Measuring the density of ultra-pure water

December 18, 2007

This release is also available in German.

For oceanography - and there in particular for the description of ocean currents - accurate measurements of the density of sea water are of great importance. For this purpose, measuring instruments are needed which reach an uncertainty of approx. 0.001 kg/m3 (relative 1 · 10-6). To be able to calibrate these measuring instruments, ultra-pure water is required as a reference fluid - the density of which can now be measured with the required accuracy over a large temperature range by means of the Magnetic Flotation Method which has been further developed at PTB.

Normally, the hydrostatic weighing method is used for measuring the density of liquids. Thereby, the density of the liquid is determined by means of Archimedes' principle from the buoyancy which is experienced by a sinker that plunges into the fluid. This method has several disadvantages which become apparent especially when water is to be measured. For this method, an open fluid tank is required into which the sinker - hanging on a wire - plunges. At the point where the wire passes through the surface of the liquid into the water, a meniscus forms which, in the case of water, is extremely difficult to be reproduced and therefore contributes significantly to the measurement uncertainty. Along the wire, a temperature gradient occurs which, too, increases the measurement uncertainty. Due to the open system, the gas content of the water is difficult to control, but it alters the density.

In order to eliminate these sources of uncertainty, an apparatus has been developed in which the wire has been replaced by a magnetic coupling. In this magnetic flotation system, a small magnet is mounted at the holder of the sinker. By means of this magnet, and with the aid of a controllable magnetic field produced by an electromagnet, the sinker is kept in a fixed position. The current needed for this purpose is a measure for the buoyancy that is experienced by the sinker. The fluid tank can almost be shut as the liquid is linked with the outside world only via a thin pipe by means of which the pressure can be regulated. In this way, it is possible to measure also with fully degassed water.

Thanks to the fact that the above-mentioned sources of uncertainty are avoided, measurements can be carried out with a repeatability standard deviation of approx. 2 · 10-7. The total measurement uncertainty of the water density measurement therefore reaches a value below 1 · 10-6. The measurements carried out with this new apparatus could confirm to a large extent the values delivered by foreign colleagues. However, there are still discrepancies in the temperature range around 4 °C, which is of great importance especially in oceanography. Therefore, there is still a great need for further research in this field.

Physikalisch-Technische Bundesanstalt (PTB)

Related Water Articles from Brightsurf:

Transport of water to mars' upper atmosphere dominates planet's water loss to space
Instead of its scarce atmospheric water being confined in Mars' lower atmosphere, a new study finds evidence that water on Mars is directly transported to the upper atmosphere, where it is converted to atomic hydrogen that escapes to space.

Water striders learn from experience how to jump up safely from water surface
Water striders jump upwards from the water surface without breaking it.

'Pregnancy test for water' delivers fast, easy results on water quality
A new platform technology can assess water safety and quality with just a single drop and a few minutes.

Something in the water
Between 2015 and 2016, Brazil suffered from an epidemic outbreak of the Zika virus, whose infections occurred throughout the country states.

Researchers create new tools to monitor water quality, measure water insecurity
A wife-husband team will present both high-tech and low-tech solutions for improving water security at this year's American Association for the Advancement of Science (AAAS) annual meeting in Seattle on Sunday, Feb.

The shape of water: What water molecules look like on the surface of materials
Water is a familiar substance that is present virtually everywhere.

Water, water everywhere -- and it's weirder than you think
Researchers at The University of Tokyo show that liquid water has 2 distinct molecular arrangements: tetrahedral and non-tetrahedral.

What's in your water?
Mixing drinking water with chlorine, the United States' most common method of disinfecting drinking water, creates previously unidentified toxic byproducts, says Carsten Prasse from Johns Hopkins University and his collaborators from the University of California, Berkeley and Switzerland.

How we transport water in our bodies inspires new water filtration method
A multidisciplinary group of engineers and scientists has discovered a new method for water filtration that could have implications for a variety of technologies, such as desalination plants, breathable and protective fabrics, and carbon capture in gas separations.

Source water key to bacterial water safety in remote Northern Australia
In the wet-dry topics of Australia, drinking water in remote communities is often sourced from groundwater bores.

Read More: Water News and Water Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to