John Ward,
I don't believe anything I wrote earlier contradicted what you wrote below. Specifically I do not believe I said that the litre is deprecated for non-precision use. I said that I understood it to be advisable not to use the SI prefixes with litre; for example, since 1000 L = 1 m^3, therefore the cubic metre should be used, not the kilolitre.
It is true that when coherent units are needed for calculations, the litre cannot be used and any litre values must be converted to cubic metres. But I did not intend that to mean the litre should not be used at all. It is just the litre with SI prefixes that should be avoided, whether coherence is a consideration or not.I said that there are good reasons to use the litre, in agreement with your statement below.
If you thought that something I wrote was not in agreement with your thoughts on the matter, please tell me what it was so I may correct myself.
Regards, Bill Hooper
On 2004 May 16 , at 11:11 PM, J. Ward wrote:
Let's examine more closely the BIPM recommendation regarding liters.
The problem in 1961 was that the new definition of the liter (a cubic
decimeter) differs from the 1901 definition by 28 parts per million.
Therefore, for high precision measurements made in the early 1960s there was
the chance of confusion in terms of whether the measurement was based on the
new definition or the old definition of liter. As a result, the 12th CGPM
"recommends that the name litre should not be employed to give the results of
high-accuracy volume measurements."
This statement does not imply that the use of liters be deprecated for non-precision use. Now, 44 years later, the definition of liters is unambiguous and the 28 ppm confusion is purely historical.
On the other hand, there are good reasons to use liters. For example, using
liters keeps prefixes simple and convenient. We teach people that milli is
1/1000. A liter is 1000 ml. Simple! However, a cubic meter is 1 000 000
000 times larger than a cubic millimeter. Furthermore, it is inconvenient to
use prefixes when when they are spread out a factor of 1 000 000 000 apart!
Take for example, a cubic micrometer and a cubic millimeter. "Liter" sure
rolls off the tongue better than "cubic decimeter." Finally, nearly the
whole world uses and understands liters for volume measurement, at least for
voluments smaller than a meter cubed.
John
On Saturday 15 May 2004 12:48, Bill Hooper wrote:On 2004 May 15 , at 2:09 AM, Pat Naughtin wrote:Keep in mind that Australia chose the simple conversion table:
1000 millilitres = 1 litre 1000 litres = 1 kilolitre 1000 kilolitres = 1 megalitre
for measuring volume and capacity.
Interesting! But BIPM recommends not using kilolitres or megalitres (or
even millilitres). The litre is just a special name for the cubic
decimetre (according to BIPM) and its common multiples already have
other names which conform better to SI organization.
1 kilolitre = 1 cubic metre 1 millilitre = 1 cubic centimetre
(Admittedly, 1 megalitre is not just one cubic SI length unit, but it is just 1000 cubic metres. And the cubic metre is the basic and coherent SI volume unit.)
But the main reason to avoid the litre and it's multiples is that the litre is not coherent with the other SI units. (I discussed the importance of coherence in another email some time ago.)
I think it is unfortunate that Australia promotes non-SI units like the
kilolitre and megalitre. Everything else in Australia metrication seems
to have been done so admirably.
Regards, Bill Hooper Fernandina Beach, Florida, USA ======================== SIMPLIFICATION begins with SI ========================
