Hi,
I see a lot of discussion about non-SI metric units, and I'm not sure I agree
with the general message that non-SI units (like centimeters or hectares)
should never be used. I'm challenging you purists to convince me!
I think the non-SI units that aren't based on factors of 10 of the base units
should be completely eliminated, including
Torr & millimeters of mercury
horse power (defined in terms of kilograms of force)
calories and Calories
Also, for scientific work, I think we should all use mks units (which is SI)
and not cgs units (which are not SI). So I would dump derived units like
ergs and dynes.
However, in daily use, I think that units like cm can be handy. The reason is
that most normal humans want to use a nice round integer that is as small as
possible without losing required accuracy. Decimals are somewhat clumsy for
casual use.
For starters, the SI unit of kPa is not as well suited for barometric pressure
as the millibar. It feels a lot more "human" to say 1014 millibars than
101.4 kPa.
A second example is height. Precision at the millimeter level is completely
unwarranted. (Measure your height when you first wake up and then before
going to bed and see for yourself how meaningless that last digit is!) So I
think it's simpler and more convenient to say someone is 181 cm tall than
1810 mm.
Beverages are a third example: 2 dL, 3 dL, 4 dL, or 5 dL. Nice, clean simple
integers give a good range of beverage sizes from small to large.
Farm size is a fourth example. Farms tend to be a few hundred hectares in
size. It's convenient to buy, sell, and price land in hectares. I think you
would be crazy to suggest farmers measure, buy, and sell land area with 7
digit numbers of meters sqaured. How much should a square meter of farm land
cost? Metric prefixes don't work with square meters. Is a "mega meter
squared" the size of a small farm or larger than Texas? Finally, note that
"square kilometer" is NOT and SI unit.
John
