured. The pressure has an effect on the air value and therefore on all measurements after this. Learn more about adjustment in the section 7 below.
  • To identify whether the measuring cell is completely clean.

In order to ensure cleanliness and support during instrument adjustment, modern digital density meters features a built in barometer to measure the local air pressure that automatically sets the reference air density value.

4. Viscosity

If a substance is viscous, is it denser than a less viscous liquid?

Viscosity describes the resistance of a liquid in flowing, informally described as the "thickness" of a fluid, and in principle it has no direct relationship with density.

However, the viscosity has influence over the density determination depending on the method used.

Method

Viscosity influence?

Comment

Pycnometer

No

But tedious sampling and cleaning, and slow temperature stabilization when using water bath.

Hydrometer

No

But tedious sampling, value reading and cleaning.

Digital Density meter

Yes

Viscous sample dampens the oscillation frequency of the vibrating U-tube


As seen in chart, as the viscosity increases, the density error also increases.

However, modern digital density meters automatically correct errors due to viscosity, ensuring maximum accuracy and fast measurements.

chart that displays density error for a respective viscosity
Expand to View

Relative density definition

Relative density is the ratio between absolute densities of two substances, where the divisor is considered the reference substance, which if not specified, it is assumed to be water at 3.98ºC and therefore has a density of 0.999972 g/cm3 or 999.972 kg/m3. As per the equation below, the relative density has no units.

formula of relative density

Substance

Relative density

Specific gravity

Air (20 ºC)

0.001199 g/cm3

0.001201 g/cm3

Water (20 ºC)

0.998203 g/cm3

1.000000 g/cm3

Specific gravity definition

Specific gravity (SG) is the ratio between the densities of a substance by exclusively the density of water. If the temperature is not specified, it is assumed to be water at 3.98ºC and therefore has a density of 0.999972 g/cm3 or 999.972 kg/m3.

formula of specific gravity

True density or absolute density definition

True density is the ratio between the mass and volume of a substance at a given pressure and temperature, corresponding to weight in vacuum.

This is the concept also used in density measurement by digital density meters.

formula

Apparent density definition

Apparent density is a property of partitioned solids, such as powders and granules, and is often used in the mining, food and chemical industries.

By definition, apparent density is the ratio between mass and volume, but it corresponds to weight in air.

apparent density formula

Substance

True or Absolute density

Apparent density

Air (20 ºC)

0.00120 g/cm3

0.00000 g/cm3

Water (20 ºC)

0.99820 g/cm3

0.99715 g/cm3

Many official density tables are still based on apparent density.

Digital density meters deliver results in different units and concentrations, check out their specifications.

Mass in vacuum True density
Mass in vacuum. True density
Mass in air. Apparent density
Mass in air. Apparent density

Step 1 – Determine your sample type / sample preparation

Before you begin your measurement, it is important to understand the necessary precautions to take depending on your sample type.

Sample type (add an image/icon?)

Tips

Viscous

  • Ensure no air bubbles are present upon introduction into the measuring cell
  • Shear forces can rise to inaccurate results, so make sure your instrument has a viscosity correction

Aggressive / toxic

Volatile

  • Samples containing dissolved organic gas (e.g. butane), show a tendency to degas, so they should be cooled down to avoid bubble generation.

Sample with dissolved gases

  • Carbonated samples, for example soft drinks must be degassed prior to measurement. To do so, stir them for several minutes until bubbling stops.
  • For samples with dissolved air, an ultra-sonic bath can be used, or the sample can be boiled for several minutes.
  • Remove CO2 or dissolved gas with a combination of air flow and stirrer with an automated density system

Non-homogeneous or with suspensions

  • Avoid solid material settle or a concentration gradient formation
  • Stir the sample well before taking a sample. Make sure that no air bubbles are introduced during stirring.
  • If you can't completely homogenize the sample, repeat the measurement several times and calculate the mean value.

Step 2 – Test Your Density Meter

Regularly verify the measurement performance of your system by measuring a sample of accurately known density (e.g. distilled water or a liquid standard), this procedure which is called test, calibration or check.

