What is a digital density meter?
A digital density meter (also called densimeter) uses the oscillation tube technology to measure the density or related values of a liquid sample. This measurement is done digitally, reducing operator influence considerably. Accuracy of up to 0.000005 g/cm3 can be reached, with small sample volume (approx..3mL) and short measuring time (less than a minute).
How does a digital density meter work?
Digital density meters use a U-shaped hollow glass tube, which is put into oscillation. The oscillation frequency of the tube filled with the sample is measured. 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.
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Which instruments are used to measure density of liquids?
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).
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
What is the density of air?
The density of air is 0.00120 g/cm3 at 20°C and under atmospheric pressure of 101.325 kPa ((i.e. at sea level). This atmospheric pressure changes with the weather conditions (lower pressure when rainy, or snowy weather) and with the altitude (lower pressure at high altitude than at sea level). At an elevation of 440 m above sea level for example, the atmospheric pressure (yearly mean) is 96.12 kPa only and the mean air density becomes 0.00114 g/cm3 at 20°C.
What is the density of water?
The density of water is 0.99820 g/cm3 at 20°. The density of water changes with the temperature, it increases from 0°C to 4°C (where it is nearly 1) and then decreases from 4°C to higher temperatures.
How much sample volume do you need for a digital density meter?
The typical volume of a U-shaped hollow glass tube used as a measuring cell in a digital density meter is around 1mL. For trouble free filling a bigger volume is recommended, typically 3 mL to 5 mL would be used with a syringe in order to have part of the drain tube also filled with the sample.
How much time do you need to measure a sample with a digital density meter?
This depends very much on the temperature difference between the sample and the measuring cell and on the sample type. It depends also on the required accuracy of the measurement. This measuring time can typically range from a few seconds with a portable digital density meter without temperature control to about 2 to 5 minutes with a bench top instrument with active temperature control.
How do you adjust a digital density meter?
Usually density meters are adjusted with air and water, at a specific measuring temperature. This temperature is controlled with a Peltier element, which can cool down or heat up the measuring cell to a specific temperature (e.g. 20°C). First, the frequency of the measuring cell filled with air is measured. This first frequency is assigned to the density value of air. Then the measuring cell is filled with water and the frequency of the measuring cell filled with water is measured. This second frequency is assigned to the density value of water. This two point adjustment can also be done with air and a standard, or with two standards covering different density ranges.
Which samples can be measured with digital density meters?
These density meters are designed and intended for measuring homogeneous liquids originally. In reality many other samples are measured as well with good success. If the sample viscosity can be brought down to below 36'000 mPa*s by heating at the pumping temperature (e.g. paraffine), they can even be measured fully automatically with a heated automation unit like the SC1H or SC30H. Samples with higher viscosity can be measured manually, if they can be filled into a syringe without trapping air pockets, and if the operator can then press these samples out at a controlled speed through a narrow, about 15 cm long tube. Samples which attack the glass U-tube of the measuring cell (e.g. hydrofluoric acid, liquids or pastes used for etching glass) should not be measured with such density meters.
What is the influence of viscosity on digital density measurement?
The viscosity of the samples has an impact on the density measured with a digital density meter. The higher shear force which occurs between the fluid and the tube wall results in slowing down the oscillation frequency, thus showing a higher density value. Modern digital density meters therefore have a built-in viscosity correction, which compensate this effect and show correct results.
