Density measurement basics
This section gives you a first insight into the basics of density measurement. You will learn that density is a temperature and pressure-dependent substance property which is often specified with the unit kg/m3 or lb/ft3. The density value is required for determining concentration, average molecular weight and content. For finding the density of gases, it must be noted that this density depends on the respective pressure. The density of liquids depends on the temperature.
- What is density?
- What do you need density information for?
- Which measuring methods are available for determining the density?
- Comparison of density measuring methods
What is density?
of two substances of the same quantity is compared, the substance with the higher weight has the higher density.
The density ρ (rho) is defined as mass m per volume V.
The density depends on pressure and temperature
The SI unit of density is kilograms per cubic meter (kg/m³). The US unit of density is expressed in pounds per cubic foot (lb/ft³).
There are also product or industry-specific units. For example, the degree Oechsle (°Oe) or the degree Brix (°Bx). The units indicate the density or sugar content of must or sugar/water solutions.
Conversion of SI units: 1 kg/m³ = 1000 g/m³ = 0,001 g/cm³= 0,000001 kg/cm³
The degree of temperature and pressure dependence is much higher for fluids than for solids. In order to obtain a precise density indication, the associated temperature and pressure must be known, especially with fluids.
The volume and the density change with a change in temperature and/or pressure. The mass always remains the same
In order to be able to compare substances better, the density of a substance can be converted into a so-called standard density or into a specific density
The standard density of substances or mixtures of substances can be taken from so-called density tables. Examples of density tables can be found here, amongst other sources:
- Alcohol: Standard OIML R 22 “International Alcoholmetric Tables” dated 1973 (http://www.oiml.org/en)
- Sugar: Standard ICUMSA “Densimetry and Tables: Sucrose -Official; Glucose, Fructose and Invert Sugar – Official ICUMSA Method SPS-4” dated 1998 (http://www.icumsa.org)
- Water: PTB notices Wagenbreth, H.; Blanke, W.: “The Density of water in the international system of units and the international practical temperature scale” dated 1968 and Bettin H.; Blanke W.: “The density of water as a function of temperature after the introduction of the 1990 International Temperature Scale
- Gases: NIST DATABASE “NIST Reference Fluid Thermodynamic and Transport Properties Database” (http://www.nist.gov)
The specific density is a dimensionless quantity
What are density specifications required for?
The density value makes it possible to derive various parameters which allow conclusions to be drawn about the composition of a mixture or a compound.
Very often the density is used to determine the concentration of a substance in an aqueous solution. The quantitative amount of a (pure) substance in a mixture can be specified in volume percent, mass fraction or as substance quantity concentration.
In addition, the quality of a mixture of substances or a compound of substances is often determined by the mean molar mass. The mean molar mass can also be determined with the help of density and enables a characterization of natural gas, for example.
Which measuring methods are available for the determination of density?
Prof. Dr. G. Hradetzky (Merseburg University of Applied Sciences)
Prof. Dr. K.-D. Sommer (PTB Braunschweig)