Hola Herbert and others interested,
Thanks for the very kind words. I actually received a couple of emailed
questions on this (one naughty and one nice) from other list members so instead
of answering individually it is probably worth taking a moment to post what you
already knew if you won't mind, in order to clarify why the units in specific
gravity "don't matter for list purposes". That was probably a tiny
overstatement on my part and I probably should have just said if one is
somewhere in the neighborhood of ambient earthly conditions and comfortable
using grams and cubic centimetres (=millilitres). Here is an illustrated
example to save some further bandwidth:
water density is:
=1.000 grams per cubic centimetre
=1000 grams per cubic metre
=0.578 ounces per cubic inch
=62.4 pounds per cubic foot
but specific gravity for water is still a unitless 1.000 in humane ("somewhat
standard")conditions.
An iron meteorite has, say, has the following equivalent densities:
=7.8 grams per cubic centimetre
=7800 grams per cubic metre
=4.5 ounces per cubic inch
=486 pounds per cubic foot
but specific gravity for the iron meteorite is still a unitless 7.8 in humane
conditions.
And if you like unwieldy units of ounces and inches, you can get back to
specific gravity by dividing:
(4.5 ounces/in3) / (0.578 ounces/in3)
The units cancel and you get a not so surprising 7.8...
So it is no wonder why they are always being mixed up. In outer space ice
probably has a density much closer to 0.917, and the meteorite probably
wouldn't change much at all, so a geologist who hypothetically was floating in
a mixed meteoroid stream might do some fancy things with a bucket of creatively
crushed ice to "dunk" meteoroids in it to measure their density, so the same
iron now meteoroid, would come out to have a specific gravity of 8.5 using
water at frozen temperatures. But that is silly, the specific gravity business
is most meaningful for geologists as a quick and dirty measurement in the field
or perhaps precisely, in a comfortable laboratory, so while I don't know why it
might be needed below freezing reference temperatures, nor whether the
"definition" excludes solid ice as silly, perhaps a case could be invented to
have fun with specific gravity and g per mL being different. Like maybe
something to do with research on the formation of the dirty snowball in a
rotating granular comet. Heck that is a wild stab at it, so back to the
original point, specific gravity and g/cc (=g/mL, = kg/L) are the same for list
purposes with the one exception that choice of exotic measurement units will
not affect the specific gravity. Now all we need is to be sure the Sri Lankan
professor didn't mix up the terms as well,,,but I suppose that is the least of
the questions right now about that decimal/comma deal,,,I agree with Rob on
that one, but I checked the classified in the Sri Lankan newspaper and they
seemed to be using the US convention at least there. Makes a little
possibility that he has a nuclear powered scale to get milligrams when weighing
a 47 kg object:)
Saludos, Doug
Herbert wrote in the most friendly manner:
Doug,
yes, indeed, I wrote my message carelessly. I mixed up "density" (which has a
unit) with "specific gravity", which has, as you correctly stated, no units. I
promise to use my brain *before* I complain next time... :-)
Greetings,
Herbert
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