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Subject: UnitSystem/@type : possible values
As we've talked about pre-populating the UnitSystem/@type attribute with predefined values (to be union'd with either an xsd:string or a xsd:token), here are some suggestions for possible values (way more and more fine-grained than likely helpful, see end of message for a boiled down suggestion): - SI (probably too general) - non-SI (probably too general) - SI base (or SI_base or SI-base) (cf. NIST SP330-2008; 1.2; p.10) - SI derived (or SI_derived or SI-derived) (cf. NIST SP330-2008; 1.4; p.12 & 2.2, p.23) - SI coherent derived or SI coherent (cf. NIST SP330-2008; 1.4; p.12 & 2.2; p.23) - SI special or SI special derived or SI special coherent or SI special coherent derived (cf. NIST SP330-2008; 2.2.2; pp. 24,25) - non-SI acceptable or accepted (or ..) (cf. NIST SP330-2008; 4.1 / Table 6; pp. 31,32) - non-SI experimental(ly determined) (cf. NIST SP330-2008; 4.1 / Table 7; pp. 33; 34) [!! "Because the quantity systems on which these units are based differ so fundamentally from that on which the SI is based, they are not generally used with the SI, and the CIPM has not formally accepted them for use with the International System" -- p.33 !!] - non-SI temporarily acceptable (cf. NIST SP330-2008; 4.1 / Table 8 & 9; pp. 32; 35; 37) - non-SI non acceptable (cf. NIST SP330-2008; 4.2 ; p. 37 & http://www.bipm.org/en/si/si_brochure/chapter4/conversion_factors.html [btw, this ends up on physics.nist.gov -- isn't the world a small place?]) - (SI) acceptable units (cf. NIST SP811-2008; 5.4; p.11 : "The term 'acceptable units', which is introduced in this Guide for convenience, is used to mean the SI units plus (a) those non-SI units accepted for use with the SI (...) and (b) appropriate multiples and submultiples of such accepted non-SI units") ----------- about the term "coherent": (all following quotes from NIST SP330-2008; 1.4 p. 12, typos mine; not Taylor/Thompson's) "Derived units are defined as products of powers of the base units. When the product of powers includes no numerical factor other than one, the derived units are called coherent derived units" Note: numerical factor, not exponent. Clarifying this is the note on the word coherent further down in that paragraph: "The word coherent is used here in the following sense: when coherent units are used, equations between the numerical values of quantities take exactly the same form as the equations between the quantities themselves. Thus if only units from a coherent set are used, conversion factors between units are never required." "The base and coherent derived units of the SI form a coherent set, designated the set of coherent SI units" ----------- what I don't really understand is where to fit in multiples and submultiples, as they don't seem to be (explicitely) in either of the mentioned ones so far (except for the broad "SI" one). ----------- So for my personal preference (disclaimer: I'm no SI expert!) I think for UnitsML fitting choices would be: SI, non-SI, SI base, SI derived, non-SI acceptable, non-SI temporarily acceptable, non-SI non acceptable (not necessarily in this wording) plus "US", "inch pound" or whatever the US calls "their" system. SI and non-SI cover basically all the units (if you want or have to remain vague); SI is partitioned into SI base and derived (I'm leaving out coherent derived as even in the committee the meaning of 'coherent' wasn't inherently clear); non-SI is partitioned into the accepted, temporarily accepted and non accepted classes. If that's what the (outside of the TC's scope lying) UnitsDB is using is another question -- but for UnitsML I think these 7 (8) values would be a good population for the enumerated list in UnitSystem/@type. Regards, -Martin
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