To summarize my position (as prelude to a long discussion): K, degrees C, and degrees F can ALL be used as EITHER an element of a temperature
scale OR as a
unit of differential temperature. This stands in contrast to another forum member’s position that C and F are only scales (not units) and therefore cannot be used for differential temperatures .... and that K is only a unit and not a temperature scale.
Great link above by Rafiq confirms that according the US metric association, K can be used as a scale.
I also think it was good for stevenal to post some excerpts from IEEE 100 and I would like to spend some time (the bulk of this post) talking abouut that. I suspect the reason the IEEE100 definitions were posted is because they don’t explicitly state that C can be used as unit or scale and K can be used as unit or scale. But considering there is only one or two sentences per definition, that omission is certainly not a contradiction to my position. And closer inspection will in fact reveal that IEEE100 supports my position (that either K or C can be used as a unit or a scale) based on examination of the definitions themselves and also by examination of useage of those terms in other parts of the document.
First look at the IEEE100 definition for C
IEEE100 said:
degree Celsius (metric practice) It is equal to the kelvin and is used in place of the kelvin for expressing Celsius temperature(symbol t) defined by the equation t = T – T0 where T is the thermodynamic temperature and T0 = 273.15 K by definition. (QUL) 268-1982s
Now look at the phrase
“It [degree Celsius] is equal to the Kelvin” and ask yourself whether “it” [degree Celsius] is being used as an element of a scale (to indicate temperature) or a unit (such as to indicate differential). It is clearly not being used as a scale because a temperature expressed in Celsius is not the same as a temperature expressed in Kelvin. So it is being used as a unit in the same way we can use C as a unit to express a differential temperature. It is perfect and complete agreement with my position.
Now look at the IEEE100 definition of C and work backward to understand the terminology used in the standard. How is it that we know that the “T” in the above equation “t=T-TO” is an absolute temperature (we already know from our prior knowledge of these relationships, but what word in this particular definition tells us that T is absolute) ? It must be by the use of the phrase “thermodynamic temperature”. Thus
in the usage of IEEE100, the phrase “thermodynamic temperature” denotes absolute temperature. And where else is that phrase used within IEEE100? In the definition of Kelvin (
“unit of thermodynamic temperature” ). But that distinction between absolute and relative temperature would not be important if K were used soley as a unit, and so there is no reason to use the phrase “unit of thermodynamic temperature” if K were used soley as a unit. It is thus clear the authors anticipated K would be used as a scale (in addition to a unit) and they are defining for us that the K scale is a thermodynamic temperature.
Now this last point I will certainly agree is more convoluted than the first, but such is the nature of trying to draw conclusions from a document that does not explicitly address the question. More importantly we don’t have to stop with the definitions, we can look at how the terms C and K are used elsewhere in the IEEE100 standard and we find as expected that it is 100% consistent with my position.
There are many IEEE100 usages of Kelvin as a temperature scale. I will quote just one but you can look for yourself to find many others:
ieee100 said:
“chromaticity….(4) (electric power systems in commercial buildings) The measure of the warmth or coolness of a light source, which is expressed in the Kelvin (K) temperature scale.”
Note also the works “Kelvin temperature scale”… that’s what I have been saying… Kelvin can be used as a unit or a scale.
And now more directly related to the question at hand:
ieee100 said:
“insulation class …. NEMA Class B. An insulation system (130 C temperature limit including a 40_C ambient or 90C rise)”
Above we are using C as a unit for a rise (not as a temperature scale). Again, that’s exactly what I’ve been saying since my very first post.
I notice wikipedia was singled out for comment, but wikipedia was certainly not the only reference that was quoted. There was IEEE-1-2000, there was the dimensional analysis book by those Canadian authors, and there were many other textbook links provided. And now add to that the link by Rafiq.
I think my point is very well supported, but it occurs to me that for someone reading casually this appears to be a question of “dualing standards”: The IEC gives rise in K and the IEEE gives rise in C….. so which one do we believe? Well…. that is certainly a contradiction to the position of others who stated K is the only unit for rise. But it is 100% consistent with my position that either C or K (or F) can be used as either a unit (such as for rise) or a scale (for temperature measurement).
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