What is a Density Meter?

Density is a characteristic property of materials and is the ratio of mass (m) to volume (V).

However, density is NOT a fundamental property of a material as it varies according to other physical factors, the most significant of which is temperature. In high precision measurement work, this is extremely critical, as a temperature change of 0.1 degC influences density as much as 0.1 to 0.3kg/m³, depending on the material.

It is essential therefore that modern density meters operate with efficient temperature control of the measurement conditions.

Main applications of Density Measurement

Density measurement is often used for determining the concentrations of mixtures of fluids. For mixtures of two substances, sometimes known as binary systems, extensive concentration look-up tables can be created to facilitate everyday measurements.

  • Drinks industry: Beer, flavourings, fruit juice, syrups, sugar, soft drinks, spirits, milk drinks.
  • Chemical industry: Acids, alkalis, salts, solvents
  • Food industry: Jams, preserves, honey, glucose or fructose syrups, jelly, mustard, ketchup, sauces, soups, convenience products, mayonnaise, ice cream, baby food, confectionery production, milk products.
  • Automotive industry: Oils, lubricants, battery acid, antifreeze.
  • Metalworking: Cooling lubricants and their emulsion solutions with water.
  • Petrochemicals: Oils, lubricants, quality control of fuels and additives.
  • Cosmetics and pharmaceuticals: Creams, ointments, pastes, emulsions, lotions, beauty products, perfumes, aromas, solvents, cleaners, shampoos, soaps, infusion solutions, urine, quality control of liquid medications and agents.
  • Wine-growing: Grape juice, cider, wine, champagne, liqueur.
  • Sugar industry: purity of sugar and syrup concentrates.

How Does a Density Meter Work?

These are the alternative procedures commonly in use in industry for measuring density:

Simple buoyancy measurement

A glass body of defined volume is weighed in air, dipped into the sample and weighed again in the sample. The difference divided by the volume of the glass body is the density of the sample. The method can be used for viscous and porous materials, while a special holder can also be used to measure solids, using a reference liquid.

  • No manual reading
  • GLP printout
  • Relatively inexpensive
  • Disadvantage: Long and difficult setup
  • Disadvantage: Large sample volume required

Hydrometer (also known as an aerometer)

This is a glass body which is simply dipped into the sample. The device works on the Archimedean principle of buoyancy in proportion to mass - after a short equilibration time it will float at a certain level. The higher the density of the sample, the less the hydrometer will sink. A calibrated scale links the level to the density. The method is best suited for rough calculations or where the operating range is narrow, such as alcoholic drinks.

  • Low-cost, simple instrument
  • Special instruments available with direct readings in alcohol%, sugar% (BRIX) or other density related values
  • Disadvantage: Breakable glassware
  • Disadvantage: Small measuring range - may need many units to cover required range
  • Disadvantage: Not suitable for GLP (no measurement protocol).
  • Disadvantage: Difficult to read, particularly in highly viscose or dark samples
  • Disadvantage: High degree of precision (maximum 0.001g/cm³) requires precise temperature regulation

Pycnometer

Provides a gravimetric determination of density. Used on education, analytical labs where GLP is not required, and production control.

Features include:

  • Higher degree of precision than the hydrometer.
  • Disadvantage: Breakable glassware
  • Disadvantage: Difficult to clean and dry
  • Disadvantage: Not suitable for GLP (no measurement protocol).
  • Disadvantage: Large sample volumes of at least 100ml required, so unsuitable for expensive samples.
  • Disadvantage: Complex weighing procedures and calculations make measurement slow (hours) and require trained personnel.

Digital measurement system - Oscillating U-tube

Digital density measurement instruments are principally used for quality control of final products.

A digital density meter can be connected to a PC or linked to a network via an Ethernet interface. If there is internet access, remote maintenance and fault diagnosis are also possible.

Optional security functionality can protect the settings from being changed unintentionally.

The instrument can be controlled from a PC, where more historical records may be stored than on the instrument itself.

The oscillating U-tube principle is the most common digital measurement method. It works in a similar way to a tuning fork - applying a mechanical impulse generates a sound with a characteristic pitch. This sound is the result of the oscillation created by deflecting the prongs of the tuning fork. The pitch of the note obtained - and thus the oscillating frequency – depends on the mass of the tuning fork.

Extremely fine capillaries are made to oscillate by a piezoelectric or magnetic transducer with a characteristic frequency. The resulting resonant frequency of the U-tube will depend on the mass of the filled sample.

  • Easy to use
  • Good repeatability - physical relationship of oscillation frequency to density is very simple and linear.
  • High precision - frequency can be measured very accurately and used to calculate the density of the sample. Accuracies are possible up to five decimal places for measuring density, relative density and concentration of aqueous solutions in a range of 0.0000 – 1.9999g/cm³.
  • Fast (minutes) at a defined, regulated temperature - The device has a high-precision recirculation air temperature control, based on a Peltier element.
  • Device calibration is normally possible with just two standards – dry air and pure water.
  • Tubing material optionally available in a chemical resistant materials
  • Injection filling possible in some versions for higher viscosity specimens.
  • Can be automated or extended to measure other parameters simultaneously.
  • Built-in Standard Operating Procedure (SOP)
  • Disadvantage: Most expensive method of density measurement

Glossary of Terms

ASTM Standard D4052 - The Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter. Gordon J. Hookey, US National Transportation Safety Board

ASTM D5002 - 99(2010) - Standard Test Method for Density and Relative Density of Crude Oils by Digital Density Analyser

Density - a characteristic property of materials. It is the ratio of mass m to volume V.

D=m/V - Typical metric units include g/cm³, kg/m³.

Frequency (or temporal frequency) - the number of occurrences of a repeating event (cycle) per unit time. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency.

GLP (Good Laboratory Practice) is a quality system concerned with the organisational processes and conditions under which non-clinical health and environmental safety studies are planned, performed, monitored, recorded, archived and reported. An internationally recognised definition of GLP can be found on the website for the Medicines and Healthcare products Regulatory Agency-UK.

Peltier effect – the cooling of one junction and the heating of the other when electric current is maintained in a circuit of material consisting of two dissimilar conductors; the effect is even stronger in circuits containing dissimilar semiconductors. Hence a Peltier element usually refers to an electronic device consisting of metal strips between which are connected alternate strips of n-type and p-type semiconductors.

Period – see Frequency

Standard Operating Procedure (SOP) - detailed, written instructions to achieve uniformity of the performance of a specific function. SOPs are often applied in pharmaceutical processing and for related clinical studies.

What does a modern Digital Density Meter look like?

The DS7800 from German manufacturer A.KRÜSS Optronic is perhaps the leading instrument in the world today, offering user-friendly measurement through oscillating U-tube technology. Click on the brochure (left) to read more about it.