After completing the check or test, the measured density is compared to the known nominal value of the sample.

Checklist

Additionally, it is a good practice to check that the measuring cell is fully clean and dry between the analyses of different substances. It is recommended that on a daily basis you complete a test of your density meter with water (0.998203 g/cm3 at 20.00 °C).

  • Which tolerance should be applied for a test with deionized water?
  • And for organic standards?
  • What do to if the test fails?

See the answers for these questions and much more detailed information in our Density Measurement Guide

Step 3 – Prevent sampling errors

Did you know that a bubble of 2 mm diameter can cause an error of 0.0008 g/cm3 on your result?

If you are measuring with a syringe:

  • We recommend “overfilling” the cell by at least an extra 5 cm at the outlet
  • Fill the measuring cell at a slow speed and with a laminar flow (5–10 mm per second) to ensure complete wetting  of the cell walls (no trapped bubbles along the walls)

When filling automatically with a pump or autosampler:

  • Adjust the sampling speed according to the sample viscosity

Prevent bubbles

  • After filling, check if the cell is bubble-free
  • Easily identify bubbles with a live video of the measuring cell and automatic Bubblecheck™ with a digital density meter

sampling video

Step 4 – Measure and document your results

Still converting your results manually?

Looking up in or interpolating from a table is error-prone and time-consuming. In addition, hand-written results bear the risk of transcription errors. Depending on the environment, there is no guarantee that this kind of documentation fulfills regulatory requirements.

The most convenient solution is an automatic conversion using built-in tables (e.g. alcohol, Brix, temperature compensation according to API), or also user defined, which prevents reading or calculation errors and saves time.

A digital density meter allows the use of built-in conversion tables to show the result directly in the desired unit.

Your Results

Step 5 – Clean your density meter

Inappropriate cleaning is the most common source of erroneous results!

Make sure that the measuring cell does not contain any residue from previously measured samples or rinsing solutions

To prevent this, the cell should be cleaned with suitable rinsing solutions and dried – preferably after each measurement.

Some general recommendations for the rinsing solutions:

Finally, make sure to dry the cell completely with dry air. In many cases there is no need to use 2 solvents when using METTLER TOLEDO Density Meters. Thanks to the powerful DryPro pump you can save time and solvent consumption.

chart density oscillation
Expand to View

Important: Adjusting a density meter

Frequent adjustment of the instrument does not necessarily guarantee accurate results.

Any adjustment results in changes being made to the instrument’s internal settings. If an adjustment is performed incorrectly, this will then lead to incorrect measurements.

Therefore we recommend the adjustment of the density meter only if the test has failed several times repeatably.

Which temperature and substances should be used for the adjustment? How to proceed?

See the answers for these questions and much more detailed information in our Density Measurement Guide

‏تنزيل دليل قياس الكثافة، والحصول على نصائح وتلميحات لتجنب الأخطاء عند قياس كثافة السوائل
دليل قياس الكثافة - كيفية تحقيق أفضل النتائج في قياسات الكثافة اليومية

Applications

Brix Measurement

Brix is the measurement in percentage by weight of sucrose in a pure water solution. For sugars others than sucrose it is called "apparent Brix" and is always a relative value. Brix measurements are mostly done in the food industry for quality control reasons e.g. sucrose solution in beverages. Sugar syrup intended for beverages and similar products is sold by weight and sugar content. The exact determination of Brix is therefore very important for cost and quality control. Although the designation of Brix is valid strictly for solutions which are entirely sucrose, the food industry uses the measurement to refer to any sweet solids in a product. Most often High Fructose Corn Syrup (HFCS) is measured and expressed in Brix°.

What is HFCS?

High Fructose Corn Syrup (HFCS) is a liquid sweetener that is extracted from cornstarch containing mainly fructose and D-glucose. HFCS is used to replace sucrose in beverage and food industries. HFCS is available in three different fructose levels: HFCS 42%, 55%, and 90% weight percent, and is mainly used in the USA.

