Featured Research

from universities, journals, and other organizations

Measuring The Density Of Ultra-pure Water

Date:
December 24, 2007
Source:
Physikalisch-Technische Bundesanstalt
Summary:
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 a small uncertainty. 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 a method which has been further developed at PTB.

View of the magnetic flotation equipment. The sinker lies on a holder whose weight is compensated by the buoyancy of a hollow sphere and of an adjusting weight.
Credit: Image courtesy of Physikalisch-Technische Bundesanstalt

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. 0.001kg/m3 (relative 110–6). 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In order to eliminate these sources of uncertainty, an apparatus has been developed in which the wire has been replaced by a magnetic coupling.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. 210–7. The total measurement uncertainty of the water density measurement therefore reaches a value below 110–6.

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 4C, which is of great importance especially in oceanography. Therefore, there is still a great need for further research in this field.


Story Source:

The above story is based on materials provided by Physikalisch-Technische Bundesanstalt. Note: Materials may be edited for content and length.


Cite This Page:

Physikalisch-Technische Bundesanstalt. "Measuring The Density Of Ultra-pure Water." ScienceDaily. ScienceDaily, 24 December 2007. <www.sciencedaily.com/releases/2007/12/071218101202.htm>.
Physikalisch-Technische Bundesanstalt. (2007, December 24). Measuring The Density Of Ultra-pure Water. ScienceDaily. Retrieved September 18, 2014 from www.sciencedaily.com/releases/2007/12/071218101202.htm
Physikalisch-Technische Bundesanstalt. "Measuring The Density Of Ultra-pure Water." ScienceDaily. www.sciencedaily.com/releases/2007/12/071218101202.htm (accessed September 18, 2014).

Share This



More Matter & Energy News

Thursday, September 18, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Virtual Reality Headsets Unveiled at Tokyo Game Show

Virtual Reality Headsets Unveiled at Tokyo Game Show

AFP (Sep. 18, 2014) Several companies unveiled virtual reality headsets at the Tokyo Game Show, Asia's largest digital entertainment exhibition. Duration: 00:48 Video provided by AFP
Powered by NewsLook.com
Stocks Hit All-Time High as Fed Holds Steady

Stocks Hit All-Time High as Fed Holds Steady

AP (Sep. 17, 2014) The Federal Reserve signaled Wednesday that it plans to keep a key interest rate at a record low because a broad range of U.S. economic measures remain subpar. Stocks hit an all-time high on the news. (Sept. 17) Video provided by AP
Powered by NewsLook.com
Space Race Pits Bezos Vs Musk

Space Race Pits Bezos Vs Musk

Reuters - Business Video Online (Sep. 16, 2014) Amazon CEO Jeff Bezos' startup will team up with Boeing and Lockheed to develop rocket engines as Elon Musk races to have his rockets certified. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

Newsy (Sep. 16, 2014) MIT developed a robot modeled after a cheetah. It can run up to speeds of 10 mph, though researchers estimate it will eventually reach 30 mph. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins