Bill,
There is no such term as "absolute temperature" in use anymore. We use
the fundamental physical quantity known as "thermodynamic temperature",
<i>T</i> to describe how hot something is. Because of a long history of
previous practice, thermodynamic temperature may also be expressed as
Celsius temperature, <i>t</i> such that
<i>t</i>/°C = <i>T</i>/K - 273.15 .
The CGPM also provides, via CIPM and BIPM, realizations of physical
quantities. In this case, we currently use ITS-90 as the scale
descriptor for temperature realizations. Temperatures measured with
respect to this may be stated as
<i>t</i>_90/°C = <i>T</i>_90/K - 273.15 .
It is realized that there are differences between <i>t</i> and
<i>t</i>_90 just as there are differences between <i>T</i> and <i>T</i>_90.
It is also well stated that the kelvin and the degree Celsius are of the
same magnitude and thus interchangeable for temperature intervals.
Please see the SI Brochure, p. 164, CIPM 1989 Recommendation 5. Also see
http://www.bipm.org/utils/common/pdf/its-90/ITS-90.pdf
I also recommend the BIPM page at
http://www.bipm.org/en/publications/its-90.html
You will see from these references that either scale may be used. Also,
"[a] difference in temperature may be expressed in kelvins or degrees
Celsius". So, if you want to tell your neighbors that the temperature in
your living room is "26 kelvins above the freezing point of water", have
at it. It's OK for you to say that.
But it's also OK for me to say the temperature in my living room is "26
°C". It's OK for me to say that.
If you want the Celsius option to go away and no longer be available for
use by those of us who are fond of Celsius temperatures, you are barking
up the wrong tree to post things here on that topic. The USMA cannot
change this situation for you. You should be writing to the Consultative
Committee for Thermometry (CCT). Appropriate contact information is on
page 18 of
http://www.bipm.org/utils/en/pdf/directory.pdf
and at
http://www.bipm.org/en/committees/cc/cct/ .
It should ease your concerns a bit to know that I have found BIPM
personnel to be quite responsive to my communications in the past. You
might also inquire at NIST to see who there is our representative on the
CCT and work through them.
Personally, I like the Celsius temperature scale just fine, thank you.
Good luck.
Jim
On 2010-09-01 1218, Bill Hooper wrote:
On Sep 1 , at 2:49 AM, Stan Doore wrote (asking about temperatures in
photography):
I guess they are degrees C like any other temperature. Right?
Yes, but all such temperatures could be stated as 20 k above freezing.
We don't really need degrees Celsius.
It might be helpful to agree that, whenever absolute temperature is
intended (when we mean how many kelvins above absolute zero), then one
would need to specify "absolute temperature". Then, the phrase "above
freezing" might be omitted whenever absolute temperature is NOT intended
(which is usually the case).
It would be the same as the elevation above sea level example I keep using.
We usually measure elevations above sea level so we do not usually
specify "above sea level". It is "understood" to be the case.
In those case where we are actually measuring distances from the center
of the Earth, it is then necessary to specify "from the center of the
Earth".
Getting back to the temperature case, what I am suggesting is simply
that we replace "degrees Celsius" by "kelvins above freezing", plus the
possibility that we could normally omit the phrase "above freezing" when
that is understood to be the case (which is most of the time).
Bill Hooper
1810 mm tall (when I'm standing up)
Fernandina Beach, Florida, USA
==========================
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