Density measurement
The density can be used to characterize substances, determine their quality or purity and measure concentrations in binary or quasi-binary mixtures. Density determination is also an important means of checking the identity of raw materials and pure substances.
There are different methods for measuring density
The hydrometeris inexpensive, but difficult to read with high viscosity or dark samples and very fragile. In addition, a sample volume of at least 100 ml is required, and the maximum measurement accuracy of 0.001 g/cm³ requires lengthy, precise temperature control.
The pycnometer – a glass vessel originally known as a “density vial” with a very precise and reproducible internal volume – is used for the measurement of the gravimetric density. The measurement is complex, time-consuming and requires qualified specialists.
Our easy-to-handle density meters measure using U-tube oscillators. These deliver reproducible, fast results, precise temperature control and require a maximum sample volume of 1 ml.
U-Tube Oscillator
This method takes advantage of the fact that the oscillation frequency of a body is a function of its mass. A U-shaped capillary is filled with the liquid sample and piezoelectric or magnetic oscillations are induced. The mass and thus the density of the sample can be calculated from the resulting eigenfrequency of the U-tube oscillator. Density meters using the oscillating U-tube method allow a highly accurate measurement and require a small sample volume and produces quickly results.
Density as a value parameter of a sample
Density is a characteristic parameter and describes the ratio of mass (m) to volume (V). It is specified in g/cm³ or kg/m³. Briefly saying: density indicates the spatial proximity of the molecules of a substance. The Greek letter Rho "ρ" is generally used as an abbreviation for density. For example, the density of ultrapure water at 20.00 °C = 0.998203 g/cm3.
Binary systems
Density measurement is also often used to determine the concentration of liquid mixtures. This applies strictly to binary mixtures, also known as binary systems. Comprehensive data tables of concentrations are stored in the DS7800. So it is easy to obtain a direct reading of the concentration. However, density is also one of the most important reference parameters when analyzing complex solutions such as beer or fruit juices.
How does a density meter work?
Density measurement with the U-tube oscillator method
The U-tube oscillator method is used to measure the density of liquids. The sample is filled into a U-shaped transducer, which is electronically excited to oscillate at its proper frequency. The resonant frequency of the U-tube changes depending on the density of the sample filled in. The density is therefore determined by measuring the resonant frequency. Generally speaking: the mass of the sample is proportional to the frequency.
The temperature of the sample is one of the most important influencing variables for a high-precision measurement, which is why the measuring cell must be very precisely tempered. Other influencing factors are: Properties of the liquid to be measured, such as viscosity and compressibility, homogeneity and sedimentation properties as well as specific heat and thermal conductivity.
Temperature and density
In many cases, the higher the temperature, the higher the volume and the lower the density, it decreases. If the temperature drops, the volume also decreases and the density increases. There is no effect on the mass of the substance.
Temperature is important because it influences how much space atoms take up in a molecule. Oscillations become stronger as the temperature rises, causing the atoms to move away from each other, which in turn leads to a lower density value. A change of 0.1 °C means a deviation in the measured value of 0.0001 g/cm³ to 0.0003 g/cm³.
Liquid water behaves slightly differently: the density of water is highest at 4°C and then drops again as the temperature continues to fall to 0°C.
Click here and learn everything you need to know about correct sample preparation
Good sample preparation guarantees precise density measurements
Did you know in a sample that even contains an air bubble with a diameter of 1 mm, can cause an error of 0.000052 g/cm³ in the measurement result? Very small bubbles or air packs are often difficult to recognise with the naked eye. These can occur due to insufficient wetting of the cell. Bubbles are also difficult to see in dark samples. To be sure of your results, carry out several comparative measurements and take the average value.
We also recommend daily checks of your density meter system. Ideally, this should be done by measuring a sample with a precisely known density, e.g. distilled water (0.998203 g/cm³ at 20°C) or a standard.
Sample preparation at a glance
Viscous samples
The challenge with creams, shampoos, soaps or honey is to prevent the sample from containing air bubbles when it is fed into the measuring cell. We recommend heating the sample in a closed vessel and waiting a few minutes before measuring. This reduces the viscosity of the sample and air bubbles disappear more easily.
Volatile samples
Examples of samples with a tendency to degas are winter fuel containing dissolved butane. Air bubbles are often formed as a result of sample degassing. We recommend storing such samples in a refrigerator and measuring them at lower temperatures (below room temperature), because the solubility of a gas in a liquid increases with decreasing temperature.
Samples with diluted gases
If measuring carbonated soft drinks with dissolved gases, the samples must be degassed before measurement. Otherwise the measurement results will be incorrect. We recommend stirring the samples (for several minutes) until the bubbles stop forming. Alternatively, the sample might have to pass through a paper filter.
Inhomogeneous samples/suspensions
If suspensions such as mortar or concrete (mineral suspensions), ink, wall paints are left for a while, these solid materials can settle and/or form a concentration gradient. These samples should be stirred before measuring. Please always ensure that no bubbles are added to the sample during stirring.
Complex samples and average value measurements
Complete homogenisation of samples such as ketchup or creams is often not possible. In addition, entering of air bubbles during the measurement cannot always be avoided.
In these cases, we recommend to repeat the measurements several times and then calculate an average value of the individual measurements to ensure reliable results.
Samples and measured values
Density is an important reference value
As a basic physical parameter, density is an important reference value for many other parameters and conversions in everyday laboratory work.
Density identifies substances in terms of their quality and purity. Particularly important: it enables highly accurate concentration and content measurements.
