Gene Mechtly's note in USMA 11891 set me to digging into my documentation. Gene wrote: >> On Tue, 27 Mar 2001, Marcus Berger wrote: >> > ... send me a small essay containing the definition of coherence >> > and consistency in metrology,... >Marcus, > Here is a statement on "coherence" from NIST and ISO: > > "A system of units is coherent with respect to a system of >quantities and equations if the system of units is chosen in such a way >that the equations between numerical values have exactly the same form >(including the numerical factors) as the corresponding equations between >quantities." > > This statement is quoted from Footnote 2, on Page 3 of the 1995 >Edition of NIST SP 811, which references [6: ISO 31-0]. I dug into ISO 31-0, which uses about 4 pages to explain what is coherent. I think an example would help. We learn that force = mass X acceleration. Those are quantities. When we bring units into the picture we get force in pounds-force = 1/32.2 mass in pounds-mass X acceleration in feet per second squared. Those units are not coherent. But force in newtons = mass in kg x acceleration in metres per second squared is in coherent units. SI is a coherent system. Imperial is not. Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