Alcohol Measurement

Density measurement is a reliable method for determining alcohol content. On digital density meters, the measured density is directly converted into alcohol in Vol% with the built-in table OIML R-22 (International Organization of Legal Metrology, Recommendation 22: alcohol tables). For straight-forward measurement of distillates an accurate measurement system is required, and careful sample preparation.

Applications in Beer Production

Beer is mainly made by fermenting water, malts, and hops. During the fermentation process the major part of the malt (sugar) is converted into alcohol. During the brewing process the value of sugar concentration (called wort) is one major control parameter as this will be directly relate to the end value of alcohol concentration. For the end product, the values of sugar (original wort) and alcohol are the major control values.

Applications in Wine production

The sugar in wine juice, also called must, directly affects the alcohol concentration of the final product and therefore must be monitored in order to achieve optimal results. This monitoring of sugar content, as well the alcohol concentration in the final product, can be performed using density measurements.

Oils and Paraffins

Density is a fundamental physical property that can be used in conjunction with other properties to characterize the quality of crude oils. Therefore, it is used during the exploration process to monitor the quality during production and to establish the price in trading.

The density and derived quantities such as the specific gravity and API gravity are used worldwide to characterize liquid petroleum products and to compare their quality. Many heavy oils, lubricating oils and paraffins are highly viscous or solid at room temperature. Density meters are sometimes combined with heating devices to perform these measurements more rapidly, accurately and reproducibly.

Flavors and Fragrances

The flavor and fragrance industry produces ingredients for a wide range of products. In some cases, the fragrance is the final product (e.g. perfumes), in other cases a small amount of fragrance or flavor is added to the final product (e.g. soaps, creams, snack foods). Certain raw materials, particularly natural substances, have a high price per kg, therefore, strict quality control is essential.

Techniques such as GC, GC-MS, HPLC or IR-technology are able to differentiate every single compound within a complex mixture; however, such techniques require a lot of time and highly skilled operators. Many producers therefore rely on density, among other physical values, for the quality control of raw materials, bulks, and finished products.

Battery Acid

The lead acid battery used in the automobile industry is made of plates, lead, lead oxide, and a solution of electrolyte. Specific gravity measurements are used to determine the density of the battery solution which consists of 65% water and 35% sulfuric acid. The higher the specific gravity of the battery solution, the lower the remaining charge in the battery.

Fertilizers

Nitrogen is the most important nutrient for the healthy growth of plants and is a common component of most fertilizers. Density, combined with refractometry, can be used to measure the nitrogen (N) concentration in the fertilizer solution Urea Ammonium Nitrate (UAN).

For applications, methods, and instructions in density measurement, visit our Applications Library.

معرفة الكيفية
دليل قياس الكثافة
دليل قياس الكثافة
‏تفضل بتنزيل نسخة مجانية من دليل قياس الكثافة لمعرفة كيفية تجنب الأخطاء عند قياس الكثافة، أو الجاذبية النوعية أو تركيز السوائل، وكيفية تحقيق أفضل النت...
density measurement
3 Ways to Measure Density
Hydrometers, pycnometers, and digital hydrometers are used in quality and process control to determine the density of liquids. Learn more here.
Densitometer

Excellence Density Meters

The All-Rounder Benchtop Density Meter for a Variety of Applications

EasyPlus Density Meters

Standard EasyPlus Density Meters

Affordable Density Meters for Daily Applications

portable density meter

Portable Density Meter

Fully Automatic Portable Density Meter for the Lab or the Field

What is density?

Density is the mass of a substance per unit of volume. It is used to measure the compactness of a substance, and is commonly expressed in g/cm³, or lb/ft³.

The density of a sample is expressed with the Greek letter "ρ" (the Latin letter D can also be used) and is calculated as:

ρ = m / v

For example, if you fill 500cm³ of a sample into a glass beaker, and the mass weighs 1000g, then the density would be:

ρ = 1000g/500cm³ = 2g/cm³

What is the difference between density and mass?

