Hi Bill et al
Your points are well taken. Science uses cubic meter instead of kL which is
fine and cubic meter still could be used everywhere.
However, volumes for barrels and other containers in the public market place
usually are expressed in the US in terms of gallons with the liter
equivalent in parens or vice versa. I thought it would be useful to help
the public correlate large volumes with things it encounters in stores. I
have not seen volume expressed in cubic meters in stores but I've seen it
expressed in liters.
My wording could be improved but the key message is to understand that kL
= m^3 and that the force of moving air and water can be a safety concern.
I would not deviate from expressing densities in kg/m^3. However, I would
try to accommodate the public by using L and kL for volumes in the
marketplace. Also, think of throwing a liter bottle of water (1 kg) at
someone. That's like a m^3 (kL) of air (approximately) coming at you in a
more dense form. Air density decreases with height and not all of us live
at sea level at 60 degrees F. That's the reason for using the words "about"
and "approximately" for air in public discourse .
Stan Doore
----- Original Message -----
From: "Bill Hooper" <[EMAIL PROTECTED]>
To: "U.S. Metric Association" <[email protected]>
Sent: Saturday, May 05, 2007 7:58 PM
Subject: [USMA:38595] Re: People oriented metric use
On 2007 May 5 , at 8:57 AM, STANLEY DOORE wrote:
1 kg/kL of air at the surface is about one kilogram (2.2 pounds) per
kilolitre (cubic metre)
1 t/kL of water has a mass of one metric tonne (1000 kilograms or 2200
pounds) per kilolitre (cubic metre)
These are good suggestions for ways to use SI metric to provide
useful information. There are a couple ways (two) it might be
improved or usefully extended.
1. The phrasing of the two lines seems awkward and even confusing.
May I suggest the following modifications (which change only the
sentence structure, not the data):
"Air has a density of about 1 kg/kL at the surface of the earth; that
is, one kilolitre (cubic metre) of air has a mass of about one
kilogram (2.2 pounds)."
and
"Water has a density of 1 t/kL; that is, one kilolitre (cubic metre)
of water has a mass of one metric tonne (1000 kilograms or 2200
pounds)."
2. However, I'm not sure that the data itself is expressed in the
most convenient way either. This takes a bit more lengthy discussion
so skip to the end if you just want my final conclusion.
lengthy discussion
==========================
I would suggest that air density be given as 1 kg/m^3 (rather than 1
kg/kL).** Why bother with kilolitres (or litres, for that matter)
when the basic SI unit of volume is of an appropriate size. Litres
were created to bridge the extreme gap between cubic metres and cubic
centimetres (or cubic millimetres). When the information of interest
is not in that gap, why use litres at all. The cubic metre is the
basic SI unit of volume; use it.
I would further suggest that the density of water be given as 1 kg/L
(rather than 1 t/kL). Again, it is simpler and yet, if necessary, it
is still easy to see from this that 1000 kg (1 tonne) would occupy a
space of 1000 L. The units of mass in SI are the kilogram and all its
multiples and submultiples*, so why interject the non-SI unit "tonne"
into the discussion at all. In addition, this allows us to avoid
kilolitres in accord with the above comments regarding the kilolitre.
If for whatever reason it would be appropriate in a particular
discussion to refer to larger volumes like the kilolitre, is is more
appropriate to call it a cubic metre. Even then, the larger mass in a
cubic metre (1000 kg) may be expressed in SI units of megagrams
(since 1 Mg = 1000 kg). This also allows us to avoid the non-SI tonne.
Finally, I would omit the reference to pounds entirely. For audiences
who might be unfamiliar with metric masses, the information than 1 kg
is about 2.2 lbs might be given, but should not clutter up the basic
ideas by mixing the metric information with the conversion-to-Ye-Olde-
English information. (And, once noted that 1 kg is about 2.2 lbs, it
should not be necessary to point out that 1000 kg is about 2200 lbs.)
end of lengthy discussion
==========================
Let me then rephrase the original information once more so it would
look like this:
"Air has a density of about 1 kg/m^3 at the surface of the earth;
that is, one cubic metre of air has a mass of about one kilogram."
and
"Water has a density of 1 kg/L; that is, one litre of water has a
mass of one kilogram."
I am sure there are other ways of presenting this information using
SI, any one of which may be as good as or better than what I'm
suggesting. My main point, I think, is to avoid non-SI units as much
as possible. Litres are a useful exception to this thought when that
is in the appropriate range of discussion, but extending that to
kilolitres (or even millilitres) is not justified.
Regards,
Bill Hooper
*Yes, I know that technically the various unit names for mass are
multiples and submultiples of the gram not the kilogram. That has no
bearing on my reasoning above.
** My reference gives air density
at 20 °C and normal atmospheric
pressure to be 1.21 kg/m^3. The
value of 1 kg/m^3 used above is OK for crude approximations but is
quite a bit too low. That fact, however, does not affect my other
opinions expressed above.
