[USMA:32568] RE: Definition of L and dm3 , Was: Re: kilogram redefinition - density of water

[Philip S Hall]

A very good account to which I would add:

 

1. There is no prospect of the current (and the only valid) definition of the litre as one dm3 or 1000 cm3, changing in future
   http://www.bipm.org/en/si/si_brochure/si-brochure.pdf, section 4, table 6, page 105 (english version)
   
2. The issue is not that the definition of the kilogram as a litre of water has "gone out of fashion" it's to do with being able to reproduce it with sufficient accuracy to meet contemporary needs.
   
3. The present definition of the kilogram based on a (highly stable) physical prototype is the best that could be achieved to date. However the effect of very slow long term changes are starting to become significant (to scientists working at very high levels of precision) hence the need to review it. The proposed redefinition is an attempt to crack the (long standing) problem of basing it on natural constant(s) so that apparatus that can be constructed to reproduce it from a set of instrucions. This is the manner in which the second and metre are now defined.

Phil Hall

-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]On Behalf Of H. Maenen
Sent: 24 March 2005 15:59
To: U.S. Metric Association
Cc: [EMAIL PROTECTED]
Subject: [USMA:32565] Definition of L and dm3 , Was: Re: kilogram redefinition - density of water

The difference between the liter and the dm3 was only 28 parts per million; meaning that for the vast majority of applications this difference could safely be ignored. By the French law of 1793 and their later laws the definition was and remained 1 L = 1dm3.

In the beginning of the 20th century the old definition1 L = 1dm3 was changed to 1 L = 1.000028 dm3 by the BIPM, under many protests by people (among them Mr. B.A. Gould, an American co-worker at the BIPM at that time) who saw this proposed change as a blight on the integrity of the metric system. I would 100% agree with them. However, in 1964 this definition was revoked and 1 L  became equal to 1 dm3 again.  1 m3 = therefore 1000 L, and that has remained so.

It was exactly the density of water that had caused the problem with the liter and the dm3; the proposed change of the definition of the kilogram is fully justified.

1 US gallon is 231 in3; 1 ft3 = 1728 in3; dividing 1728 by 231 yields 7 US liquid gallons and 111 in3. I cannot regard this as simpler than the (no longer valid) meter - liter relationship, which, I state that again, differed only by 28 ppm and could be ignored in daily life; only in scientific measurements it had to be taken into account.

Han

 

========================================
Message date : 24-03-2005 16:29
From : "rocky the squirrel" <[EMAIL PROTECTED]>
To : "U.S. Metric Association"
Copy to :
Subject : [USMA:32562] kilogram redefinition - density of water
It is a pity that the definition of a kilogram has to
be some huge number of wavelengths or carbon atoms or
what have you.  Is the idea of it being the mass of a
cubic decimeter of water at the triple point too old
fashioned now?

Relating the units of length and mass by means of the
density of water was a beautiful plan, but now they
have only some arbitrary chunk of metal or a "close
enough" number of atoms of the right isotope.

While i am at it ;), since a liter is only "close to"
1000 cubic centimeters, even the supposedly identical
qualities of capacity and volume are not related.
This is as bad as having both gallons and dry gallons!
 Ironically, the relationship between feet and US
gallons (231 cubic inches) is simpler than the
meter-liter relationship.

rocky


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