Mass is a measure of how much matter there is within an object or liquid while density expresses how much mass there is per a certain amount of volume.

For example, 10kg of steel and 10kg of feathers have the same mass, but different volumes therefore they have different densities.

What are some everyday examples of density?

Helium filled balloons fly because the density of helium is lower than the density of air.

Oil floats in water because it has a lower density. This property is helpful if an oil spill happens in seawater. The fact that the oil floats makes it easier to clean up.

What is the relationship between density and temperature?

The density of a sample is very much dependent on the temperature, therefore the temperature must be recorded and controlled very precisely.

The graph below shows changes in the density of water based on the temperature it is measured at.

Usually, density is measured at a reference temperature of 20°C.

The density of water at 20°C is:

Water ρ 20= 0.99821 g/cm

What is specific gravity?

Density and specific gravity are not the same. Specific gravity, or relative density, is the density of a sample divided by the density of a reference substance. The reference substance for liquids and solids is usually water at its densest (4°C), and for gasses, it is air at 25°C.

The density of water at 4°C is 1g/cm3 so for example, if a liquid sample has a specific gravity less than 1, which means that it is less dense than water and it will float.  If the sample has a specific gravity greater than 1 it is denser than water and it will sink in it.

Why can density be used to identify a sample?

What is density? Why can it be used to identify  sample?

Density can easily be used to identify a pure sample because each element has a unique density. After a measurement, the density of the sample in question can be looked up to see what it corresponds to.

How is the density of solutions measured?

What is density and how can it be measured in dissolved solutions?

Let's take a solution of ethanol in water as an example.

As shown before, at 20°C pure water has a density of ρ=0.9982 g/cm3, and pure ethanol has a density of ρ=0.9839 g/cm3 at 20°C. A solution of ethanol/water will have a density value which depends on the concentration of the solution.

Let's see what is the density of an ethanol/water solution with 20% ethanol concentration:

What is viscosity?

What is density? Are density and viscosity related?

Density and viscosity are not related. Viscosity is simply a measure of a fluids resistance to flow.

For example, honey is more viscous than saline water, but it is less dense.

The SI unit for viscosity is the pascal second (Pa·s)

Which instruments are used to measure density?

What is density and how can it be measured?

The density of a solution can be measured manually or digitally. For more detailed information on how to measure density with pycnometers, hydrometers and digital density meters please see our 3 Ways to Measure Density guide.

Manual methods like pycnometers and hydrometers are widely used for the determination of density and related values, e.g. specific gravity, alcohol%, BRIX°, API degrees, Baumé, Plato, etc. Although these methods are easy to use and quite inexpensive, they require expertise in every step of the manual operation protocols, which often affects the accuracy and reliability of the results.

A Pycnometer is a glass beaker of defined volume. It is weighed without the sample (M1), then filled with the sample and weighed again (M2). The difference between M1 and M2 divided by the volume of the beaker is the density of a sample.

A Hydrometer is a glass body which is dipped into the sample. After a short equilibration time it will float a certain level. The higher the density of the sample, the less the hydrometer will float. The level of equilibration reads the density.

A Density Kit is used together with a balance. A glass body of defined volume is weighed in air (M1), dipped into the sample and weighed again in the sample (M2). The difference between M1 and M2 (buoyancy) divided by the volume of the glass body is the density of the sample. A special holder can also be used to measure density of solids, using a reference liquid (water, ethanol or user defined).

density measurement instruments
density measurement instruments

Digital Density Meters are available as benchtop and handheld instruments. They use oscillation tube technology to very accurately measure the density of a sample in a short time. A hollow glass tube vibrates at a certain frequency. This frequency changes when the tube is filled with the sample: the higher the mass of the sample, the lower the frequency. This frequency is measured and converted to density. In addition, benchtop digital density meters use a built-in Peltier thermostat to control the temperature of the sample.

For more information on digital density meters and how they compare to manual methods, please see our Comparison of different measuring techniques

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