Why not publish that air has a density of about 1 g/L. Most people can visualize both a litre and a gram.
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Bill Hooper Sent: 06 May 2007 00:58 To: U.S. Metric Association 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.