In close cooperation with industry and science, we have developed easy-to-use, robust density meters with oscillating glass measuring cells that meet high requirements in terms of accuracy, speed and required sample volume. We offer suitable density meter sets for every working method and depending on the type of sample or accuracy requirements.
Please feel free to find out more: In our table you will find an alphabetically sorted overview of typical samples and substances for your orientation. For most data, 3 decimal places of the density in g/cm³ at a reference temperature of 20 °C are recorded.
Standards and guidelines
Density measurement according to standards and guidelines
Standards in metrology describe terms, measurement methods and units of measurement. However, standards also regulate networking by standardizing interfaces, communication and secure data transmission. They enable a quality agreement for standards and tolerances. Although not legally binding, normative action is associated with a high degree of legal certainty for the user. It facilitates the selection of devices and thus ensures the company’s own processes and product quality.
Density meters are also designed and used in accordance with standards. Standards in density measurement regulate the environmental conditions and sample preparation as well as measurement tolerances, measurement conditions and properties of a device or calibration medium under the aspect of “best practice”. Those who use density meters in accordance with standardized specifications can ensure that measurements are correct and reproducible.
Note:This is a general overview of the standards. We can also discuss which specifications the different density meter models from A.KRÜSS meet in a personal consultation. Please feel free to contact us.
Typical areas of application
Density measurement is used in many industries
Based on the measured density, different parameters of samples can be calculated. In quality control, density is used to check whether a product is within defined limits or not.
In conjunction with other methods such as refractometry (determination of the refractive index), density measurement also allows precise statements to be made about quality at every stage of the production process. For example, it is possible to indirectly determine the flow characteristics of ketchup, analyse rubber mixtures of car tyres or determine asphalt mixtures.
Find out more: In our overview, we have listed various application examples with typical measuring substances. The various requirements and standards are also listed. We have also noted recommended density meters. This summary is intented to give you a guide. We can also discuss which devices are suitable in a personal consultation.
Cleaning tips
The most common reason for incorrect measurement results is incorrect cleaning
Make sure that the measuring cell does not contain any residues from previously measured samples or rinsing solutions. Deposits from previously measured products cannot always be seen. If, for example, products containing oil or grease are measured, a very thin film of oil can be deposited on the measuring cell. For this reason, immediate cleaning and subsequent control measurement after the measurement is particularly important. Depending on the sample measured, a suitable solvent must be selected for cleaning. It may make sense to clean with two different solvents in order to save time during the subsequent drying process. This can be particularly important when operating an autosampler. Replacing previous samples with a new one is also a frequently used and time-saving Application.
Cleaning: The U-tube oscillator is flushed with the appropriate medium using a syringe or peristaltic pump. All you have to do is press a button and the drying unit removes all liquid residues. Cleaning processes with semi-automatic or fully automatic drying can also be configured.
Tip: A test measurement can be used to determine whether the U-tube oscillator is clean and dry. Dry air at 20 °C and normal air pressure should give a measured value of approx. 0.0012 g/cm³.
Open by flipping and see what are the best cleaning agents:
For cleaning, the U-tube oscillator is flushed with the appropriate medium using a syringe or peristaltic pump. All you have to do is press a button and the drying unit removes all liquid residues. Cleaning procedures with semi-automatic or fully automatic drying can also be configured.
Tip: A test measurement can be used to determine whether the U-tube oscillator is clean and dry. Dry air at 20 °C and average air pressure (1013.25 hPa) has a measured value of 0.0012 g/cm³.
| Sample | Cleaning agent 1 | Cleaning agent 2 |
|---|---|---|
| Industry: food, beverages and spirits | ||
| Fruit juices, orange juice, | Water | Ethanol |
| Soft drinks | Water | Ethanol |
| Salad dressing, mayonnaise | White spirit | Ethanol |
| Beer, beer wort | Water, enzymatic laboratory cleaner (3 % Mucasol) | Ethanol |
| Spirits | Ethanol | – |
| Industry: chemicals, cosmetics, pharmaceutical industry | ||
| Flavors, fragrances, perfume | Ethanol, isopropanol | _ |
| Shampoo | Water | Ethanol |
| Cleaning agents, liquid soap | Water | Ethanol |
| Suntan lotion | White spirit | Ethanol |
| Polyamides, polymers | Cresol | – |
| Wax, parafine | Toluene | Xylene, white spirit |
| Tturpentine-based wood preservative | White spirit | Ethanol |
| Water-based wood preservative | Water | Ethanol |
| Industry: petrochemicals | ||
| Engine oil, lubricating oil | White spirit | Acetone |
| Brake liquid (ethylene glycol) | Xylene | _ |
| Diesel, paraffin, aviation fuel, heating oil | White spirit, petroleum ether | Acetone |
Automation of the density measurement
With our DS7000 series density meters, the entire measuring process from sample removal to cleaning and drying can be carried out semi-automatically or fully automatically and without manual intervention.
With full automation, measurements can be user-independent, overnight, or on weekends. This increases productivity and reduces costs per measurement.
In the fully automatic solution, the density meter is supplemented by an integrated sample pump and an autosampler. Individual settings for measurements can be summarized in methods via the density meter’s user interface and any number of cleaning processes and sample templates can be created. This enables the unattended measurement of up to 89 samples with small space requirement for automated density measurement.
How to Use
Perform density measurement
Density meters that measure according to the U-tube oscillator principle are characterized by their ease of use. Their use is worthwhile for just a few samples a day, but the devices are also well suited for quick routine measurements with larger numbers of samples. Application and cleaning is easy, see for yourself in our videos. Learn all about measuring with the U-tube oscillator method, with manual, semi-automatic and automatic sample supply and details on easy cleaning.
