Clarify why you say the litre is not part of SI.
Because the official declaration of the international bodies that have the authority to make such decisions has said so.
Why did they do it? Because the litre is not coherent with the rest of the SI units. The SI volume unit that is coherent is the cubic metre. The SI is a system of measurment set up with the explicit goal of having all the units coherent with each other.
Whether a unit is or is not coherent with others is a critically important characteristic. It may be of real importance only in technical work, but that does not make it less important. However, it does mean that for common use, one ordinarily does not have to worry about it. That is why litres are permitted; because they are a convenient size for measuring volumes. However, anyone doing work in thermodynamics, for example, would need to know that in calculating work done by an expanding gas (e.g. in the cylinder of an automobile engine), the increase in volume needs to be expressed in cubic metres, not in litres.
If this does not make sense to you or if you don't understand the argument, then you need to find out what coherent means.
Has it (the litre) been replaced with the
cubic decimeter because of the slight difference between the two (as detected
by precise modern equipment) ...
No, because there is no difference between the litre and the cubic decimetre. The SI definition of the litre is that it is just a special name given to the cubic decimetre. It has nothing to do with the precision of any measurements.
... or is it because cubic decimeters and cubic meters make the litre unneeded?
It is not because the cubic metre makes the litre unnecessary, it is because the cubic metre is coherent and the litre is not.
As an aside, I need to point out that the centimetre is not coherent either. Technically it is called "a submultiple" of the SI unit. Only the metre is the coherent unit. Thus also, the cubic metre and not the cubic centimetre is the coherent unit. Only the unprefixed unit is the coherent one. This is true for ALL SI units (with one exception). The multiples and submultiple are available for convenience in expressing larger and smaller sizes but only the unprefixed units should be used in calculations because only they are the coherent ones.
(Mass units are the only exception. There still is only one coherent unit but it is the kilogram rather than the gram. The multiples and submultiples, however, are formed by adding prefixes to the name "gram". This is done for historical reasons; the kilogram had long been used as a coherent unit in much of what was called metric before SI was created. SI tried to keep as much of the older systems as possible in order to avoid drastic changes in then existing measurements. How much this exception bothers you or me is a personal thing.)
Gavin continues:
Likewise why is the hectare not to be used?
Same reason. it's not coherent.
The hectare is listed in the official SI publications in a table headed "Other non-SI units currently accepted for use with the international System". The only difference between this table and the one in which the litre is listed is that the table containing the hectare carries the explanatory note that the use of these units "is not encouraged", a note that does not appear in the table where the litre is listed. There is also a footnote indicating it is to be used only for land area.
Are we supposed to say 10,000 square meters ...
Yes, or maybe 0.01 square kilometres. In this example, 10 000 m^2 would be more convenient than 0.01 km^2. But for a much larger areas, say 25 000 000 square metres, one would say 25 km^2. (Or one could say 2500 hectares.)
... or (are we supposed to say) 100 ares instead of hectare?
No! The "are" is listed in he same SI table as the hectare; namely, "non-SI unit", "currently accepted", "not encouraged" and only "for land area".
Regards, Bill Hooper
============================================= Below is my earlier note which raised this question for Gavin.
Quoting Bill Hooper <[EMAIL PROTECTED]>:
PS Some other units that you may be surprised to learn are NOT units of
the SI system are:
the litre
the tonne (metric ton)
the hectare
the bar (and, by inference, the millibar)
the calorie and
the micron.
